India's Space based Telescopes and Astronomical spacecraft

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2. Observing cycles​

In this second AO cycle, 35% of observing time is available for AO proposals and 5% time for Targets of Opportunity (ToO). Rest of the 60% time in this cycle is allotted for payload teams and calibration.

2.1 AO cycle

AstroSat is operated in a pre-planned manner i.e proposers are not present at Mission Operations Complex during the execution of their observations. Thus, all observations must be specified in full details in advance.

• The percentage of observing time for executing AO proposals during April to September 2017 is ~35% and is termed as second AO cycle.

• This 35% time is exclusive for Indian proposers as Principal Investigators (PIs) to utilise AstroSat observatory time. They could be interested researchers, scientists and astronomy community at large, involved in scientific research in the field of astronomy and are equipped to submit proposals as Principal Investigators (PIs) for specific target observations with necessary scientific and technical justification and can analyse the data, if the target is observed based on approvals.

• In order to expand the AstroSat user community, it is recommended that those who are submitting proposals as PIs in Guaranteed Time (GT) cycle, encourage other astronomers to be PI in AO proposals. If a proposer from a university, college or any institution submitting a proposal under AO cycle wishes to have a Co-I from any payload teams to get help in data analysis or needs support on science related issues, he/she may do so in consultation with the payload team leads as provided in the handbook.

• All the selected AO proposals will be inserted into the observing schedule. However, few observations approved in this AO cycle may be scheduled outside of the above period, in case there is operations requirement, which will be provided by AstroSat Mission.

• Proposers are requested not to duplicate any of the planned observations and should carefully justify duplications if any, with performed observations. Proposers have to check the list of objects and instrument parameters for observations already/ being carried out using AstroSat. A Red book containing these targets will be made available in the ISSDC website. Proposers wishing to observe any of these targets should justify why it is important to do so and what additional information will come from the proposed observations. See also section 4.

• Checks for duplications will be performed by ATAC while processing the proposals during the scientific review.

2.2 Targets of Opportunity (ToO) cycle

• Proposals that require observation of phenomenon like outburst of a supernova or nova, observation of a new transient source or X-ray nova or study of characteristics of a source when it makes transition to a different state etc. and for which one cannot predict in advance the time of occurrence, must be submitted as ToO proposal and they will be reviewed by a separate ToO Committee.

• A ToO cycle is always open for submitting proposals at present for any Indian proposer. A provision of 5% observing time is reserved for ToO proposals.

• Data of observations made using a ToO proposal during the allotted ToO time will be made public immediately.

• If this time is not fully subscribed, it may be added to the time for the AO proposals.

• Anticipated ToOs are ToOs whose source position is known but time of observation is not known or unpredictable. The proposal for anticipated ToOs can also be submitted through APPS, which will be reviewed by ATAC committee. The scientific justification needs to be really strong to clear the proposal and also has to get priority “A” for acceptance (refer APPS proposer's guide available in ASC website for details on anticipated ToOs and priority ranking for proposals). However when the event occurs, a ToO trigger proposal by the PI of the original proposal has to be submitted under ToO cycle to schedule the proposal. The data rights are same as that for AO proposals.

 

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3. Overview of proposal preparation, submission and selection​

Proposers/ guest observers will have to submit proposals to ISRO by the deadline given in the ISSDC website using AstroSat Proposal Processing System (APPS) software. APPS is available online through http://www.issdc.gov.in. APPS is not downloadable and cannot be used off-line. An APPS proposer’s guide will be made available in ISSDC and ASC websites which elaborates on the proposal submission procedure. A summary is provided in this section.

3.1 Proposal preparation pre-requisites

Depending on the scientific requirement, proposals to AstroSat can be submitted for observation with a single or more instruments. Proposals are to be made as per APPS proposer’s guide and this procedures document. Proposers can refer to redbook for the list of observed targets.

AstroSat proposals will require the following information at the minimum.

• Source coordinates, source angular size if extended, V magnitude, 2-10 keV flux, estimated count rates for different instruments, exposure times, UVIT bright source list, Astroviewer output for feasibility of observations of the target in pdf format. (use AstroSat tools listed in section 3.3)

• Instrument configuration parameters such as instrument mode, filter, etc. (Ref. Handbook)

• Scientific and technical justification.

3.2 APPS Instructions

Instructions to fill various entries within APPS to prepare proposals are available online. APPS proposer’s guide can also be referred for this purpose. Queries on APPS can be mailed to [email protected] for proposal preparation and submission. Queries will be answered on best effort basis.

3.3 Proposal Preparation Tools

Proposers can use the following tools in order to prepare an AstroSat proposal.

• Astroviewer (http://issdc.gov.in/astroviewer/index.html) to determine Astrosat visibility period for a target of interest.

• PIMMS (http://astrosat-ssc.iucaa.in:8080/WebPIMMS_ASTRO/index.jsp) to estimate source count rates.

• UVIT Exposure Time Calculator (http://uvit.iiap.res.in/Software/etc) to estimate required exposure time for an astronomical source.

• UVIT Bright Source Warning Tool (http://uvit.iiap.res.in/Software/bswt) to set filters of the VIS channel for ensuring: a) safety of the detector against overexposure, b) adequacy of signal from stars in the field for tracking drift of the S/C.

• AstroSat Schedule viewer (https://astro.issdc.gov.in:9321/MCAP/) to check Tentative/Confirmed Target Schedule of Astrosat.

• Spectral simulation tools such as X-ray Spectral fitting package (XSPEC), Integrated Spectrographic Innovative Software (ISIS), Modelling and fitting package (Sherpa) which are available online.

3.3.1 ASTROVIEWER - Tool to aid Celestial Source Viewing

The tool gives the view periods of a selected celestial source for a prolonged period of one year maximum. Also, the view periods that satisfy all the constraints are provided orbit-wise so that the proposer can plan their observations more accurately and also season-wise. For UVIT payload users, view duration timings during eclipse that satisfies all envisaged constraints are available in a separate file as UVIT is expected to observe only in eclipse. The Tool has been designed to use the latest orbit information available on a daily basis and provides the various constraint angle characteristics in graphical plots so that proposer can visualize the situation while planning for observations. The view period of the selected source is stored orbit wise and is made available in tabular form for the proposer to use. Since the ram angle constraint for certain sources are on and off due to the closeness to the orbit inclination, this output contains flags ‘0’ for satisfying the constraint and ‘1’ for violating the constraint in the table. A Graphical User Interface program allows the user to interact remotely and obtain the required details. Additional information like Eclipse and occult Entry/Exit is also made available.

Geometrical Constraints

• RAM angle (+ROLL and velocity vector) > 12˚
• Terminator (+ROLL and Bright Earth Limb) > 12˚
• Sun Angle (+ROLL and SUN) > 65˚
• Angle b/w +YAW and SUN > 90˚
• Angle b/w Star Sensor and SUN > 50˚ Angle b/w +ROLL and Albedo > 12˚

3.3.2 Portable Interactive Multi-Mission Simulator (PIMMS)

The AstroSat PIMMS package (downloadable from http://astrosat-ssc.iucaa.in/ or accessed online at http://astrosat-ssc.iucaa.in:8080/WebPIMMS_ASTRO/index.jsp) is an implementation of the Portable Interactive Multi-Mission Simulator package, originally distributed from NASA/GSFC High Energy Astrophysics Science Archive Research Centre (HEASARC). This implementation includes the effective area of AstroSat X-ray instruments and can be used to estimate source count rates in LAXPC, SXT, CZTI and SSM for a variety of input spectral models. A user manual is distributed with the downloadable version, and online help is available for the WebPIMMS version.

Response files: Response Matrix files and estimated background spectra are provided for LAXPC, SXT and CZTI payloads at the website http://astrosat-ssc.iucaa.in. These may be used to carry out spectral simulations for X-ray sources, for example with the fakeit command in HEASOFT XSPEC.

3.3.3 UVIT Exposure Time Calculator (ETC)
Help Page: http://uvit.iiap.res.in/Software/etc/Help Present Version: 2.0.0 (03 May, 2016).

The UVIT Exposure Time Calculator (ETC) will help assess the feasibility of an observation. It calculates the expected count rate from a source in various UVIT filters, followed by either i) The Signal-to-Noise Ratio (SNR) achieved for a given observation time, or ii) The time required to reach a given SNR. Users may choose from a range of astronomical sources/spectra such as a star, black-body, galaxy, power law, etc. or choose to upload their own source spectrum.

3.3.4 Bright Source Warning Tool (BSWT)
Help Page: http://uvit.iiap.res.in/Software/bswt/Help Present Version 1.6.1 (4 July 2016).

Aim of the tool is to inform the proposer whether the region of the sky around a science target is safe / unsafe for UVIT to take observations. The program scans for stars brighter than the safety threshold and lists out the count rates of these bright stars in all the 10 filters in the FUV and NUV telescopes. This program identifies all the bright stars within 20 arcmin radius of the target object. See also guidelines document at the same website. Please note that as per the latest process used by UVIT this output is only used to check for filters of VIS (320-550 nm) channel; the checks for NUV/FUV filters are not made with this list. Hence the following mandatory checks are necessary.

Mandatory checks to be done for UVIT observations

The UVIT is not designed to observe very bright sources and the presence of a bright source in the UVIT field of observation can cause “Bright Object Trigger” in the hardware that would switch OFF all three detectors. In addition, the presence of an ultra-bright source near the UVIT field of view will scatter excessive radiation beyond the allowed limit. One of the UVITs, the VIS channel is primarily used for the spacecraft tracking. It is the proposers’ responsibility to ensure smooth tracking during their proposed observations. Hence the proposers need to exercise extra caution in preparing a proposal for UVIT observations. It is strongly recommended that the proposers follow the guideline described in detail in the document (ref:http://uvit.iiap.res.in/sites/uvit.iiap.res.in/files/Guidelines_for_proposal_submission_3.pdf, v 1.0, 15 November 2016) for mandatory checks to be done for UVIT observations.

3.4 Preparing an ASTROSAT proposal

Proposers will need to register into the APPS before they can prepare proposals.

Proposers may go through the APPS help document regarding submission of proposals.

3.5 Proposal handling in APPS

The receipt of each incoming proposal will be automatically acknowledged. At the end of submission date, the APPS will forward them to the ATAC for scientific review, while performing some assessments and preparing overall statistics on the response. ATAC is already constituted by Chairman, ISRO.

The ATAC will assign priorities to each proposal as A, B and C (and, as needed, grade individual observations within a proposal). The ATAC may ask some proposers to reduce the observing time or the number of targets in a proposal. Such proposals will be made available for revision to the PIs. The proposers will be able to submit a revised proposal before the set deadline only for changes recommended by the ATAC. Such partially allocated proposals, if not revised before the deadline, will be excluded from the list of successful proposal.

The technical feasibility of making the observations will be conducted by AstroSat Technical Committee (ATC) with support from Mission operations team.

One of the parameters used to plan which observations will be carried out during a particular orbit, is the priority of the observations as allocated by the ATAC and ATC. However, for operational reasons, no guarantee can be given that a particular observation will in fact be executed, regardless of its grade.

 

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4. Aspects of duplication​

The general policy of the ASTROSAT is to avoid repeating the same observation, i.e. to avoid duplications.

In general, a duplication is determined by consideration of the target coordinates and of the main observing parameters (especially the instrument (s) and the observing modes). A proposed observation duplicates another one if the expected science data are essentially the same or of lower requirement (e.g. lower exposure time) and is therefore discouraged. It is, however allowed, to observe the same target with the same instrument configuration several times for variability studies. In addition, in large extended objects several pointings in the vicinity of a source may become necessary e.g., to image Coma Cluster of galaxies out to 2 deg. diameter so as to cover its virial extent, and these may have co-ordinates that are not too different from that of the previous observation of a source.

The responsibility for defining and resolving cases of duplication rests with the ATAC in consultation with the Principal Investigator, AstroSat as needed. The ATAC can allow duplications between a proposed observation and an observation of a previous cycle. These should be restricted to proposals which provide convincing evidence that additional data are of scientific relevance.

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5. Data processing, data rights and publication​

After the completion of observation, the raw data received will be converted to Level-1 data at Indian Space Science Data Center (ISSDC). ISSDC is responsible for governing the ingest, Quick look Display (QLD), processing (for level-0/1), archival (all levels, along with the auxiliary data) and dissemination of payload data. The data will be in standard FITS format.

Level-1 data can be downloaded from the ISSDC website by the payload teams or Principal Investigators (PIs) of the proposals for science analysis as well as to produce higher level data products. Sample data, software and utilities are provided in the ASC website.

The PI will be informed, after the completion of successful observation for the downloading of processed Level-1 data. The standard pipeline software from Level-1 to Level-2 and any other higher level standard products will be made available to the Guest Observers proposers through ISSDC website.

5.1 Proprietary period

There shall be a Proprietary period associated with observational data from all AstroSat instruments and in all phases and years after launch. This "proprietary period" would begin from the date the Level-1 data is made available to the payload teams and /or PIs of AO proposal.

During this proprietary period, the data will NOT BE USED by any persons or teams other than those who submitted the proposal(s) for the observations, except in cases where the proposers or PI themselves involve such other persons.

The proprietary period for AO cycle data is 12 months. After the proprietary period, all data will be kept in ISSDC public archive which is accessible both nationally and internationally. It is the responsibility of the Payload Operation Centres (POCs) to provide Level-2 products with a quality report to ISSDC.

Target of Opportunity (ToO) observations which are taken from ToO observation time will be processed immediately to Level 1 data and will be placed in ISSDC archive. These data are non-proprietary and are open to public immediately after observation.

5.2 Data rights & obligations

The Principal Investigators (PIs) of all the proposals will have exclusive rights to all the data from all the co-aligned instruments (namely LAXPC, CZTI, SXT and twin telescope UVIT) for those fields that are observed with AstroSat against their proposals, unless they forgo this right by allowing piggy back setting.

Data rights for other objects detected within the observed field of observation also belong to the PI of the proposal, unless they communicate not to have it. At present there is no way to separate target data and field data. The proposal PI may collaborate with payload teams (and vice versa) for analysis of data on field objects other than primary target.

Proposal PI can relinquish the data rights from other instruments if he/she does not want to use it, by not configuring those payloads. This will allow for piggy back setting by payload teams. Such data will be provided to the payload teams and proprietary period remains the same as AO proposal.

Any instrument team or PI has the right to reduce the proprietary period by sending an email to [email protected] with cc to ISSDC team ([email protected]) recommending for placing the data in ISSDC data archive before the end of the proprietary period.

5.3 Publication

The proposers /guest observers shall make available the salient results of the data analysis to the scientific community through publication in appropriate journals.

All the publications shall acknowledge the AstroSat data, by including a phrase “AstroSat -along with the name of the payload(s)” whose data is used for analysis/ interpretation in the abstract.

When publishing a paper using AstroSat data, please include the following acknowledgement.

“This publication uses the data from the AstroSat mission of the Indian Space Research Organisation (ISRO), archived at the Indian Space Science Data Centre (ISSDC)”.

If a user has used already published AstroSat results and carried out further interpretation or modeling, the following statement may be included in the acknowledgement.

“The research is based (partially or to a significant extent) on the results obtained from the AstroSat mission of the Indian Space Research Organisation (ISRO), archived at the Indian Space Science Data Centre (ISSDC)”.

ISRO may use any/all results that are derived from AstroSat data and published through academic papers in journals or any other publications by the user, for its own use, in its reports and publications with due reference/ acknowledgements to such journals and publications.

 

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Announcement of Opportunity (AO) soliciting proposals for second AO cycle observations

AO procedures

Criteria for applying:

• This announcement is open to Indian scientists/ researchers residing in India and working at institutes/Universities/colleges in India, who
• are involved in research in the area of astronomy and
• are equipped to submit proposals as Principal Investigators (PIs) for specific target observations with necessary scientific and technical justification and
• Can analyse the data, if the target is observed based on approvals.

Announcement of Opportunity (AO) soliciting proposals for second AO cycle observations

• 1. Introduction and Schedule
• 2. Observing cycles
• 3. Overview of proposal preparation, submission and selection
• 4. Aspects of duplication
• 5. Data processing, data rights and publication

 

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Astronomers discover ‘powerful cosmic double whammy’ with help of India’s GMRT
A supermassive black hole and the collision of giant galaxy clusters have combined to create a cosmic acceleration.
By: PTI | Washington |
Updated: January 7, 2017 5:06 PM

The two clusters are both very massive, each weighing about a quadrillion – or a million billion – times the mass of the Sun.

Astronomers, using data from India’s Giant Metrewave Radio Telescope (GMRT), have discovered two of the most powerful phenomena in the universe – a supermassive black hole and the collision of giant galaxy clusters about two billion light years from Earth.

The two phenomenon have combined to create a stupendous cosmic particle accelerator, researchers said.

By combining data from NASA’s Chandra X-ray Observatory, the Giant Metrewave Radio Telescope (GMRT) in Pune and other telescopes, researchers found what happens when matter ejected by a giant black hole is swept up in the merger of two enormous galaxy clusters.

“We have seen each of these spectacular phenomena separately in many places,” said Reinout van Weeren of the Harvard-Smithsonian Centre for Astrophysics (CfA) in the US, who led the study.

“This is the first time, however, that we see them clearly linked together in the same system,” said van Weeren. This cosmic double whammy is found in a pair of colliding galaxy clusters called Abell 3411 and Abell 3412 located about two billion light years from Earth.

The two clusters are both very massive, each weighing about a quadrillion – or a million billion – times the mass of the Sun.

The comet-shaped appearance of the X-rays detected by Chandra is produced by hot gas from one cluster plowing through the hot gas of the other cluster. Optical data from the Keck Observatory and Japan’s Subaru telescope, both on Mauna Kea, Hawaii, detected the galaxies in each cluster.

First, at least one spinning, supermassive black hole in one of the galaxy clusters produced a rotating, tightly-wound magnetic funnel.

The powerful electromagnetic fields associated with this structure have accelerated some of the inflowing gas away from the vicinity of the black hole in the form of an energetic, high-speed jet.

These accelerated particles in the jet were accelerated again when they encountered colossal shock waves – cosmic versions of sonic booms generated by supersonic aircraft -produced by the collision of the massive gas clouds associated with the galaxy clusters.

“It’s almost like launching a rocket into low-Earth orbit and then getting shot out of the Solar System by a second rocket blast,” said Felipe Andrade-Santos, also of the CfA. “These particles are among the most energetic particles observed in the universe, thanks to the double injection of energy,” said Andrade-Santos.

This discovery solves a long-standing mystery in galaxy cluster research about the origin of beautiful swirls of radio emission stretching for millions of light years, detected in Abell 3411 and Abell 3412 with the GMRT.

The team determined that as the shock waves travel across the cluster for hundreds of millions of years, the doubly accelerated particles produce giant swirls of radio emission. The study appears in the journal Nature Astronomy.

 

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Party ho gayi yaar!:biggrin2:
Discovery of a hot companion associated with a Blue Straggler in NGC-188 using AstroSat UVIT data

An Open Star Cluster consists of hundreds to thousands of stars, which are loosely bound. They are formed most likely from a single gas cloud, and are therefore roughly of same age. Open clusters and particularly old open clusters therefore are ideal sites to study the stellar evolution for both single and binary stars. Most stars evolve away from the main sequence once their hydrogen burning phase is over. The turn over point of the Hertzsprung – Russell (HR) diagram of an open cluster is indicative of its age.

Blue Straggler Stars (BSS) are members of old clusters that are brighter and bluer than stars on the upper main sequence. They appear to 'extend' the main sequence in a HR diagram of the cluster and appear as if they are 'younger' stars. They are termed stragglers because they do not move away from the main sequence like the other stars in the same cluster.

NGC-188 is a well-studied old open cluster with an estimated age of 7 Gyr (Billion year, astronomically known as Giga Year- Gyr) and exhibits high metallicity. It is located about 5000 light years away and has about 1050 stars as its members with 20 BSSs confirmed. WOCS-5885, most likely a member of NGC-188 (with a high probability of 53 to 80% quoted in literature), was one of the 3 objects identified with exceptionally blue color. Various classifications -a BSS or a sub-dwarf or a binary with a red giant and a pre-white dwarf to name a few- were attributed to this object, because its spectrum did not match with any single identification. This could only be resolved if the hot (UV, blue) and the cool (red, IR) part of the spectrum of this object could be fitted together with spectral models of stars. This had been done with observations from space (GALEX, UIT, UVOT, SPITZER, WISE) and several ground based observatories, spanning the IR, optical and UV bands.

The UV band observations from the Ultraviolet Imaging Telescope (UVIT) on ASTROSAT have provided additional points in the Spectral Energy Distribution (SED) thus resulting in a much better spectral fit over the wavelength range of 0.15 µm to 7.8 µm. With this data set, WOCS-5885 has been classified as a binary consisting of a BSS and a hot star which is either a post Asymptotic Giant Branch or Horizontal branch (post-AGB/HB) star.

The UVIT contains two 38-cm telescopes; one for the far-ultraviolet (FUV) region, the other for the near-ultraviolet (NUV) and visible (VIS) regions. These are divided using a dichroic mirror for beam splitting. UVIT is primarily an imaging instrument, simultaneously generating images in the FUV, NUV and VIS channels over a 28 arcmin diameter circular field. Each channel can be divided into smaller pass bands using a selectable set of filters.

UVIT observed NGC-188 both as a first light object and for regular calibration. The observations have been done in both NUV and FUV filters in the wavelength band of 0.3 to 0.15 µm. With these observations, it is found that the SED can only be fit with spectra consisting of 2 stars. The cooler star is found to be a BSS with a temperature of 6,000+150 K, and the temperature of the hotter star is 17,000+500 K. The estimated size and luminosity of the hotter star rule out a white-dwarf or a sub-dwarf classification and hence it is proposed that it could be a post AGB/HB star. If the membership of WOCS-5885 to NGC-188 is confirmed, it could be a rare BSS + post AGB/HB binary, the first of its kind to be identified (for which probability is high) in an open cluster. This system therefore provides a great opportunity to constrain theories of BSS formation via mass transfer.

Thus, observations from the UVIT were used to solve the puzzle of a star WOCS-5885 which appeared as a single star but whose spectra did not match with this identity.

For details:

Subramaniam Annapurni et al., A Hot Companion to a Blue Straggler in NGC-188 as Revealed by the Ultra-Violet Imaging Telescope (UVIT) on ASTROSAT-The Astrophysical Journal Letters, Volume 833, No. 2, 19 December 2016.

FUV (left) and NUV (right) images of NGC-188 obtained on 18 February 2016.
WOCS-5885 is marked as red square

Spectral Energy Distribution (SED) for WOCS-5885. Points in light green are from UVIT

Story of the Week - Archive
Jan 09, 2017 : Discovery of a hot companion associated with a Blue Straggler in NGC-188 using AstroSat UVIT data
Jan 02, 2017 : Golden Jubilee of Composite Materials Activities at ISRO
Dec 26, 2016 : Indigenous Development of 4.5 ton Vertical Planetary Mixer
Dec 19, 2016 : First National Finite Element Developers/FEASTSMT Users’ Meet at VSSC
Dec 05, 2016 : National Rollout of GeoMGNREGA held at Vigyan Bhavan, New Delhi
Nov 28, 2016 : Applications of Unmanned Aerial Vehicle (UAV) based Remote Sensing in NE Region
Nov 21, 2016 : ISRO Telemetry, Tracking and Command Network (ISTRAC) Celebrates Ruby Year

• 1 of 12
  
• next ›

 

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'Vampire' star caught in the act by Indian space observatory

India's first dedicated space observatory, ASTROSAT, has captured the rare phenomenon of a small, 6-billion-year-old "vampire" star "preying" on a bigger celestial body.

Scientists say the smaller star, also called a "blue straggler", feeds off its companion star by sucking out its mass and energy, causing its eventual death.

"The most popular explanation is that these are binary systems in which the smaller star sucks material out of the bigger companion star to become a blue straggler, and hence is called a vampire star.

"The small star becomes bigger, hotter and bluer, which gives it the appearance of being young, while the ageing companion burns out and collapses to a stellar remnant," said Annapurni Subramaniam, a Professor at the Indian Institute of Astrophysics.

Though this phenomenon is not unheard of, the observation of the entire process through the telescope will provide insights that will help scientists in studying the formation of 'blue straggler' stars.

This discovery also highlights the capabilities of the telescopes on ASTROSAT, a dedicated space observatory satellite launched in September 2015.

The study was recently published in Astrophysical Journal Letters by a team of scientists from IIA, Inter-University Centre of Astronomy and Astrophysics (IUCAA), Tata Institute of Fundamental Research (TIFR), Indian Space Research Organisation (ISRO) and the Canadian Space Agency (CSA).

Scientists are now looking to understand the chemical composition of the 'blue straggler' using high resolution spectroscopy, which could reveal more about the evolution of these peculiar celestial objects.

The stars are part of a "cluster" called NGC 188 formed some 6 billion years ago, and are much older than the sun, which is believed to have come into existence nearly 4.5 billion years ago.

"As the sucked up material from the ageing star will be polluted with material processed within the ageing star, this blue straggler will throw light on the kind of nuclear processing that happens in ageing stars", Subramaniam, also the Calibration Scientist of UVIT on board the ASTROSAT, said.

"The data from six filters of UVIT were used to estimate the precise temperature of the companion star, its size and brightness. This has been possible only due to the excellent capability of the UVIT telescope," Subramaniam added.

The UVIT is made of twin telescopes with an effective diameter of 375mm each and records images in the Far UV, Near UV and visible frequencies. UVIT provides very sharp UV images over a field of view as large as the Moon.

"The companion (the bigger star) is still going through ageing and has not yet become a remnant. It is also hot and large. Therefore it appears very bright in ultraviolet image, but not so bright in an image taken with an optical telescope looking at visible light," she said.

This is why previous studies of the 'blue straggler' in the optical range could not detect the companion. "This pair thus becomes a rare sample to study the details of the formation of blue straggler stars," Subramaniam added.

The purpose of ASTROSAT is to understand high energy processes in binary star systems containing neutron stars and black holes, estimate magnetic fields of neutron stars, study star birth regions and high energy processes in star systems lying beyond our galaxy.

It is also tasked to detect new briefly bright X-ray sources in the sky and perform a limited deep field survey of the universe in the Ultraviolet region.

Apart from the UVIT, the ASTROSAT also includes a Large Area X-ray Proportional Counter (LAXPC), a Soft X-ray Telescope (SXT), a Cadmium Zinc Telluride Imager (CZTI) and a Scanning Sky Monitor (SSM).

These instruments observe the sky for electro-magnetic radiations in the visible, ultraviolet and X-ray frequency ranges coming from distant celestial sources.

 

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Not orbital but related from Bhabha Atomic Research Center (BARC)...

In view of the present developments in the field of GeV/TeV astronomy, the Division proposes to set up a 21m diameter imaging telescope at the high altitude (4200m) observatory site at Hanle in the ladakh region of northern India. Operating at an energy threshold of ~ 20 GeV this telescope is expected to detect a large number of sources in the GeV sky.

http://www.barc.gov.in/pg/nrl-harl/mace.html

The telescope is named after the Soviet scientist Pavel Cherenkov, who predicted that charged particles moving at high speeds in a medium emit light. The high-energy gamma rays emitted from black holes, centers of galaxies and pulsars do not reach the land as they get absorbed in the atmosphere. Upon interaction with the atmosphere, these photons produce electron–positron pairs, leading to a cascade of particles which while moving at very high speed give rise to Cerenkov radiation.

Very-high-energy (VHE) gamma rays offer a unique insight into some of the most extreme phenomena of our Universe. Detection of celestial VHE gamma rays allows the study of exotic objects like pulsars, pulsar wind nebulae, super nova remnants, micro-quasars, active galactic nuclei etc where particles are accelerated to TeV (10 exp 12 eV) energies and beyond. These exceptionally energetic photons are detected on the Earth by an indirect process which uses the Earth's atmosphere as a transducer. The Cherenkov light is beamed around the direction of the incident gamma ray and covers an area of around 50,000 square meters on the ground. This effective area is far larger in magnitude than the area of satellite instruments used for detecting gamma rays directly. To detect these flashes of Cherenkov light, photomultiplier tube cameras are used at the focus of large tracking light collectors. The intensity of the image recorded by the telescope is related to the energy of the incident gamma ray photon.

The MACE Telescope consists of a large-area tessellated light collector of 356 m², made up of 356 mirror panels. A high-resolution imaging camera weighing about 1200 kg, for detection and characterization of the atmospheric Cherenkov events, forms the focal plane instrumentation of the telescope. The elevation over azimuth mounted telescope basket structure has two axes movement capability of ± 2700 in azimuth and -260 to +1650 in elevation for pointing towards any source in the sky and tracking it. The telescope, which weighs about 180 tons, is supported on six wheels which move on a 27-metre-diameter track.

The telescope has an integrated imaging camera, which contains 1088 photo multiplier-based pixels and all the signal processing and data acquisition electronics. The camera communicates the acquired data to the computer system in the control room over optical fiber.

The main features of the telescope include safe and secure operation of the telescope remotely from anywhere in the world, and its structure is designed to operate in winds of speed up to 30 km/h and retain its structural integrity in the parking position in winds of speed up to 150 km/h.

https://en.wikipedia.org/wiki/Major_Atmospheric_Cerenkov_Experiment_Telescope

At 21m, I believe this the world's second-largest Gamma Ray Telescope, after the HESS Telescopes in Namibia (operated by a 13-nation collaboration), where the largest one (known as HESS-II) has a 28m mirror. The MACE is expected to begin operations sometime this year.

 

[AnantS]

@gadeshi: Look above Indian Tribute to Russian Scientist

 

[Indx TechStyle]

X-ray Polarimeter Satellite (XPoSat) Mission

Imaging, timing and spectral studies are being carried out by various X-ray satellites, but X-ray polarisation studies of celestial objects has been minimal. The X-ray Polarimeter Satellite (XPoSat) mission is the first dedicated mission for polarisation studies and was approved in April 2016. Polarimeter Instrument in X-ray (POLIX) payload will study the degree and angle of polarisation of bright X-ray sources in the energy range 5-30 keV. Payload development is in progress at Raman Research Institute (RRI), Bengaluru.

QM mechanical components (detectors and collimator) are ready. Electronic packages are under fabrication. Prototype of Be / Li scatterer is under development at BARC.

Source: Annual Report 2016-17

 

[Chinmoy]

We could say that XPOSat would be enhanced version of one aspect of ASTROSAT. In itself, ASTROSAT is already doing X Ray imaging.

 

[brational]

Yeh kya ghanta Chal raha hai? Black holes, vamp stars, seems everything is terrorized in the deep space too... Bhai log Kuchh achcha news post karo... Like sunny leone got impregnated by solar radiation. Something like that...

Totally tired after month end closure...

 

[Indx TechStyle]

We could say that XPOSat would be enhanced version of one aspect of ASTROSAT. In itself, ASTROSAT is already doing X Ray imaging.

It's AstroSat-2.
AstroSat performs multiple observations while Polix is dedicated for specific bodies.

 

[Indx TechStyle]

India's AstroSat catches astronomical imposter
India’s AstroSat catches astronomical imposter Download PDF

Indian scientists leading an international team of astronomers showed that a new object discovered in the sky, believed to be related to the latest gravitational wave discovery, was in fact an unrelated gamma ray burst.

The LIGO scientific collaboration’s discovery of GW170104 led to a frenzy of activity among partner astronomers around the world, each trying to find any associated explosions in the sky. The Hawaii-based ATLAS group found a source that was in the right place in the sky, and was fading fast – causing excitement all around. But was it really associated with GW170104? Was it the first discovery of an optical source related to a gravitational wave detection? No, according to a study by the AstroSat CZTI team and the international GROWTH collaboration.

While studying observations of the source – named ATLAS17aeu – the team noticed something odd about how fast it was fading. “Analysing the data, I concluded that ATLAS17aeu must be related to some explosion on 5th January, not the 4th”, says Varun Bhalerao (IIT Bombay), the lead author in this study. The team had already used CZTI to look for X-rays coming from GW170104, and not seen any. CZTI (Cadmium Zinc Telluride Imager), a gamma ray telescope on ISRO’s maiden space observatory AstroSat, proved to be the most sensitive instrument in the world to find transient sources with sub-second durations. So if there was another burst in the sky, they were sure they would find it in CZTI data. Varun continues, “I shot off an email to my student Sujay, asking him to search for a burst in CZTI data in the calculated time window. And then I noticed an email from Vidushi (another student) in my inbox: she had found the burst I was looking for!”

The culprit seemed to be a gamma ray burst GRB 170105A – that exploded in the same part of the sky 21 hours later. But to be sure of this, astronomers needed more data. Enter GROWTH: Global Relay of Observatories Watching Transients Happen. This multi-national team had already swung into action, observing ATLAS17aeu with optical, X-ray and radio telescopes. Team member Dipankar Bhattacharya (IUCAA) said, “The team studied the source with radio, optical and X-ray telescopes for a few days, till it faded away into oblivion. Based on its behaviour we concluded that this event signalled the birth of a new black hole when a massive star imploded in a galaxy several billion light years away.”

This gamma ray burst was missed by several other international satellites which were pointing at other parts of the sky at that instant. It was detected only by the Cadmium Zinc Telluride Imager (CZTI) on AstroSat, and by the Chinese-European POLAR instrument. “This is the result of insightful instrument design, imaginative onboard software, and collaborative data analysis from a nationwide team”, says A.R. Rao (TIFR). But the team has its eyes on the prize: finding the first electromagnetic counterpart to a gravitational wave source. And the highly sensitive CZTI might not be enough. Rao adds, “We need wide angle detectors scattered over interplanetary space to discover X-rays from LIGO sources. The CZTI team has proposed a small sized instrument called MOTIVE to ISRO as a likely payload for a future interplanetary mission. Together, CZTI and MOTIVE can revolutionize the field!”

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CZT-Imager is built by a consortium of Institutes across India. The Tata Institute of Fundamental Research, Mumbai, led the effort with instrument design and development. Vikram Sarabhai Space Centre, Thiruvananthapuram provided the electronic design, assembly and testing. ISRO Satellite Centre (ISAC), Bengaluru provided the mechanical design, quality consultation and project management. The Inter University Centre for Astronomy and Astrophysics (IUCAA), Pune did the Coded Mask design, instrument calibration, and Payload Operation Centre. Space Application Centre (SAC) at Ahmedabad provided the analysis software. Physical Research Laboratory (PRL) Ahmedabad, provided the polarisation detection algorithm and ground calibration. A vast number of industries participated in the fabrication and the University sector pitched in by participating in the test and evaluation of the payload. The Indian Space Research Organisation funded, managed and facilitated the project.

This work was supported by the GROWTH project funded by the National Science Foundation under Grant No 1545949. GROWTH is a collaborative project between California Institute of Technology (USA), Pomona College (USA), San Diego State University (USA), Los Alamos National Laboratory (USA), University of Maryland College Park (USA), University of Wisconsin Milwaukee (USA), Tokyo Institute of Technology (Japan), National Central University (Taiwan), Indian Institute of Astrophysics (India), Inter-University Center for Astronomy and Astrophysics (India), Weizmann Institute of Science (Israel), The Oskar Klein Centre at Stockholm University (Sweden), Humboldt University (Germany). GROWTH is supported by the Science and Engineering Research Board, Department of Science and Technology, India.

More information about the discovery of the gravitational wave source GW170104 can be obtained at https://www.ligo.caltech.edu/page/press-release-gw170104, and in the IndIGO press release.

 

[Indx TechStyle]

Jun 01, 2017
AstroSat CZTI and the search for X-ray counterparts to gravitational wave sources

The Ligo scientific collaboration has discovered the third confirmed gravitational wave event, GW170104. Several Indian and international astronomy groups searched for corresponding electromagnetic signals in the sky. The Cadmium Zinc Telluride Imager (CZTI) on AstroSat conducted the most sensitive search for short duration X-ray flashes associated with this event, but did not find anything. The Hawaii-based ATLAS group found a transient optical source, which was thought to be related to GW170104.
The CZTI team, in collaboration with the international GROWTH collaboration, studied this transient extensively, and proved that it was not the counterpart of GW170104. Instead, it was a Gamma ray burst event caused by the explosive death of a massive star in a galaxy several billion light years away giving birth to a new black hole.

 

[Indx TechStyle]

AstroSat Picture of the Month - Sep 2017

Stellar Porn!

 

[Indx TechStyle]

AstroSat Science Meet​

The first dedicated Indian Astronomy mission, AstroSat which was launched on September 28, 2015 completed two years in orbit. A unique feature of Astrosat mission is that it enables simultaneous multi-wavelength observations (optical, UV and X-rays) of various astronomical objects with a single satellite.

The satellite is being operated as an “Observatory”, in which observing time is allotted based on the proposals received from interested researchers and scientists in the country, through ISRO’s Announcements of Opportunity (AO). More than 50 papers have been published in peer reviewed journals and one special issue of Journal of Astrophysics and Astronomy (JAA) has been published (Ref: Vol 38, No.2, June 2017). From October 2017, the observatory is open to Indian and International astronomy community.

In order to commemorate two years completion of AstroSat in orbit, Space Science Programme Office (SSPO), ISRO Headquarters organised an “AstroSat Science Meet” at ISRO HQ, Bangalore during 26 – 27 September, 2017. The inaugural session was graced by Dr.K. Kasturirangan, Honorary Distinguished Adviser, ISRO and former Chairman ISRO and Secretary DOS, who gave the keynote address. Dr. M. Annadurai, Director, ISAC inaugurated the session and Dr.P.G. Diwakar, Scientific Secretary, ISRO released the AstroSat picture of the month.

Dr. S. Seetha, Director, SSPO welcomed all the dignitaries and participants for the science meet. Prof. U.R. Rao, a Space scientist of International repute and Chairman, Advisory Committee for Space Science (ADCOS) was remembered for his contribution to Indian Space science programme. Dr. Seetha mentioned that AstroSat has made 772 individual pointings to observe 378 distinct sources. 110 Gamma-ray bursts were detected.

Dr. Annadurai in his address, appreciated the involvement of young members in the AstroSat project and mentioned that AstroSat has provided next level learning experience for the satellite team. He brought out the comparison between Mars Orbiter Mission and AstroSat in terms of technological advancement, payload mass, orbit of the satellite, payload and satellite realization schedule, science focus etc. He also emphasized that future mission should take forward what is learnt from AstroSat. The instruments, spacecraft and pipeline for AstroSat-2 have to be made in the bigger way. Utilising AstroSat-1to the fullest, longevity of the mission, niche areas for the next mission are to be thought about.

Dr. K. Kasturirangan delivered the Keynote address. He congratulated ISRO and payloads teams who have been successful in meeting the significant challenge of design and development of spacecraft and payloads. Within two years, AstroSat has observed around 400 sources, 110 GRBs, polarisation in GRBs, Quasi-Periodic oscillations, search for X-ray analogues of gravitational waves etc. This indicates the kind of science which can be envisaged from the payloads. He presented the legacy of Indian astronomy activities since 1950s from cosmic rays studies using balloons, rockets, satellite borne payloads on SROSS, IRS-P3 and the learning from which led to the realisation of the dedicated astronomy mission. He recalled the rationale behind proposing AstroSat as a “Multi-wavelength” satellite. He commended all the scientists involved in reviewing the design of payloads. He mentioned that AstroSat-2 should have next level of science and engineering which could be anything from Gravitational wave counterpart detection, polarization of GRBs, IR telescopes or multi-wavelength capability, bigger telescope with grazing incidence optics etc. Universities have to be brought into data analysis and in Big data analytics. Formation flying concept with AstroSat 2A, 2B etc having international competence can be thought of in future. He stated that AstroSat is attributed to the undying spirit of ISRO and more than a mission, it is a culture.

“AstroSat Picture of the Month” is an initiative taken by the AstroSat Training and outreach team, ISRO and Public outreach and education committee of Astronomical Society of India. Dr. Diwakar released the First Poster for the month of September 2017.

Dr. Diwakar mentioned that two cycles of AO have been completed and third cycle observations commence from October 2017. The observatory is open to Indian and International community and more research outcomes are expected from this mission. He also requested researchers to communicate the results to the public.

Prof. Srianand, IUCAA is the Chairman of Astrosat Time Allocation Committee (ATAC). He made a presentation on “Overview of Science from AstroSat and Time Allocation”. He briefed on the proposals received for the third AO cycle and the niche science areas from the proposals. He also gave suggestions to improve the proposals and process.

Around 150 participants attended the science meet.

In the first session on status and science from payloads, presentations were made by the payload managers and mission operations on the status of payloads, calibration and science highlights.

The sessions are arranged astronomy theme-wise and are based on the abstracts received from the AO cycle users for this science meet. 25 researchers presented their work using AstroSat data during the science meet.

A panel discussion on “Space astronomy beyond AstroSat – path ahead” was held. Directors or representatives from various national institutions presented their ideas for the next astronomy mission. After the deliberations, it was decided that the panel will submit a report to ISRO.

Dr. K. Kasturirangan made pertinent suggestions to the panelists which were discussed in detail for possible consideration. Other senior members in the audience also provided relevant points to be taken up by the panelists.

Shri. A.S. Kiran Kumar, Chairman, ISRO/ Secretary, DOS, addressed the gathering and appreciated the teams of AstroSat, which in two years has changed the outlook of Indian space astronomy. He also mentioned that advanced capabilities in terms of technology and computing exist within the country, which can be brought forth for future projects. This could consist of few small satellites with a turnaround time of about two years followed by a full-fledged mission. He said the community should not be constrained by any limitations and encouraged the community to look beyond and come up with new original ideas. The primary goal of next mission is to be science-driven and hence he requested the panel to have in-depth discussion, and recommend pioneering concepts which can be considered for future missions.

Inaugration of the event by Dr. K. Kasturirangan

Dr. Diwakar releasing AstroSat Picture of the Month

 

[Indx TechStyle]

AstroSat contributes to the saga of Gravitational Wave Astronomy
On August 17, 2017, scientists seeking the holy grail of gravitational wave (GW) astronomy struck gold. The elusive and long sought after GW signals from merging binary neutron stars were found and multi-messenger observations provided tell-tale signs of this merger to clinch the issue without any qualms. Two of the GW detectors in the US picked up the signal and a third, working in Europe, confirmed it. Several of the satellites in the sky detected signals from this event across various bands of the electromagnetic spectrum, and a vast array of optical and radio telescopes worldwidetrained their vision into this new phenomenon, finding a variety of corroborating signals.

The AstroSat scientists, who pitched in with their efforts, today stand shoulder to shoulder with a few thousand scientists across the globe (including three Nobel Prize winners and a few scores of other Indian scientists) to announce this momentous discovery and an `open sesame’ moment of staring at the huge cache of scientific discovery that this new era of`multi-messenger, time-domain astronomy’ opens up.

Gravitational Wave Astronomy: the last frontier

Any accelerated electronic charge emits electromgnetic radiation: scientists routinely use this to generate and send electromagnetic waves like radio waves, optical light, and X-rays. Any moving mass disturbs the space time and a `quadropole’ moment in the moving mass should generate gravitational waves: theorised Albert Einstein a hundred years ago. Einstein’s words are treated as Veda Vakya or Gospel Truth, and astronomers routinely use this to understand the dynamics of compact large masses in the cosmos. Russell A. Hulse and Joseph H. Taylor, Jr discovered two radio pulsars going around each other, slowly hurtling towards each other, and they invoked Einstein’s gravitational wave theory to understand their behavour: they were duly awarded a Nobel Prize for this work. This opens up an interesting question-shouldn’t astronomers, who use every branch of electromagnetic radiation from radio to gamma-rays to prise open the secrets of the Universe, use gravitational waves to understand exotic features of the cosmos- like the ripples of the Big Bang or merging of black holes when galaxies collide ?

Well, they should,but the catch lies in the fact that the gravitational force is extremely weaker than the electromagnetic force, and common sense deems that even the most sensitive detectors that humans can build cannot detect the most exotic gravitaional wave sources that we can imagine. However, during the past few decades, a huge number of dedicated scientists have built the most sophisticated detectors capable of measuring infinitesimal movement of mass corresponding to a tiny fraction of a nanometer in kilmoter sized objects so that they would be sensitive to the gravitational waves from outer space. Year after year, they kept looking for signs of merging neutron stars, but the quest was in vain !

Mother Nature usually likes to keep surprises up her sleeve! When the GW detectors with highly improved sensitivity were switched ON in 2015, they found something: not a neutron star-neutron star merger, but a totally unexpected event of two massive black holes merging and spewing out energy equivalent to the complete burning out of mass corresponding to two Suns. This is indeed a momentous discovery, and the architects of this humongous human effort, Kip Thorne, Rainer Weiss, and Barry Barish, duly got this year’s Nobel Prize.

What about the elusive case of the merger of two neutron stars anticipated from the discovery of Messers Hulse and Taylor?During the past two years, four GW events were discovered, however, they were all due to mergers of black holes. The problem with merging black holes is that they are, as apparent from the name, `black’;i.e., apart from the GW events,there are no tell tale signs of the merger in any other branch of electromagnetic radiation. So, we cannot determinewhere they are coming from, or what are their progenitors. This is not the case for neutron star mergers. It was firmly believed that when GW events are discovered from neutron star mergers, they would be accompanied by huge amounts of electromagnetic radiation, which will help us pin down the sources of these events.

The whole scientific community was eagerly waiting for this much anticipated event.

CZT Imager of AstroSat pitches in

AstroSat was launched on September 28, 2015 and the CZT Imager (CZTI) instrument of AstroSat was the first instrument to be made operational. On October 6, 2015, the first day of operation, CZTI detected a gamma-ray burst (GRB) and proved to be an efficient GRB detector. The scientists working with the CZTI data realised that it would be a wonderful instrument to detectany gamma-ray events accompanying the GW sources.

The problem with detecting such gamma-ray events is that they are rare, unpredictable, and can come from any direction in the sky. Hence, the detectors need to have all sky sensitivity, and generally, there is a trade off in their observing capabilities. Currently, there are three sensitive operating GRB monitors, along with a few more less sensitive detectors, each having their own capabilities and limitations. The most sensitive GRB monitor currently operating is the Swift satellite, however, it can observe only one tenth of the sky at any given time. CZTI and the Fermi satellite, on the other hand, are sensitive to much larger regions in the sky, but have very limited capability to localise these events. The anti-coincidence shield of the INTEGRAL satellite, too, can act as a GRB monitor.

Each of these instruments played their part in the race to detect gamma-ray signals accompanying the GW events. During the very first GW event on September 14, 2015 (before the launch of AstroSat), Fermi claimed that it haddetected a GRB like event within 0.4 s of the GW event. Observations from theINTEGRAL satellite, however, disagreed: the consensus was that this could be some unrelated spike in the background. During another GW event detected in January 2017, optical astronomers saw, the very next day of the event, some source gradually diminishing in brightness. Could this be the tell-tale signs of something happening in the GW source? CZTI chipped in with a firm No! It had detected a GRB, 21 hours after the GW event. The fading optical source was shown to be this GRB, unrelated to the GW event.

Aug 17, 2017: a red letter day

On August 17, 2017, the much anticipated event occurred.

The GW detectors in US registered a very long series of signals, or `chirps’, closely resembling what the scientists have simulated for decades to be coming from neutron star coalescence. Even before they could announce this discovery, the Fermi satellite had detected a GRB at the same time: in fact within a couple of seconds of the GW event. Could this also be an unrelated background fluctuation event? Very unlikely, because, at exactly the same time, the anti-coincidence shield in the INTEGRAL satellite had also detected this GRB. What about Swift and CZTI? They didn’t detect any! The event should probably be outside the narrow field of view of Swift. What about CZTI? It was active and operating and the GRB should have been detected. The only way to reconcile was to assume that the source was blocked by the Earth: this helped to narrow down the possible source regions of the GW event.

Soon, the GW detectors from Europe too pitched in, and the region of the sky responsible for the GW event and GRB was narrowed down to a small region. Optical telescopes around the world scanned each and every galaxy in this region and, lo and behold, there indeed was a bright optical object, not seen before, near a galaxy called NGC 4993.

The rest, as they say, is history. Soon, infra-red and ultraviolet emissions were seen from this source. Nine days later, an X-ray source was detected, and fifteen days later, radio emission was also observed. From such vast multi-wavelength data, the physics of colliding and merging neutron stars were studied in depth. An exciting find is that the material ejected in the event is rich in heavy elements, so much so that, colliding neutron stars can account for the entire supply of precious metals, like gold, platinum and silver, in the universe. Production of these elements have been difficult to understand, and now the source has been found!

The story of GW170817 bears testimony to the amazing outcome possible when all the world’s best instruments are combined for a single purpose. The collaborative efforts of a number of teams worldwide lends an added credibility to this exciting and substantial discovery and ushers in a new era in multi-messenger, time-domain astronomy!

Scientific curiosity: never satiated

The GW detectors are taking a year off to return with an improved sensitivity. Neutron star merger events and the accompanying `kilonova’ should be fairly common observations during the next run. There should be more of black hole merger events as well. Scientists are already dreaming about the rich future harvests:

Can we get any tell-tale signatures of black hole mergers to identify where they are coming from? Perhaps more sensitive all sky detectors would help with an answer.

Can these events be used as a tool for distance measurement to refine cosmology? A massive collaboration between GW theorists and kilonova observers should be able to do it.

Can we learn anything about the regions close to black hole? Possible.

Are there some strange stars among the neutron stars? Certainly more such objects will tell us. Finally, has Mother Nature more surprises up her sleeve? Only the future will tell us!

What next? Significant next steps will involve making detectors more sensitive, improving localisation capability and most importantly, continued collaboration of observatories worldwide spanning all electromagnetic bands, neutrinos and gravitational waves.

In the Indian space science context, the capability of CZTI would certainly be improved through better algorithms and simulations: it should be possible to independently confirm and localise gamma-ray events for future GW associations. Perhaps, even a much improved CZTI like all sky monitor could be designed and flown!

Multi-messenger studies of GW170817 incorporating the contribution of AstroSat CZTI are published in the journals Science and Astrophysical Journal Letters.

Papers

• Illuminating gravitational waves: A concordant picture of photons from a neutron star merger - Science 10.1126/science.aap9455 (2017).
• Multi-messenger Observations of a Binary Neutron Star Merger - The Astrophysical Journal Letters, 848:L12 (59pp), 2017 October 20

https://www.isro.gov.in/update/17-o...butes-to-saga-of-gravitational-wave-astronomy

 

[Indx TechStyle]

Story of the Week
CZT Imager of AstroSat measures first phase resolved X-ray polarisation of Crab pulsar
X-ray polarisation: All types of electromagnetic radiation, like X-rays and optical light, are bundles of energy called photons defined by an electric vector, and an orthogonal magnetic vector. The electric vectors are mostly random in orientation, but quite often, they are aligned to a particular direction depending on the conditions in the source of these photons. For example, optical light scattered in the sky are aligned, or polarised, in the plane of the scattering and you can take a simple optical polariser to look at the sky and determine the direction of the incident photons (prior to scattering) from the source – in this case the Sun. The polarisation properties of electromagnetic radiations have been regularly used by astronomers to study the conditions of the cosmic sources emitting these radiations. For example, the direction of the magnetic field in our galaxy, Milky Way, can be precisely mapped using polarisation measurements.

Strong X-ray emission from astrophysical sources often signifies the presence of exotic compact objects in the universe: neutron stars and black holes. The X-ray emission from these objects traces the regions of particle acceleration and understanding the conditions of particle acceleration can tell us about several exotic phenomena: for instance, whether the black hole is spinning, does the strong gravity near black holes obey Einstein’s equation, or if neutron stars are made up of ordinary matter or strange matter and so on. It has long been thought that X-ray polarisation properties will tell us more about the mysteries of these strange objects.

Immediately after the birth of X-ray astronomy in 1962, due to the serendipitous discovery of an extra-solar X-ray source in a rocket flight, there was a flurry of activity in all aspects of X-ray astronomy, including polarisation measurement. Scientists at Columbia University flew a simple scattering based X-ray polarimeter in a rocket flight in 1969 to look at the pulsar in the Crab Nebula: they could not detect any polarisation – less than 36% of X-ray photons had their electric vectors aligned. They sent another rocket in 1971 with an improved version with graphite crystals to look at X-rays of specific energy at 2.6 keV and discovered X-ray polarisation in Crab: about 20% of the photons at this energy show aligned electric vectors. Buoyed by this success, they sent a polarimeter in the 8th Orbiting Solar Observatory (OSO-8) in 1975: it confirmed the X-ray polarisation in Crab and put quite stringent (5 – 10%) upper limits on the polarisation of several bright X-ray sources.

Exploring X-ray Polarisation using AstroSat
AstroSat, being India’s first dedicated astronomy mission, was a massive effort involving the participation of many ISRO centres, academic institutes, and universities. Tata Institute of Fundamental Research (TIFR), Mumbai led the scientific effort by shouldering the responsibility to deliver three of the five major payloads of AstroSat.

Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, founded for the explicit purpose of promoting “the nucleation and growth of active groups in astronomy and astrophysics at Indian universities’’, was an indirect participant in the AstroSat instrument development: Dipankar Bhattacharya (IUCAA) designed the specialised `coded aperture masks’ for two instruments of AstroSat. Since AstroSat is supposed to be an observatory class satellite catering to all users, including those from the universities, it was thought that a formal participation of IUCAA in several allied tasks like software development, student training, etc. would enhance the utility of AstroSat data.

Arthur Holly Compton had discovered, in 1923, that X-rays can deposit part of their energy in a material and undergo `Compton’ scattering. The CZT detectors are pixellated (and hence, have position information) and at certain energies, X-rays will interact primarily by Compton scattering. If the instrument can be made sensitive to measure the incident X-rays as well as the scattered X-rays, then the distribution of the scattered X-rays in the neighbouring pixels of the CZT detector will show some tell-tale signs of the polarisation properties of the incoming X-rays. Such pixellated X-ray detectors are thought to work as X-ray polarimeters, but experimentally no one, so far, had demonstrated the polarisation properties of pixellated X-ray detectors. Since X-ray polarisation measurement is a long sought after experimental technique in X-ray astronomy, examining the utility of CZTI as an X-ray polarimeter looked attractive.

Polarisation Experiments
There followed a flurry of activity to demonstrate that CZTI can be used as an X-ray polarisation instrument. Rao ascertained from the supplier of the CZT detectors that the properties of incident and scattered X-rays are measured and retained, and this was quickly demonstrated by some simple experiments using radio-active sources of known energies. Santosh used a sophisticated code to simulate this behaviour and satisfied himself that polarisation measurements can indeed be made. Then followed a series of Varkari between Ahmedabad and Mumbai: Santosh and his student Tanmoy Chattopadhyay (currently working as a post-PhD researcher at Pennsylvania State University) made controlled experiments on the flight models of CZTI detectors. Several improvisations were part of the testing: like the use of simple thermocol pieces as experimental set-up, use of very strong radio-active sources of X-rays and making them scatter from blocks of aluminium to polarise them - but using sophisticated simulations to precisely calculate the energy of scattered X-rays.

All these activities were done while the fabrication and testing of the flight models were going on ! First a single X-ray detector, called a module, was bombarded with X-rays of known energies and its properties were measured. A rigorous modelling, based on these experimental results, showed that CZT Imager as a whole, with 64 detector modules, will have enough sensitivity to measure X-ray polarisation from bright cosmic X-ray sources. Then started a series of controlled experiments for the proof of concept: beams of X-rays were generated, with or without polarisation, and the polarisation signal in the detector was measured and compared with the results from the simulations. All the tests were repeated for X-rays of a single energy, as well as continuum of X-rays (as expected from Cosmic X-ray sources). Finally, some precious time was readjusted from the very tight delivery schedule of the flight model to repeat these tests in a fully flight-like environment.

This is only the beginning of the story. Because, all the tests in the lab are made with small beams of X-rays and in flight, while observing X-ray source situated at thousands of light years away, one expects parallel beam of polarised X-rays. To test with similar large area polarised beam of X-rays in the lab, one needed to build a humongous facility – perhaps taking several years of effort just to fabricate it. The whole detector geometry was simulated and sophisticated simulations were carried out to predict the behaviour of the detector. These were compared with the controlled lab experiments of small beam, at different experimental configurations, to satisfy that the simulation results are indeed as observed in the lab. This gave a confidence for the prediction of the parallel beam of large area and the results indeed showed that CZT Imager is a very sensitive X-ray polarimeter.

Finally, on 2015 September 28, a fully ground calibrated X-ray polarimeter was flown into space.

The special source ‘Crab’ and its polarisation
When a new era of astronomy was ushered in by telescopes in the 17thand 18th century Europe, it was realised that the sky consists of several fuzzy objects, apart from point-like stars, and they were named ‘Nebula’. The Crab Nebula, in the constellation of Taurus, is the first object identified by Messier, a French astronomer, who meticulously catalogued such objects. Crab Nebula cannot be seen by the naked eye, but can be seen as a fuzzy crab-like feature (hence the name) by a simple binocular. Among such nebulae, supernova remnants form a large fraction. Soon, it was realised that the Crab Nebula is none other than the `Guest Star’, or the supernova recorded by the Chinese astronomers in year 1054.

Within a year of the discovery of pulsars (rapidly rotating highly magnetised neutron stars) in 1967, a pulsar with a period of 33 milliseconds was discovered in the Crab Nebula. It was, however, surmised that the explosion of the supernova that occurred about 1000 years ago has long lost its energy and currently, the nebula is powered by the monster sitting at its centre: the fast spinning neutron star. Crab Nebula, along with its pulsar, became the darling of all observers: it is the only pulsar observed to be pulsating in all branches of the electromagnetic radiation, from radio to even ultra high energy gamma-rays. Since the pulsations are due to simple light-house effect of the rotating neutron star, the period should be, and indeed is, the same in all wavelengths - hence the Crab pulsar became a calibrating source to test the timing accuracy of any new window of observation. The emission mechanism is understood as due to the movement of particles in a magnetic field, the particles being accelerated to humongous energies in the polar cap of the highly rotating magnetic neutron star. Since the magnetic field and the rotation period are reasonably stable for several years, the emitted flux, in hard X-rays, is expected to be a constant : hence it also became a good flux calibrator for any new observations. The realisation that particles are accelerated to very high energies has made a strong case for such young supernova remnants as sources of the mysterious particles bombarding the Earth: Cosmic Rays.

In recent years, Crab Nebula and pulsar are the subjects of diverse observations and theorising to understand the particle acceleration mechanism in fast spinning neutron stars. Measurement of polarisation as a function of pulse phase in optical and radio wavelengths and detailed theoretical modelling are hinting that particle acceleration happens outside the `light-cylinder’ of this source and scientists across the globe are pondering over the nature of such acceleration

Crab polarisation using AstroSat
Crab Nebula was the first source AstroSat stared at post switch ON and verification, and, it kept looking at this source quite often for a periodic calibration of the X-ray instruments, as well as to measure its polarisation. There followed a very meticulous analysis. Every recorded photon was examined, stringent criteria were imposed on them to be validated as `Compton-scattered’ events, their distribution among their neighbours were examined, for consistency, among different detectors in the instrument. Regions in the sky were identified and the position of the satellite was manoeuvred such that the instrument looks at a `source-free’ region in the sky at similar satellite orientation with respect to Earth. The behaviour of the detector was studied in these `background’ regions and the resultant profile was subtracted from the source observations. To establish that these `Compton-scattered’ photons were really from Crab, their arrival times were investigated and found to be beating at the standard Crab clock of 33 milli-seconds.

Yes, X-ray polarisation was clearly detected even in individual observations lasting about half a day and when all the eleven observations were added together, the measurement is clearly the most precise hard X-ray polarisation measurements till date The results were submitted to the British journal Nature.

It is true that measuring polarisation with the highest accuracy possible is a technical challenge: but what is the new thing we are learning ? After a few weeks of intense debate among the researchers, it was decided to collect more data and attempt a very precise measurement of the variation of the polarisation properties when the pulsar beam sweeps across us, the observers. For this, data from different observations spanning across 18 months had to be added. Do we know the pulsar period accurately for this ? The Crab pulsar was observed in radio wavelengths from the Ooty radio telescope every day and also many times during these months from the Giant Meter-wave Radio Telescope (GMRT) at Khodad, near Pune. The pulse of pulsar was accurately measured, the X-ray photons were assigned a pulse phase and the change in the polarisation property as a function of the pulsed beam emission was studied.

When we think that the pulsar beam is shining elsewhere, the remainder of the beam has sufficient X-ray emission, with the polarisation increasing and also showing a sharp change. What it means is that the light house is leaking and emitting high energy X-rays all the time. Most magnetospheric theories predict that the polarisation of X-ray radiation will show changes only during the emission of a pulse, but not at other times. The new observations thus support the view that the particle acceleration is happening outside the conventional boundary of the magnetosphere, in a region where the charged particles generated by the pulsar are spiralling out in the form of a wind. The surprising observation by CZTI of a sharp change of polarisation in the “off pulse” region is clearly a big challenge to theorists.

This result is published in Nature Astronomy on November 6, 2017, publication doi: 10.1038/s41550-017-0293-z

Left panel: The grey line shows the brightness of the Crab pulsar as observed by AstroSat CZTI. The horizontal axis (phase) represents time expressed in units of the pulsar’s spin period. Phase 0.0 to 1.0 stands for the full rotation cycle of the pulsar. The same result is shown repeated between phase 1.0 and 2.0, for a clear demonstration of the periodic pattern. Colored bars indicate how strongly polarized the observed radiation is. Sharp variation of polarization when the brightness is low is the surprising discovery by AstroSat.

Right panel: The angle of X-ray polarization measured by AstroSat CZTI shown superposed on a composite optical and X-ray image of the Crab nebula, taken by NASA’s Hubble and Chandra telescopes respectively. The white arrow represents the projected spin axis of the pulsar located at the center of the nebula. Other arrows display the orientation of the observed polarization. The color of an arrow indicates the range of phase it belongs to, being equal to that spanned by bars of the corresponding shade in the left panel.
Story of the Week - Archive
Nov 06, 2017 : CZT Imager of AstroSat measures first phase resolved X-ray polarisation of Crab pulsar
Oct 30, 2017 : 38th Asian Conference on Remote Sensing
Oct 23, 2017 : ETS - A System for Transportation of Small Satellite and Flight Hardware
Oct 17, 2017 : ISRO Celebrates World Space Week-2017
Oct 09, 2017 : Successful completion of One Year of Service by SCATSAT-1 Scatterometer
Oct 03, 2017 : Many Satellites Celebrated Birth Anniversary during last week of September
Sep 25, 2017 : Operationalisation of Thunderstorm Nowcasting Services over NE Region using DWR data

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