XPoSat

Studying how radiation is polarised gives away the nature of its source, including the strength and distribution of its magnetic fields and the nature of other radiation around it. XPoSat will study the 50 locally brightest (known) sources in the universe consisting of, variously, pulsars, black hole X-ray binaries, active galactic nuclei, neutron stars and non-thermal supernova remnants.{{Cite web |title=5 Unique Space Science Missions That ISRO Will Be Flying in the Near Future |url=https://thewire.in/the-sciences/space-science-aditya-isro-xposat-nisar |access-date=2023-12-25 |website=The Wire}} The observatory was placed in a circular low Earth orbit of {{cvt|500-700|km}}. The payloads onboard XPoSat will observe the X-Ray sources during its transit through the Earth's eclipse period.{{Cite news |last= |first= |date=2023-11-30 |title=India set to launch its first X-Ray Polarimeter Satellite: ISRO |language=en-IN |work=The Hindu |url=https://www.thehindu.com/sci-tech/science/india-set-to-launch-its-first-x-ray-polarimeter-satellite-isro/article67589615.ece |access-date=2023-12-25 |issn=0971-751X}}

The XPoSat project began in September 2017 with the Indian Space Research Organisation (ISRO) grant of {{INR|95,000,000}}. Preliminary Design Review (PDR) of the XPoSat including the POLIX payload was completed in September 2018, followed by preparation of POLIX Qualification Model and beginning of some of its Flight Model components fabrication.{{cite web |title=XPoSat |url=https://www.isro.gov.in/XPoSat.html |website=isro.gov.in}}{{cite web|url=http://www.rri.res.in/htmls/gen/RRI2019annualreport_english.pdf |title=Raman Research Institute, Annual Report 2018-19|url-status=live|archive-url=https://web.archive.org/web/20200809025628/http://www.rri.res.in/htmls/gen/RRI2019annualreport_english.pdf|archive-date=9 August 2020}}

XPoSAT was successfully launched aboard the PSLV-C58 on 1 January 2024 at 9:10 am IST. The launch was precise, leaving only a deviation of (±) 3 km. Following the launch, the final 4th stage of the PSLV dropped to a 350 x 350 km orbit to facilitate its use as the PSLV Orbital Experimental Module POEM-3.{{cite web |title=New Year's Day Launch, The XPoSAT mission by ISRO |url=https://www.brutimes.com/news/india/new-years-day-launchthe-xposat-mission-by-isro |website=BruTimes |language=en}}{{Citation |title=Launch of PSLV-C58/XPoSat Mission from Satish Dhawan Space Centre (SDSC) SHAR, Sriharikota |url=https://www.youtube.com/watch?v=H10igyNHsr4 |access-date=2024-01-01 |language=en}}

File:PSLV-C58, XPoSat - Lift off from First Launch Pad at SDSC-SHAR 05.webp

The XSPECT payload on XPoSat captured its first light from the Cassiopeia A (Cas A), a supernova remnant somewhat over 11,000 light years away on 5 January 2024. During its performance verification phase, XSPECT was directed towards this standard celestial source used for instrument evaluation which is among the brightest radio frequency sources in the sky. The observation commenced on 5 January 2024, capturing the supernova remnant's emission lines corresponding to elements such as magnesium, silicon, sulphur, argon, calcium, and iron.{{Cite web |date=11 January 2024 |title=XSPECT payload performance verified with Cas A observation |url=https://www.isro.gov.in/XSPECT_payload_performance.html |access-date=2024-01-11 |website=Indian Space Research Organisation}}{{Cite web |date=11 January 2024 |title=ISRO Payload "Aces" Performance Check, Captures Light From Supernova Event |url=https://www.ndtv.com/science/isro-payload-aces-performance-check-captures-light-from-supernova-event-4842800 |access-date=2024-01-12 |website=NDTV.com}}

XPoSat's POLIX sensor has started making scientific observations including first-ever data of x-ray polarisation of the Crab Pulsar, its first subject. The observation, which verified the POLIX instrument's operation, took place between January 15 and 18, 2024. POLIX monitored this fast-spinning neutron star in the Crab Nebula that releases roughly thirty X-ray pulses per second. Through the identification of polarization in its incoming X-rays, POLIX provides fresh perspectives on the physical emission processes at the surface of neutron stars. On 10 January 2024, the instrument was gradually turned on.{{Cite news |last=Kumar |first=Chethan |date=2024-02-13 |title=Isro achieves first pulsar observation from XPoSat payload |url=https://timesofindia.indiatimes.com/india/isro-achieves-first-pulsar-observation-from-xposat-payload/articleshow/107659819.cms |access-date=2024-02-16 |work=The Times of India |issn=0971-8257}}{{Cite web |date=13 February 2024 |title=Isro's black hole mission XPoSat begins active observation, looks at Crab pulsar |url=https://www.indiatoday.in/science/story/isro-black-hole-mission-xposat-begins-active-observation-looks-at-crab-pulsar-2501428-2024-02-13 |access-date=2024-02-16 |website=India Today |language=en}}

In response to a massive Solar flare in May 2024, XpoSAT, along with Aditya-L1 and the Chandrayaan-2 Orbiter collected data on the event. XSPECT was used in conjunction with data from ground based observatories to provide fast timed and good spectroscopic results in the X-Ray spectra.{{Cite web |title=ISRO Captures the Signatures of the Recent Solar Eruptive Events from Earth, Sun-Earth L1 Point, and the Moon |url=https://www.isro.gov.in/ISRO_CapturesSignaturesRecentSolarEruptive_Events.html |access-date=2024-12-10 |website=www.isro.gov.in}}

On March 19, 2025, the XSPECT instrument detected a rare thermonuclear “burst” peaking in just a few seconds and fading over about 20 seconds, followed about 16 minutes later by a much longer and more powerful event called a “superburst” from a neutron star system named 4U 160852, located about 4,000 light-years from Earth. XSPECT’s detailed observations showed the neutron star’s surface temperature during the bursts reached around 20 million degrees Kelvin, with a radius close to 9.3 kilometers.The data also suggest special processes like Compton scattering might be involved in the superburst’s high brightness and slow fade. The superburst was also observed by the MAXI experiment on the ISS.{{Cite web |date=2025-04-24 |title=Nuclear explosion that lasted three hours: Indian satellite detects mega burst |url=https://www.indiatoday.in/science/story/indias-xposat-satellite-detects-thermonuclear-explosion-that-lasted-three-hours-2714027-2025-04-24 |access-date=2025-04-25 |website=India Today |language=en}}

File:PSLV-C58, XPoSat - Render of POEM-3 and XPoSat inside payload fairing.png of the PSLV rocket.]]

Two payloads of XPoSat are hosted on a modified IMS-2 satellite bus. Primary scientific payload is Polarimeter Instrument in X-rays (POLIX) to study the degree and angle of polarisation of about 50 locally brightest astronoThe superburst was also observed by MAXI.mical X-ray sources of different types during its mission in the energy range 8-30 keV.{{cite web |url=https://currentaffairs.gktoday.in/isro-announces-mega-missions-05201968993.html|title=ISRO announces Seven Mega Missions|newspaper=GK Today|date=20 May 2019|access-date=10 December 2021}}{{cite web|url=https://indianexpress.com/article/technology/science/chandrayaan-2-launched-here-are-future-isro-missions-to-space-5842337/|title=Chandrayaan 2 launched: Here are future ISRO missions to space|date=2019-07-22|newspaper=The Indian Express|access-date=2019-07-23}} POLIX, a {{cvt|125|kg}} instrument, was developed by the Raman Research Institute.[http://rri.res.in/~bpaul/polix.html X-ray Polarimeter Experiment (POLIX)] Raman Research Institute Accessed on 2 August 2019

File:XSPECT XPoSat.jpg

POLIX is the primary scientific payload aboard XPoSat. It is a Thomson X-ray polarimeter, which measures the degree and angle of polarization (polarimetry parameters) of astronomical sources in the medium X-ray range (8-30 keV).{{cite journal |last1=Ghosh |first1=Sahana |title=India to launch world’s second X-ray polarimetry space observatory |journal=Nature India |date=11 September 2023 |doi=10.1038/d44151-023-00132-x |url=https://www.nature.com/articles/d44151-023-00132-x |language=en|url-access=subscription }} It has been developed by the Raman Research Institute.

Its science objectives are to measure:

• the strength and the distribution of magnetic field in the sources
• geometric anisotropies in the sources
• their alignment with respect to the line of sight
• the nature of the accelerator responsible for energising the electrons taking part in radiation and scattering.

The experiment configuration consists of a collimator, central low Z (lithium, lithium hydride or beryllium) scatterer surrounded by xenon filled four X-ray proportional counters as X-ray detectors which collects the scattered X-ray photons. The instrument is rotated along the viewing axis leading to the measurement of the azimuthal distribution of the scattered X-ray photons which gives information on polarisation. Polarised X-rays will produce an azimuthal modulation in the count rate as opposed to uniform azimuthal distribution of count rate for unpolarised X-rays. POLIX has four independent detectors, each with its own front end and processing electronics. Localization of the X-ray photon in the detectors is carried out by the method of charge division in a set of resistive anode wires connected in series.

The prime objects for observation with this instrument are the X-ray bright accretion powered neutron stars, accreting black holes in different spectral states, rotation powered pulsars, magnetars, and active galactic nuclei. This instrument bridges an energy gap in detection capability, between the soft X-ray polarimeters utilising Bragg reflection (OSO-8) or Photoelectron tracks (IXPE), and hard X-ray polarimeters using Compton scattering such as the Cadmium Zinc Telluride Imager (CZTI) on AstroSat.

File:POLIX XPoSat.jpg

XSPECT is the secondary payload on XPoSat. It measures spectroscopic information, timing of soft X-rays and electromagnetic spectrum generated by different types of matter.{{cite web |url=https://www.isro.gov.in/sites/default/files/flipping_book/annual_report_2019-20_english/files/assets/common/downloads/Annual%20Report%202019-20%20%28English%29.pdf|title=Government of India, Department of Space, Annual Report 2020|url-status=live|archive-url=https://web.archive.org/web/20200214130824/https://www.isro.gov.in/sites/default/files/flipping_book/annual_report_2019-20_english/files/assets/common/downloads/Annual%20Report%202019-20%20%28English%29.pdf|archive-date=14 February 2020}} XSPECT is designed to pursue timing studies of soft X-rays (0.8-15 keV), complementary to what the Large Area X-ray Proportional Counter (LAXPC) does at high energies on AstroSat, while simultaneously providing adequate spectral resolution in the 1-20 keV band. It has an energy resolution of <200 eV at 5.9 keV (-20 °C) and a timing resolution of ~2 msec. It has been developed by the Space Astronomy Group of the U R Rao Satellite Centre.

The detector achieves modest effective area without the use of focusing optics using the large area Swept Charge Devices (SCD), a variant of X-ray charge-coupled Devices (CCDs). SCDs permit fast readouts (10–100 kHz) and moderately good spectral resolution at the cost of a position sensitivity. These devices are unique in requiring very benign cooling requirement (requiring only passive cooling) unlike traditional X-ray CCDs.

Key science objectives of XSPECT include understanding long-term behavior of X-ray sources through correlation of timing characteristics with spectral state changes and emission line variations.

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