X-ray Polarimeter Satellite

X-ray Polarimeter satellite (XPoSat) in deployed configuration
Names	XPoSat
Mission type	X-ray polarimetry
Operator	ISRO
COSPAR ID	2024-001A
SATCATno.	58694
Website	www.isro.gov.in/XPoSat.html
Mission duration	Planned : 5 years Elapsed : 1 year, 10 months, 21 days[1]
Spacecraft properties
Spacecraft	X-ray Polarimeter Satellite
Bus	Modified IMS-2[2]
Manufacturer	Raman Research Institute U R Rao Satellite Centre
Launch mass	480 kg (1,060 lb)[3]
Payload mass	144 kg (317 lb)[3]
Dimensions	65 × 65 × 60 cm (26 × 26 × 24 in)
Power	1260watts[3]
Start of mission
Launch date	1 January 2024, 9:10 AMIST (3:40UTC)[4]
Rocket	PSLV-DL[5][6]
Launch site	Satish Dhawan Space CentreFirst Launch Pad
Contractor	ISRO
Orbital parameters
Reference system	Geocentric orbit
Regime	Low Earth orbit
Perigee altitude	638 km
Apogee altitude	653 km
Inclination	6°[3][7]
Period	97.61 minutes
Instruments
Polarimeter Instrument in X-rays (POLIX) X-ray Spectroscopy and Timing (XSPECT)
XPoSat Logo

TheX-ray Polarimeter Satellite (XPoSat) is anISRO-manufacturedspace observatory to studypolarisation ofcosmic X-rays. It was launched on 1 January 2024 on aPSLV rocket,^[8] and it has an expected operational lifespan of at least five years.^[9][10]

The telescope was developed by theRaman Research Institute (RRI) in close collaboration with theU R Rao Satellite Centre (URSC).^[11] Per ISRO, this mission will complement the efforts of US space agencyNASA, which launched itsImaging X-ray Polarimetry Explorer (IXPE) in 2021 by observing space events across a broad energy range of 2–30 keV.^[12][13]

Studying how radiation is polarised gives away the nature of its source, including the strength and distribution of itsmagnetic 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-thermalsupernova remnants.^[9][14] The observatory was placed in a circularlow Earth orbit of 500–700 km (310–430 mi).^[9][2] The payloads onboard XPoSat will observe the X-Ray sources during its transit through the Earth's eclipse period.^[15]

The XPoSat project began in September 2017 with theIndian Space Research Organisation (ISRO) grant of₹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.^[16][17]

XPoSAT was successfully launched aboard thePSLV-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 thePSLV dropped to a 350 x 350 km orbit to facilitate its use as thePSLV Orbital Experimental Module POEM-3.^[18][19]

The XSPECT payload on XPoSat captured itsfirst light from theCassiopeia A (Cas A), a supernova remnant somewhat over 11,000light 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 brightestradio frequency sources in the sky. The observation commenced on 5 January 2024, capturing the supernova remnant's emission lines corresponding to elements such asmagnesium,silicon,sulphur,argon,calcium, andiron.^[20][21]

XPoSat's POLIX sensor has started making scientific observations including first-ever data of x-ray polarisation of theCrab 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-spinningneutron star in theCrab 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.^[22][23]

In response to a massive Solar flare in May 2024, XpoSAT, along withAditya-L1 and theChandrayaan-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.^[24]

On March 19, 2025, the XSPECT instrument detected a rarethermonuclear “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 aneutron star system named4U 1608-52, 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 likeCompton scattering might be involved in the superburst’s high brightness and slow fade. The superburst was also observed by theMAXI experiment on theISS.^[25][26]

ISRO released the first tranche ofXpoSAT data to the public on October 13 2025, during a national-level organisational meet.The maiden Datasets release contained over 134GB of data and was followed by an appraisal of the mission.^[27]

Two payloads of XPoSat are hosted on amodified IMS-2satellite bus.^[9] Primary scientific payload is Polarimeter Instrument in X-rays (POLIX) to study the degree and angle of polarisation of about 50 locally brightest astronomicalX-ray sources of different types during its mission in the energy range 8-30keV.^[2][28] POLIX, a 125 kg (276 lb) instrument,^[9] was developed by theRaman Research Institute.^{[14][2][28][29]}

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).^[30] It has been developed by the Raman Research Institute.

Its science objectives are to measure:^[29]

• the strength and the distribution of magnetic field in the sources
• geometricanisotropies in the sources
• their alignment with respect to the line of sight
• the nature of the accelerator responsible for energising theelectrons taking part inradiation andscattering.

The experiment configuration consists of acollimator, central low Z (lithium,lithium hydride orberyllium) scatterer surrounded by xenon filled four X-ray proportional counters as X-ray detectors which collects the scattered X-ray photons.^[11] The instrument is rotated along the viewing axis leading to the measurement of theazimuthal 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 poweredneutron 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 utilisingBragg reflection (OSO-8) or Photoelectron tracks (IXPE), andhard X-ray polarimeters usingCompton scattering such as the Cadmium Zinc Telluride Imager (CZTI) onAstroSat.

XSPECT is the secondary payload on XPoSat. It measures spectroscopic and timing information ofsoft X-rays generated by celestial X-ray sources.^[31][12] XSPECT is designed to pursue timing studies of soft X-rays (0.8-15 keV),^[30] 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 0.8-15 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 theSpace Astronomy Group of theU 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-raycharge-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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