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. The observatory was placed in a circular low Earth orbit of . == History ==

The XPoSat project began in September 2017 with the Indian Space Research Organisation (ISRO) grant of . 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. Launch 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. First Light XSPECT 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. POLIX 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. == Science ==

Solar observations 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. Astronomical observations 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 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 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. Release of Data ISRO released the first tranche of XpoSAT 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. == Payloads ==

of the PSLV rocket. Two payloads of XPoSat are hosted on a modified IMS-2 satellite bus. POLIX, a instrument, Polarimeter Instrument in X-rays (POLIX) 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). It has been developed by the Raman Research Institute. Its science objectives are to measure: 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 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 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. == See also ==