A PSLV rocket launches from Satish Dhawan Space Center in India, with XPoSat.

A PSLV-DL rocket lit up the early morning skies over Satish Dhawan Space Centre in India on New Year's Day, carrying the first X-ray polarimetry astrophysics mission for the Indian Space Research Organization: the X-Ray Polarimeter Satellite (XPoSat). This kind of astronomy can only be done from space, because Earth’s atmosphere absorbs X-rays.

The launch occurred at 10:40 p.m. EST, December 31st / 3:40 UT January 1st, which makes it the final launch for 2023 and the first launch for 2024. It will likely turn up in the Space-Track catalog as COSPAR ID 2024-001A.

The mission is reported as in good health by controllers, and is now in a 6-inclination low-Earth orbit at an altitude of 650 kilometers (400-miles).

XPoSat in the clean room ahead of launch.

The science payload consists of two instruments:

  • The Polarimeter Instrument in X-rays (POLIX) developed by the Raman Research Institute, will observe X-rays in the medium-energy range (8 to 30 keV), determining the degree and angle of polarization of target X-ray sources.
  • The X-ray Spectroscopy and Timing (XSPECT) instrument will work at low-energy X-rays (0.8 to 15 KeV) providing spectroscopic context. XPECT was developed by the ISRO's U.R. Rao Satellite Centre (URSC).

“Polarization is a fundamental aspect of photons,” says Biswajit Paul (Raman Research Institute). “It is important in all areas of astronomy. In the X-ray band, it is relatively less explored.”

XPoSat, mounted on its adapter collar ahead of launch.

Polarimetry: A Key Window on the X-ray Sky

Polarized light carries clues about the nature of the source. X-ray detectors traditionally record the location of the source on the sky, the energy of the X-rays it's emitting, and the time of those X-rays' arrival. Polarimetry adds a new dimension to such observations by recording the polarization of the light. Whether the light waves are oscillating in a single plane, are circularly polarized, or are not polarized at all, polarization gives astronomers information about the source that's emitting the radiation and the medium it's traveling through.

"Polarized X-ray photons have a predictable distribution (modulation) in the azimuthal plane," says Chandreyee Maitra (Max Planck Institute). X-ray polarimeters therefore measure the azimuthal distribution of scattered photons. The detectors are usually gas-filled proportional counters, specifically a Thomson polarimeter in the case of POLIX onboard XPoSat."

XPoSat is the second dedicated X-ray polarimetry mission. The first is NASA's Imaging X-ray Polarimetry Explorer (IXPE), launched in 2021 and still operating. IXPE observes in the 2 to 8 keV range, so XpoSat will overlap and extend the observed range.

“[XPoSat] is less sensitive than the IXPE mission,” says Paul. “But it works in a complementary energy band.”

A diagram of the XPoSat observatory.

Polarimetry can reveal the orientation and rotation of an X-ray source, such as a fast-spinning pulsar or black hole. Dedicated polarimetry missions such as IXPE and XPoSat can fill in gaps in our understanding of these exotic objects. During its five-year nominal mission, XPoSat will study 50 of the brightest X-ray sources in the sky, including supernova remnants, active galactic nuclei, pulsars, and binary black holes.

IXPE 1st Image
The Cassiopeia A supernova remnant seen in IXPE's first image, with data from the satellite (in magenta) overlaid with data (in blue) from the Chandra X-ray observatory.

The mission follows the legacy of India’s first astrophysics mission, the Indian X-ray Astronomy Experiment launched aboard the Indian Remote Sensing Satellite in 1996. Next up was AstroSat, launched by the ISRO in 2015. AstroSat still observes the sky in the near- and far-ultraviolet, as well as X-rays from 0.3 to 100 keV.

The upper stage PSLV booster for the XPoSat, hoisted into position.

This caps a busy year in space for India, with more to come: the ISRO successfully launched the Aditya-L1 solar observatory in September and landed Chandrayaan-3 on the Moon in August. In 2024, watch for the launch of an ISRO rocket with ESA’s solar coronagraph mission in May, and perhaps the launch of the nation’s second Mars Orbiter, MOM-2, as well as the country’s first Venus mission, Shukrayaan 1, by the end of the year. The launch of XPoSat kicks off another busy year in spaceflight, with lots more to come.


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