Launch

XPoSAT was successfully launched aboard 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 PSLV Orbital Experimental Module POEM-3.^[18][19]

First Light

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).^[26] It has been developed by Raman Research Institute.

Its science objectives are to measure:^[25]

• 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.^[11] 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.

X-ray Spectroscopy and Timing (XSPECT)

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.^[27][12] XSPECT is designed to pursue timing studies of soft X-rays (0.8-15 keV),^[26] 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.