XPoSat is the India’s first X-ray polarimetricmission launched on 1st January 2024 by PSLV C58 rocket from Sriharikota. The satellite carries twoco-aligned instruments, Polarization in X-rays (POLIX) and X-ray SPECtroscopy and Timing (XSPECT). POLIX will investigate the X-ray polarization in cosmic X-ray source in medium energy X-rays and XSPECT will carry out X-ray spectroscopy of these sourcesin soft X-ray band.

Astrophysical sources emitting predominantly in X-rays have been typically associated with binary systems. In these systems a compact object such as black-hole, neutron star or white dwarf, an end product of stellar evolution accretes matter from a normal main sequence star. The in-falling materialloses its potential energy and angular momentum and forms an accre-tion disc around these stars. The temperature of such disc is around millions of degree K and emits primarily in X-rays. In the first year, about ten bright X-ray sources have been selected for observations, which contains, black-hole binary, X-ray pulsars, low-mass X-ray binary and crab pulsar.

Taking advantage of long duration (2-4 weeks) observation required by POLIX payloadto measure the X-ray polarization, XSPECT payload will be able to carry out continuous and long term spectral and temporal studies of X-ray sources. XSPECT payload operates in the soft X-ray band of 0.8 keV – 15 keV with good spectroscopic resolution (< 200 eV at 6 keV)and moderate timing capability. The payload was developed by Space Astronomy Group of U R Rao Satellite Centre.

Science objectives of XSPECT:

XSPECT will provide crucial information about X-ray spectral state evolution in bright X-ray binaries, long term changes in continuum emission and line fluxes, pulse-shape and pulse pe-riod evolution in X-ray pulsars. Large count rate handling capacity of XSPECT makes it ideal to probe the bright X-ray sources in soft X-rays which was not possible earlier by imaging X-ray CCDs. Some of the scientific objectives of XSPECT payload are;

• Investigation of long term changes in continuum X-ray spectra and spectral-transition in black-hole and neutron star X-ray binaries. Such a study will provide crucial under-standing about the physics of accretion processes and emission mechanisms in these systems.
• Study of spin period evolution, accretion torque history, associated changes in X-ray flux, emission mechanisms and pulse profile. Such a study will shed light on standard model of accretion on to magnetized neutron star.
• Measuring the mass and spin of the black-hole by modeling the continuum and iron-line.

Salient features of XSPECT payload: