Home : Science Promotion : Scientific Payloads : XSPECT Last updated on: 09-Jan-2024

XSPECT payload on XpoSat

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:

XSPECT has Swept Charge Devices (SCD) as its sensors, which are non-imaging CCD type of sensor, each device is of ~ 4 cm2 in geometric area. Total sixteen devices make up the payload. Two different FOV collimators are used, 7 devices with 30 x 30 and 8 devices with 20 x 20 FOV, to measure cosmic X-ray background as well as source flux of interest enabling it to correct for background for each observation independently. One of the detector is shielded by tantalum sheet to block X-rays and thus enabling it to measure the charged particle back-ground.

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Figure 1: (a) XSPECT payload detector package. Two such packages are mounted on the instrument deck of the spacecraft. (b) XSPECT electronics package

As part of Performance Verification (PV) phase operation, XSPECT payload was pointed to-Cassiopeia A (CasA), a standard source in the sky used for Instrument evaluation. The ob-servation of the source started on 05 Jan 2024. This source is a supernova remnant, which shows many emission lines in X-rays corresponding to elements such as Magnesium (Mg), Silicon (Si), Sulphur (S), Argon (Ar), Calcium (Ca), and Iron (Fe) as major lines.

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Figure 2: XSPECT sees supernova remnant Cassiopeia A (CasA). The spectrum in figure is as seen by the instrument, includes both Galactic Cosmic Ray (GCR) background and Cosmic X-ray Background (CXB).The flux above 8 keV is mainly due to both CXB and GCR. The spectrum shown in Figure is for a total integration time of 20 ksec collected over multiple orbits.