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Chennai, Dec.01 – India is set to launch its first X-Ray Polarimeter Satellite (XPoSat), aiming to investigate the polarization of intense X-Ray sources.

The mission is likely to take place by the end of this year from SHAR Range.

ISRO is expected to use its reliable launch vehicle PSLV for the mission.

ISRO said on Thursday that while space-based X-Ray astronomy has been established in India, focusing predominantly on imaging, time-domain studies, and spectroscopy, this upcoming mission marks a major value-addition.

The astronomy community is particularly enthused about the prospect of a systematic exploration into the polarization of X-Rays emitted by astronomical sources. This research, supplementing traditional time and frequency domain studies, introduces a novel dimension to X-Ray astronomy, generating anticipation and excitement within the scientific community.

The XPoSat spacecraft is designated for observation from Low Earth Orbit (non-sun synchronous orbit of Rs650 km altitude, low inclination of 6 degree), carrying two scientific payloads. With these two payloads, the XPoSat mission is capable of simultaneous studies of temporal, spectral, and polarization features of the bright X-Ray sources.

The mission objectives include measurement of X-Ray polarization in the energy band of 8-30 keV emanated from X-Ray sources, long-term spectral and temporal studies of cosmic X-Ray sources in the energy band of 0.8-15 keV.

The mission life is expected to be 5 years.

The payloads onboard XPoSat will observe the X-Ray sources during its transit through the Earth’s shadow, i.e., during the eclipse period.

At this juncture, in order to appreciate the importance of this mission, it is apt to take a retrospective look of the evolution of astronomical instrumentation and gradual unfurling of the mysteries in the cosmos.

It all started with the invention of optical telescope, more than four hundred years ago. Even our initial encounter with Astronomy, during school days, often involved optical telescopes, observing the Moon’s features and planets in our solar system. Progressing to attaching cameras for celestial photography, our capability expanded to astronomical imaging. Such imaging telescopes allowed imaging of planets, natural satellites, and monitoring starlight fluctuations. Later, we delved into analyzing the frequency components in stellar light, thus making spectroscopy a vital tool in Astronomy. Integrating imaging and spectroscopy, instruments emerged capable of capturing celestial bodies at diverse wavelengths. Notable examples include captivating images of the Sun in visible, ultraviolet, and X-Ray wavelengths, each revealing a facet of the Sun’s intricate story.

Apart from imaging, studying the fluctuations of light from a source, and spectroscopy, there emerged another tool for observational astronomy. It was ‘polarization’, which is regarded as one of the intrinsic properties of light. As every frequency band in astronomy has a story to tell about the processes that generate the radiation, the information on polarization provides a deeper insight to the processes, as well as the local anisotropies of the fields (electric/magnetic/gravitational).

To be precise, X-Ray polarization serves as a crucial diagnostic tool for examining the radiation mechanism and geometry of celestial sources. Analyzing X-Ray polarization signatures enables measurements of the mass and spin of accreting black holes, comprehension of the source’s geometric arrangement and local properties, exploration of accretion flow, outflow, and jets, investigation into the nature of X-Ray scattering and reflection mediums, estimation of strong magnetic fields, and revelation of the radiation zone and particle acceleration processes in pulsars, among other applications. Each of these processes manifests its distinct signature within appropriate energy bands, depending on the involved energetics. The XPoSat mission by ISRO is specifically designed to investigate such X-Ray polarization signatures emanating from bright X-Ray sources.

On the XPoSat Mission, ISRO said the primary payload of XPoSat, POLIX (Polarimeter Instrument in X-rays), is designed to measure polarimetry parameters—specifically the degree and angle of polarization—in the medium X-ray energy range of 8-30 keV photons originating from astronomical sources. Complementing this, the XSPECT (X-ray Spectroscopy and Timing) payload will provide spectroscopic information within the energy range of 0.8-15 keV. The POLIX payload is developed by the Raman Research Institute (RRI), Bangalore, with support from the ISRO centres. The XSPECT payload is developed by the U R Rao Satellite Centre (URSC), ISRO.

The POLIX payload serves as an X-ray Polarimeter designed for astronomical observations within the energy band of 8-30 keV. The instrument comprises a collimator, a scatterer, and four X-ray proportional counter detectors surrounding the scatterer. The scatterer, constructed from low atomic mass material, induces anisotropic Thomson scattering of incoming polarized X-rays. The collimator plays a crucial role in restricting the field of view to 3 degrees by 3 degrees, ensuring that only one bright source is within the field of view for most observations. POLIX’s primary objective is to observe bright astronomical sources across various categories during the planned 5-year lifetime of the XPoSat mission. Notably, POLIX stands out as the first payload in the medium X-ray energy band specifically dedicated to polarimetry measurements.

XSPECT is an X-ray Spectroscopy and Timing instrument, designed to offer fast timing and excellent spectroscopic resolution in soft X-rays (0.8-15 keV). XSPECT also monitors changes in line flux and profile, offering simultaneous, long-term temporal monitoring of soft X-ray emission. The instrument employs an array of Swept Charge Devices (SCDs) with an effective area exceeding 30 cm² at 6 keV and an impressive energy resolution of less than 200 eV at 6 keV. XSPECT employs passive collimators to reduce background by narrowing its field of view. This payload is anticipated to observe a variety of sources, including X-ray pulsars, black hole binaries, low-magnetic field neutron stars (NS), active galactic nuclei (AGNs), and magnetars.