Context: The Indian Space Research Organisation (ISRO) successfully placed its inaugural X-ray Polarimeter Satellite (XPoSat) into a meticulous circular orbit at an altitude of 650 km following a 21-minute flight on January 1. XPoSat represents the globe's second satellite mission exclusively focused on conducting X-ray polarimetry measurements.

About  XPoSat:

1. XPoSat, India's inaugural X-ray Polarimeter Satellite, is dedicated to studying X-ray polarization from bright celestial Sources within the medium frequency band.
2. The satellite features two key payloads, namely the Indian X-ray Polarimeter (POLIX) and X-ray Spectroscopy and Timing (XSPECT), developed by the Raman Research Institute and UR Rao Satellite Centre in Bengaluru.
3. Positioned in a low earth orbit at approximately 650 km with a low inclination of around 6 degrees, XPoSat is designed for observations of polarized X-rays.
4. The spacecraft is expected to operate for an estimated five years, during which it will focus on observing polarized X-rays emitted by Sources such as magnetars or neutron stars, known for their strong magnetic fields and diverse X-ray activities.
5. XPoSat's observations will particularly occur when these celestial objects, like magnetars or neutron stars, pass through Earth's shadow, such as during eclipse periods.

Payloads:

1. POLIX Payload:

• It is the world's pioneering instrument intended for operation within the medium X-ray range of 8 to 30 kilo electron Volt (keV).
• Components: POLIX comprises a crucial collimator, responsible for filtering light from bright Sources in the field of view, and a scatterer featuring four X-ray proportional counter detectors to prevent the escape of trapped light.
• Function: It is designed to observe several astronomical Sources and was entirely conceived, designed, and constructed at RRI (Raman Research Institute).

2. XSPECT Payload:

• It is designed for fast timing and high spectroscopic resolution, specifically in the soft X-ray energy band ranging from 0.8 to 15 keV.
• Observation Targets: XSPECT aims to observe a diverse range of Sources, including X-ray pulsars, black hole binaries, low-magnetic field neutron stars, active galactic nuclei (AGNs), which are compact regions at the center of galaxies emitting significant energy across the electromagnetic spectrum, and magnetars.

Significance:

1. Historical Dependency on Traditional Data Sources:

• Astronomers have traditionally relied on data from ground-based telescopes or satellite missions covering the optical to radio frequency bands of the electromagnetic spectrum.
• Celestial polarization was typically studied in the optical or radio bands.

2. XPoSat's Revolutionary Role:

• XPoSat represents a groundbreaking shift, enabling X-ray polarization measurements from bright Sources within the medium energy band of 8-30 keV, a novel endeavor in astronomy.

3. Targeted Celestial Sources:

• The XPoSat team has identified numerous X-ray-emitting Sources, categorizing them into persistent Sources (known and targeted) and transient Sources (such as pulsars, active galactic nuclei, and magnetars).

4. Deep space understanding:

• X-rays in space become polarized due to various factors, such as exposure to strong magnetic fields or interactions with material surrounding black holes.
• Studying polarized X-rays from Sources like magnetars, black holes, and neutron stars allows scientists to probe radiation nature and the intricate processes involved in radiation generation.

5. POLIX Mission Objectives:

• POLIX, as part of XPoSat, will conduct crucial measurements, including determining the degree and angle of polarization of X-ray photons in the vicinity of cosmic entities like black holes and neutron stars.

6. Comprehensive Data Integration:

• The combination of polarimetric data along with spectrographic, timing, and imaging data enhances researchers' ability to comprehensively understand celestial bodies.
• The goal is to unravel previously unknown mysteries of the Universe by improving the understanding of the radiations and processes associated with cosmic entities like black holes and neutron stars.

Need to study polarization of X rays:

• X-rays are composed of electric and magnetic waves in constant motion, characterized by sinusoidal waves lacking a specific direction.
• Polarized X-rays exhibit organization, featuring two waves vibrating in the same direction, unlike the random motion of non-polarized X-rays.
• When magnetars or black holes release X-rays, these encounter diverse materials in the Universe.
• As X-rays pass through thick material clouds, the electric component of the X-ray emits a photon in a different direction due to scattering.
• The resulting photon becomes polarized in a direction perpendicular to the plane formed by the original and scattered photon.
• Scattering during X-ray emission by magnetars or black holes alters the direction of the electric component of X-rays.
• This process results in the polarization of the new photon, perpendicular to the plane formed by the original and scattered photons.
• Polarization measurements, including angular and degree of polarization, are crucial for understanding bright X-ray-emitting Sources.
• These measurements offer insights into the nature of X-ray radiations and the intricate processes they undergo.
• By studying polarization in X-rays, scientists aim to gather valuable clues about the characteristics of Sources emitting bright X-rays, providing a deeper understanding of the radiations and the complex processes involved in their generation.

Comparasion of XPoSat with X-ray experiments in other countries:

• Few missions globally have focused on X-ray polarization measurements, such as HX-POL and XL-Calibur, which were balloon-based experiments conducted by NASA and collaborators. These were short-duration projects.
• Indian astronomers utilized AstroSat, launched in September 2015, for timing and broadband spectroscopy of X-ray Sources. However, no polarisation studies were conducted at that time.
• The scarcity of highly sensitive and precise instruments has posed significant challenges in the development of missions dedicated to X-ray polarization measurements.
• This difficulty has contributed to the limited number of missions in this domain.
• In 2021, NASA introduced the Imaging X-ray Polarimetry Explorer (IXPE), specifically designed for soft X-ray band polarization measurements (2 to 8 keV energy band).
• XPoSat, with its POLIX payload, aims to complement IXPE by extending the observational energy band. POLIX is designed to perform X-ray polarization measurements in the medium X-ray band (8 to 30 keV), providing additional insights and capabilities.
• POLIX, part of XPoSat, offers a unique contribution to X-ray polarimetry by operating in the medium energy band, differentiating it from the soft X-ray focus of the IXPE mission.