XPoSat: India’s First X-Ray Polarimetry Mission
Source: ISRO launches XPoSat: What is the mission and its significance?; IE, January 1, 2024
- Successful Launch: The Indian Space Research Organisation, ISRO, successfully launched its XPoSat mission. The satellite achieved a stable orbit around Earth at 650 km.
- World’s Second: XPoSat stands out as the world’s second mission focusing specifically on X-ray polarimetry.
- Introduction to XPoSat: The X-ray Polarimeter Satellite (XPoSat) is ISRO’s first project to study X-ray polarization. It targets X-rays coming from space’s bright objects.
- Made in Bengaluru: Two main instruments make up XPoSat: the Indian X-ray Polarimeter (POLIX) and the X-ray Spectroscopy and Timing (XSPECT). Bengaluru is home to their development sites, at the Raman Research Institute and UR Rao Satellite Centre.
- Mission Specifications: XPoSat orbits relatively close to Earth, at around 650 km. It sits in a low-inclination orbit. The mission is set to last five years.
- Focus on Polarised X-Ray Sources: During its five-year mission, XPoSat will focus on polarised X-ray sources like magnetars and neutron stars. Observation periods are timed with these objects’ movement through Earth’s shadow, like during eclipses.
XPoSat’s Scientific Instruments
- POLIX: The POLIX instrument is unique. It works with medium X-rays in the 8-30 keV range. Its main part is a collimator, which filters out unwanted light. There are also four detectors designed to stop light from leaking out. This instrument will study around tens of space objects. It was made at the RRI.
- XSPECT: XSPECT has a different focus. It’s for quick timing and detailed analysis of soft X-rays between 0.8-15 keV. This instrument will look at a variety of astronomical objects. These include X-ray pulsars, black hole systems, neutron stars with weak magnetic fields, the very energetic centres of distant galaxies (AGNs), and magnetars.
XPoSat: Pioneering X-Ray Polarisation
XPoSat is set to revolutionise the way we measure X-ray polarisation in space. This will be the first time such measurements are made in the medium energy range of 8-30 keV.
- Targeted and Transient Sources: The XPoSat mission will focus on two types of X-ray sources. These include persistent sources, which are always there, and transient sources like pulsars and magnetars.
- The Science of X-Ray Polarisation: In space, X-rays can become polarised. This happens when they face strong magnetic fields or interact with matter near black holes. Studying this helps us understand black holes, magnetars, and neutron stars better.
- POLIX Measurements: The POLIX instrument will measure how polarised the X-rays from these cosmic entities are. Understanding polarisation gives us more details about the extreme processes in space. These efforts will enhance our knowledge of the universe significantly.
- Understanding X-ray Polarisation: X-rays are made up of electric and magnetic waves. Unlike certain waves with straightforward paths, these waves move in complex patterns. Some X-rays have waves that align together, which we call polarised X-rays.
- The Journey Through Space: X-rays often come from distant cosmic objects like magnetars or black holes. They travel through space and encounter different materials. As the X-rays move through these dense clouds, they interact and scatter.
- Effects of Scattering: The scattering process affects the X-rays’ electric waves by redirecting them. This results in a photon that moves off in a new direction. This diverted photon is now polarised and sits at a right angle to the plane created by its original and new paths.
- Why Polarisation Matters: By studying the angles and degrees of X-ray polarisation, scientists can learn more about their sources. Polarisation gives insights into the X-rays’ origins and the dynamics they experience.
Global Context of X-Ray Polarisation Missions
- Past Missions and Experiments: Globally, there have been few X-ray polarisation missions. The HX-POL and XL-Calibur, organised by NASA and its partners, were limited to balloon experiments with short durations.
- India’s Contribution: India launched its first astronomical space mission, AstroSat, in September 2015. Indian astronomers were able to perform timing and broadband spectroscopy through AstroSat. However, this mission did not include polarisation studies.
- Challenges in Mission Development: Developing instruments that are both highly sensitive and precise for X-ray polarisation is a challenge. That’s why there have been so few such missions.
- NASA’s Recent Developments: In 2021, NASA introduced the Imaging X-ray Polarimetry Explorer (IXPE). This mission is focused on measurements within the soft X-ray band of 2 to 8 keV.
- XPoSat’s Unique Position: XPoSat stands out with its payload POLIX. It is set to complement NASA’s IXPE. POLIX will expand the range of observation, targeting medium X-ray bands between 8 to 30keV for polarisation measurements.