May 19, 2024

ISRO to Launch XPoSat Mission aboard PSLV on January 1 to Explore Polarization of Cosmic X-Ray Sources

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Dec 28, 2023

The Indian Space Research Organisation (ISRO) is set to launch the XPoSat mission on January 1st, 2023 onboard the Polar Satellite Launch Vehicle (PSLV). XPoSat stands for X-ray Polarimeter Satellite and aims to study the polarization of cosmic X-ray sources such as pulsars, neutron stars and black hole binaries.

XPoSat Mission Overview

As per reports, the launch is scheduled at 12:30 pm IST from the Satish Dhawan Space Centre in Sriharikota. XPoSat weighs about 328 kg and will be injected into a circular 500 km orbit by the PSLV-C58 rocket during a two-hour flight.

XPoSat carries the Polarimeter Instrument in X-rays (POLIX) payload which can measure the polarization of bright cosmic X-ray sources in the energy range of 5-30 kilo electronvolt (keV). By measuring X-ray polarization, the mission aims to answer key questions related to the geometry and emission mechanism of cosmic radio sources.

Key Details
Launch Date January 1, 2023
Launch Time 12:30 pm IST
Launch Site Satish Dhawan Space Centre, Sriharikota
Orbit Circular 500 km
Payload POLIX X-ray Polarimeter
Launch Vehicle PSLV-C58
Mission Duration 5 years

XPoSat follows ISRO’s previous astronomy missions such as ASTROSAT and will pave the way for the more advanced X-ray Polarimetry Satellite (XRPS) planned for launch in 2024.

Significance of Studying X-ray Polarization

X-ray polarimetry allows the exploration of the geometry and emission mechanism of celestial X-ray sources. While current X-ray measurements provide spectral, timing and imaging data, polarization measurements provide additional inputs to decode emission processes.

According to Dr Sunil Chandra – Head of Calibration & Payload Testing at ISRO:

“Polarisation measurement of cosmic X-ray sources can lead to unambiguous understanding of the emission mechanism from celestial bodies and the environments of neutron stars and black hole binaries that act as cosmic laboratories with conditions impossible to replicate on earth…It will also enable us to explore fundamental physics in extreme gravitational fields.”

By studying the polarization of pulsars, accretion disks around black holes and other cosmic radio sources, XPoSat will help test theories related to astrophysics, thermodynamics and quantum chromodynamics.

Previous Attempts of X-ray Polarimetry

While X-ray polarization measurements provide unique information, they require dedicated instrumentation leading to limited attempts so far:

  • 1970s – 1980s: Small pioneering experiments showed that X-ray polarimetry was feasible but lacked necessary sensitivity
  • 2006: NASA’s Gravity and Extreme Magnetism SMEX (GEMS) satellite attempted to measure 10% polarization but was cancelled due to budget issues and technological challenges
  • 2016: The European Space Agency launched the X-ray Imaging Polarimetry Explorer (IXPE) to explore high-energy astrophysical processes but with limited sensitivity

XPoSat aims to take X-ray polarimetry to the next level using an innovative Time Projection Chamber (TPC) to accurately measure cosmic X-ray polarization fractions down to 5-10%.

Key Scientific Objectives

The key scientific goals of the XPoSat mission are:

  • Measure linear polarization in bright cosmic X-ray sources down to a Minimum Detectable Polarization (MDP) of 5%
  • Constrain X-ray emission models of black holes, neutron stars, pulsars and Active Galactic Nuclei
  • Understand acceleration mechanisms in pulsars and geometry of accretion disks around black holes
  • Study quantum chromodynamics and thermodynamics under extreme conditions
  • Test General Theory of Relativity in strong magnetic fields of neutron stars

By achieving these goals, XPoSat will answer several open questions related to high-energy astrophysics and fundamental physics.

Payload Details: POLIX Instrument

The polarization measurements will be carried out by POLIX (Polarimeter Instrument in X-rays), the primary instrument onboard XPoSat operating in the energy range of 5-30 keV.

POLIX is a novel gas-based detector that can image tracks of electrons generated by X-ray photon interactions to determine polarization states. It consists of a gas chamber filled with a mixture of argon and methane at a pressure of 2 atmospheres.

An incoming X-ray photon ionizes the gas molecules and the ejected photo-electrons ionize other gas atoms leading to an electron cascade. By tracing the electron tracks and interaction locations, POLIX can derive polarization information of the incident X-rays.

Launch Vehicle: PSLV-C58

The PSLV-C58 launch vehicle will carry XPoSat into a circular 500 km orbit during its 2-hour flight. PSLV, ISRO’s trusted workhorse, is a four-stage rocket powered entirely by solid and liquid propulsion systems.

The rocket is designed to carry heavier payloads than previous versions with larger strap-ons and enhanced capability. This will be the 26th flight of the upgraded PSLV variant.

So far, PSLV has successfully launched over 345 satellites belonging to international customers indicating its versatility and reliability.

Future Prospects

Following XPoSat, ISRO plans to launch XRPS (X-ray Polarimetry Satellite) in 2024 with enhanced capacity to measure X-ray polarization fractions down to 1%. XRPS will have 3 times better sensitivity and wider energy coverage for studying neutron stars, pulsars and stellar black holes.

The successful demonstration of POLIX technology and operations aboard XPoSat will pave the way for advanced future X-ray polarimetry missions. XPoSat also adds to ISRO’s growing fleet of astronomical observation satellites taking India a step closer to becoming a global space power.


XPoSat’s launch on New Year’s day ushers in exciting possibilities for the new year for ISRO and India’s space program. The mission will attempt unique polarization measurements that have been challenging so far, helping unravel mysteries surrounding high-energy cosmic phenomena involving neutron stars and black holes.

By exploring fundamental physics in extreme environments that cannot be replicated on Earth, XPoSat has the potential to push boundaries on several theoretical fronts taking Indian space science to new frontiers. More importantly, it will lay the technology foundations for ISRO’s future X-ray astronomy missions in the coming decade.




AiBot scans breaking news and distills multiple news articles into a concise, easy-to-understand summary which reads just like a news story, saving users time while keeping them well-informed.

To err is human, but AI does it too. Whilst factual data is used in the production of these articles, the content is written entirely by AI. Double check any facts you intend to rely on with another source.

By AiBot

AiBot scans breaking news and distills multiple news articles into a concise, easy-to-understand summary which reads just like a news story, saving users time while keeping them well-informed.

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