XPoSat: India’s First X-ray Polarimeter Satellite

XPoSat (X-ray Polarimeter Satellite) marks India’s entry into the realm of dedicated polarimetry missions to study the intricate dynamics of bright astronomical X-ray sources in extreme conditions. Equipped with two cutting-edge scientific payloads, this spacecraft is set to orbit the Earth, unraveling the mysteries hidden within the vast expanse of the universe. In this article, we delve into the details of XPoSat’s payloads, their capabilities, and the profound impact they are expected to have on our understanding of astronomical emission processes.


XPoSat represents a significant milestone in India’s space exploration endeavors. As the nation’s inaugural dedicated polarimetry mission, it aims to shed light on the complex physical processes governing the emission mechanisms of various astronomical sources, including black holes, neutron stars, active galactic nuclei, and pulsar wind nebulae. While previous space-based observatories have provided valuable spectroscopic and timing information, the emission nature of these sources continues to pose challenges to astronomers. The incorporation of polarimetry measurements, which capture the degree and angle of polarization, presents a promising diagnostic tool to unravel these mysteries.

XPoSat Payloads

POLIX: Polarimeter Instrument in X-rays

Developed collaboratively by the Ramam Research Institute (RRI) in Bangalore and the U R Rao Satellite Centre (URSC), POLIX serves as XPoSat’s primary payload. Functioning within the medium X-ray energy range of 8-30 keV, this X-ray polarimeter is specifically designed for astronomical observations. POLIX comprises a collimator, a scatterer, and four X-ray proportional counter detectors surrounding the scatterer. The scatterer, composed of low atomic mass material, induces anisotropic Thomson scattering of incoming polarized X-rays. To ensure optimal observations, the collimator restricts the field of view to 3 degrees by 3 degrees, allowing for the study of a single bright source during most observations. Over the anticipated five-year lifespan of the XPoSat mission, POLIX is poised to observe approximately 40 diverse astronomical sources, making it the first medium X-ray energy band payload dedicated to polarimetry measurements.

XSPECT: X-ray Spectroscopy and Timing

Working synergistically with POLIX, XSPECT is another crucial payload onboard XPoSat. Specializing in providing fast timing and high-quality spectroscopic resolution in the soft X-ray energy range of 0.8-15 keV, XSPECT offers a comprehensive suite of observation capabilities. By leveraging extended-duration observations necessitated by POLIX’s polarimetry measurements, XSPECT enables long-term monitoring of spectral state changes in continuum emission, variations in line flux and profile, and simultaneous temporal monitoring of soft X-ray emission. The XSPECT payload employs an array of Swept Charge Devices (SCDs), boasting an effective area of over 30 cm² at 6 keV, with an impressive energy resolution of under 200 eV at the same energy level. Passive collimators are strategically employed to reduce background interference by narrowing the field of view. XSPECT’s observations encompass various sources, such as X-ray pulsars, black hole binaries, low-magnetic field neutron stars (NS) in low-mass X-ray binaries (LMXBs), active galactic nuclei (AGNs), and magnetars.

Understanding Astronomical Emission Processes

The emission mechanisms originating from diverse astronomical sources entail intricate physical processes that challenge our comprehension. While spectroscopic and timing data collected by previous observatories have provided substantial information, unraveling the exact nature of these emissions remains a profound scientific quest. The introduction of polarimetry measurements introduces two additional dimensions—degree and angle of polarization—to our understanding. These parameters serve as invaluable diagnostic tools to decipher the emission processes governing astronomical sources. By combining polarimetric observations with spectroscopic measurements, astronomers anticipate breaking the degeneracy plaguing various theoretical models, bringing us closer to unveiling the mysteries of the universe.

Research Direction of XPoSat

India’s scientific community eagerly embraces the challenge presented by XPoSat. With a keen focus on polarimetry measurements and spectroscopic data, researchers aim to make significant contributions to the field. The mission’s primary thrust lies in overcoming the limitations of existing theoretical models, propelled by the synergistic integration of polarimetry and spectroscopy. The insights gained from XPoSat’s observations are expected to revolutionize our understanding of astronomical emission processes, paving the way for novel discoveries and unprecedented advancements in astrophysics.


XPoSat’s arrival on the space exploration scene marks a crucial milestone in India’s scientific pursuit of unraveling the mysteries of the cosmos. Equipped with the POLIX and XSPECT payloads, this groundbreaking mission aims to elucidate the intricate emission processes of astronomical sources. Through the integration of polarimetry measurements and spectroscopic data, XPoSat promises to break new ground in our understanding of the universe’s hidden dynamics. The scientific community eagerly awaits the wealth of knowledge this mission will unveil.


Based on the aforesaid information, answer the following in the comment box:

What is the main objective of XPoSat?

a. To study gravitational waves

b. To explore the atmosphere of Mars

c. To investigate X-ray emission from astronomical sources

d. To search for exoplanets


Which payload of XPoSat measures the degree and angle of polarization?



c. Ramam Research Institute

d. U R Rao Satellite Centre


What is the energy range of the POLIX payload?

a. 0.8-15 keV

b. 8-30 keV

c. 30-100 keV

d. 100-500 keV


How many X-ray proportional counter detectors surround the scatterer in POLIX?

a. One

b. Two

c. Three

d. Four


What is the field of view for POLIX?

a. 1 degree x 1 degree

b. 2 degrees x 2 degrees

c. 3 degrees x 3 degrees

d. 4 degrees x 4 degrees


Which payload provides fast timing and good spectroscopic resolution in soft X-rays?



c. Ramam Research Institute

d. U R Rao Satellite Centre


What type of devices are used in XSPECT to provide spectroscopic capabilities?

a. X-ray proportional counters

b. Swept Charge Devices (SCDs)

c. Passive collimators

d. X-ray spectrometers


What is the expected impact of polarimetry measurements on understanding astronomical emission processes?

a. To discover new exoplanets

b. To break the degeneracy of theoretical models

c. To measure the exact distances of celestial objects

d. To study the cosmic microwave background radiation


How long is the planned lifetime of the XPoSat mission?

a. 1 year

b. 3 years

c. 5 years

d. 10 years


What sources will XSPECT observe?

a. X-ray pulsars, black hole binaries, and low-magnetic field neutron stars

b. Exoplanets, asteroids, and comets

c. Supernovae, galaxies, and quasars

d. Solar flares, coronal mass ejections, and auroras


1. What is XPoSat?

XPoSat stands for X-ray Polarimeter Satellite, which is India’s first dedicated polarimetry mission designed to study the emission processes of astronomical X-ray sources.

2. What are the main payloads of XPoSat?

The main payloads of XPoSat are POLIX (Polarimeter Instrument in X-rays) and XSPECT (X-ray Spectroscopy and Timing).

3. How does POLIX measure polarimetry parameters?

POLIX measures the degree and angle of polarization using a collimator, a scatterer, and four X-ray proportional counter detectors.

4. What is the field of view for POLIX?

The field of view for POLIX is restricted to 3 degrees by 3 degrees to focus on a single bright source during most observations.

5. What types of sources will XSPECT observe?

XSPECT will observe various sources, including X-ray pulsars, black hole binaries, low-magnetic field neutron stars in low-mass X-ray binaries, active galactic nuclei, and magnetars.

Also read Chandrayaan 3.

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