Why is it in the News?

Indian Space Research Organisation (ISRO) put its first polarimetry mission X-ray Polarimeter Satellite (XPoSat) in a precise circular orbit of 650 km on Monday (January 1) after a 21-minute flight.

Context:

• As the world welcomed the first morning of 2024, ISRO's launch vehicle successfully placed a new X-ray payload, XPoSat (X-ray Polarimeter Satellite), into orbit for astronomical research.
• XPoSat is the world’s second satellite-based mission dedicated to making X-ray polarimetry measurements.
• If the mission proceeds as planned, this entirely indigenous instrument in design and fabrication will mark another significant achievement for Indian astronomers.

What is XPoSat (X-ray Polarimeter Satellite)?

• X-ray Polarimeter Satellite (XPoSat) is India’s first  and world’s second mission dedicated to analysing the polarisation of X-rays emanating from bright celestial sources in the medium frequency band.
• XPoSat comprises two payloads, including Indian X-ray Polarimeter (POLIX) and X-ray Spectroscopy and Timing (XSPECT).
• They have been built by Raman Research Institute and UR Rao Satellite Centre, both located in Bengaluru.
• The spacecraft is designated for observation from low earth orbit (~ 650 km, low inclination of ~ 6 degree).
• It has an estimated mission life of about five years during which XPoSat will observe sources that emit polarised X-rays.
• The observations will be done when the magnetars or neutron stars (they are highly magnetic and display a wide array of X-ray activity) are in transit through the Earth’s shadow, for instance, during the eclipse period.

What are the Two Scientific Payloads Onboard XPoSat?

• POLIX: It is the world’s first instrument designed to operate in the medium X-ray of 8 to 30 kilo electron Volt (keV) energy band.
• It comprises a collimator, which is the key component to filter light originating from bright sources in the field of view.
• Moreover, there is a scatterer consisting of four X-ray proportional counter detectors (that prevent the trapped light from escaping).
• It will observe a few tens of astronomical sources. It was conceived, designed, and built at RRI.
• XSPECT: It is designed to conduct fast timing and high spectroscopic resolution in a soft X-ray energy band (0.8-15 keV).
• It will observe a variety of sources like X-ray pulsars, black hole binaries, low-magnetic field neutron stars, active galactic nuclei or AGNs (a compact region at the centre of a galaxy that emits a significant amount of energy across the electromagnetic spectrum) and magnetars.

Why is the XPoSat Mission Significant?

• Till now, astronomers have largely used and depended on spectroscopic, imaging and timing–based data obtained from either ground-based telescopes or satellite-based missions from the optical to the radio frequency band of the electromagnetic spectrum.
• Polarisation of celestial sources was done either in the optical or radio bands.
• XPoSat, however, will be a game-changer and facilitate X-ray polarisation measurements possible from bright sources, that too, in the medium energy band (8-30 keV) energy range – which has never been attempted ever before.
• The XPoSat team has identified several tens of sources radiating X-rays.
• XPoSat will observe two kinds of sources:
• Persistent sources (targeted and known sources) and
• Transient sources (pulsars, active galactic nuclei, magnetars).
• Out in space, X-rays get polarised due to multiple causes.
• For example, X-rays when subject to strong magnetic fields or due to the interactions with material present around black holes.
• So, by studying the polarised X-rays emanating from excellent sources like magnetars, black holes and their surrounding environments, and neutron stars, scientists can probe the nature of the radiations and the multitudes of processes involved in the generation of these radiations.
• POLIX will undertake important measurements like the degree and angle of polarisation of X-ray photons from the environment surrounding black holes, neutron stars, and other such cosmic entities.
• These two additional parameters, along with the spectrographic, timing and imaging data, will aid researchers to overall improve the present understanding of the celestial bodies and ultimately unravel some of the unknown mysteries of the Universe.

What is the Polarisation of X-rays and Why Study It?

• X-rays comprise electric and magnetic waves that are constantly in motion.
• Being sinusoidal waves, they do not follow a patterned direction of motion.
• Whereas, a polarised X-ray is both organised and has two waves vibrating in the same direction.
• When magnetars or black holes emit X-rays, they encounter a wide variety of materials in the Universe.
• As X-rays pass through the thick cloud of materials, the electric component of the X-ray emits a photon in a changed direction, as it has now undergone scattering.
• In the process, the new photon has got polarised in a direction perpendicular to the plane formed between the original and scattered photon.
• The polarisation measurements – angular and degree of polarisation – are believed to provide clues about the bright X-ray emitting sources the nature of these radiations and the complex process they undergo.
• X-rays are electromagnetic radiation whose wavelength is 0.01-10 nanometres (where 1 nanometre is one-billionth of a metre), corresponding to energies of 100-100,000 electron-volt (eV).
• Electromagnetic radiation is characterised by an electric field and a magnetic field vibrating perpendicular to each other.
• The polarisation of electromagnetic radiation refers to the orientation of these two fields as the radiation moves through space.
• X-rays can be polarised when they get scattered.
• For example, when an X-ray travelling through space encounters an atom, the electric field of the X-ray can energise an electron, which will then emit a photon.
• Since X-rays are also photons, the new photon will give the impression that an X-ray photon has been scattered.
• Polarised X-rays are also produced when the path of a fast-moving charged particle is bent by a magnetic field.
• Studying these X-rays can reveal which way the magnetic field is pointing, and tracking how these X-rays evolve in time can reveal many things about the body producing such fields, like a pulsar.

How Does XPoSat Compare With X-ray Experiments or Missions Globally?

• Missions on X-ray polarisation measurements have been a handful, the world over.
• Some like HX-POL and XL-Calibur have been balloon-based and short-duration experiments by NASA and collaborators.
• Indian astronomers, using AstroSat which is India’s first astronomy-based space mission launched in September 2015 — performed timing and broadband spectroscopy of X-ray sources but no polarisation studies were performed.
• The lack of development of highly sensitive and precise instruments makes missions for polarisation measurements of X-rays extremely challenging, thus fewer missions have been attempted so far.
• In 2021, NASA launched Imaging X-ray Polarimetry Explorer (IXPE).
• It has been designed to operate and perform X-ray polarisation measurements within the soft X-ray band (2 to 8 keV energy band).
• Besides complementing IXPE, XPoSat’s payload POLIX will offer an expanded observational energy band, as it is designated to perform X-ray polarisation in the medium X-ray band (8 to 30 keV).

Conclusion

The accomplished launch of XPoSat and the deployment of POLIX mark a substantial advancement in India's commitment to excelling in space-based X-ray astronomy. With the inventive design of POLIX and its capability to investigate lower energy X-rays, coupled with its collaboration with NASA's instrument, Indian astronomers find themselves at the forefront of unravelling the enigmas surrounding pulsars and black holes. As XPoSat embarks on its cosmic scanning mission, the scientific community is eagerly anticipating the valuable insights it is poised to deliver.