With XPoSat, India Becomes An Early Mover In A Practically Unexplored Area Of Astrophysics — X-Ray Polarimetry

India has been on a bit of a spree with space science missions of late. Its space agency, the Indian Space Research Organisation (ISRO), has remarkably launched three science missions within just the last six months.

The last of the three, following closely on the heels of the Chandrayaan-3 lunar and Aditya-L1 solar missions, launched on Monday, the new year day of 1 January 2024.

“XPoSat” — the name of the latest science satellite, short for X-ray Polarimeter Satellite — was shot to orbit so that it can spend at least five years there and collect crucial data to advance high-energy astrophysics.

Interestingly, the satellite will give India a new, super-cool X-ray look at the universe. So new is this way of investigating the cosmos that, with the launch of XPoSat, India became only the second country, after the United States (US), to have what’s called an X-ray-polarimetry space observatory in the world.

It was as recently as December 2021 that the US space agency National Aeronautics and Space Administration (NASA) launched its Imaging X-ray Polarimetry Explorer (IXPE), which was its, as well as the world’s, maiden mission to study the polarisation of X-rays from extreme objects in the universe.

ISRO’s XPoSat is a close global second in this new avenue of space-based X-ray polarimetry. What’s more, the potential for scientific discovery using XPoSat is huge, as NASA’s IXPE has demonstrated in only two years. “There can be no question that IXPE has shown that X-ray polarimetry is important and relevant to furthering our understanding of how these fascinating X-ray systems work,” said Martin Weisskopf, retired IXPE principal investigator.

As the director of U R Rao Satellite Centre, M Sankaran, said after the XPoSat launch, "From the point of catching up with the world, now we are in the trajectory to start to lead the world."

XPoSat Science

India’s first dedicated polarimeter satellite will use the two scientific instruments on board — POLIX (‘Polarimeter Instrument in X-rays’) and XSPECT (‘X-ray Spectroscopy and Timing’) — to take a really good look in a new light at some of the most powerful and mysterious objects in the universe.

You know these objects — neutron stars, black holes, active galactic nuclei, and others, which are a source of endless fascination. They are very bright X-ray sources and primary targets of the XPoSat mission.

The primary payload POLIX will probe the structure of the intense magnetic field in cosmic objects, as well as the behaviour of matter and radiation in extreme gravity. It will do so by observing bright X-ray sources in the 8-30 kiloelectron volt (keV) range.

The other instrument, XSPECT, will provide spectroscopic information for those same celestial sources in the energy range of 0.8-15 keV.

While POLIX was designed, built, and tested at the Raman Research Institute (RRI), Bengaluru, XSPECT was built by the Space Astronomy Group of the U R Rao Satellite Centre (URSC), Bengaluru.

The “XPo” in XPoSat

POLIX will study polarised X-rays, or X-rays coming from matter experiencing the most extreme conditions.

The cosmic objects expected to produce polarised X-ray radiation include accretion- and rotation-powered pulsars, magnetars, pulsar wind nebulae, non-thermal supernova remnants, black holes, microquasars, active galactic nuclei, low-mass X-ray binaries, and gamma-ray bursts.

By studying polarised X-rays of these enigmatic objects, we can learn more about what they are made of and how they work.

X-rays are a form of high-energy light that is invisible to the human eye. They come from the hottest places in the universe — typically from the grandest cosmic explosions and collisions, and strong magnetic fields. In such violent environments, X-ray polarimetry stands to provide more insight than in most other wavelengths.

Polarisation is a property of light, much like brightness and colour. A wave oscillating in one plane only is said to be polarised — apply this to X-rays and you get polarised X-rays. This is a relatively unexplored area in high-energy astrophysics.

POLIX will measure X-ray polarisation — the degree and angle of polarisation — from cosmic sources. X-ray polarisation is measured by rotating the instrument around the viewing axis. RRI has identified about 50 potential celestial X-ray sources for observation during XPoSat’s planned mission life of five years. Each source will be observed anywhere between one and four weeks.

POLIX works on the principle of Thomson scattering, the scattering of electromagnetic radiation by free charges.

Thomson scattering works well in the 8-30 keV, chosen for the XPoSat mission. For lower than 8 keV and higher than 30 keV, photoelectron scattering or Bragg diffraction and Compton scattering work better, respectively.

POLIX will be kept switched on at all times and will, thus, be generating data round the clock. Among particular X-ray sources identified for initial observations are the Crab nebula, powered by a neutron star at its core; the Cyg X1 black hole X-ray binary; the GX301-2 X-ray pulsar in a wide binary; and SCO-X1, a neutron star in a low-mass X-ray binary.

The plan in the first year is to observe 12 to 18 sources, learn from the experience, and then observe more sources, while re-observing some of the earlier X-ray sources.

From X-ray polarisation, one can expect outcomes such as the measurement of mass and spin of accreting black holes, understanding the geometrical arrangement and local properties of an X-ray source, the accretion flow, outflows, and jets, and insights into the radiation zone and particle acceleration process in pulsars.

What To XSPECT

The second payload, XSPECT, has the job of providing fast timing, of about 1 millisecond, and good spectroscopic resolution in soft X-rays. It will gather long-term observations — spanning weeks to months — of select bright sources in the X-ray energy band of 0.8 to 12 keV.

It will characterise long-term spectral or temporal evolution of bright X-ray sources, which would anyway be the target of polarisation studies. Long stares at a source will allow the evolutionary tracking of said source as it undergoes spectral and temporal state transitions.

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

Spectroscopy measurements combined with polarisation data will enable scientists to dig deeper into radiative and accretion mechanisms in X-ray sources.

Advancing X-ray Polarimetry

RRI has been working on the POLIX instrument since 2006. A prototype was ready by 2009, a modification was made to the design in 2011, a satellite proposal was submitted to ISRO in 2015 and the approval granted a year later, and then, a successful launch finally on 1 January 2024.

Although NASA overtook ISRO to install the world’s first X-ray polarimeter satellite, POLIX is still a first for polarimetry measurements in the medium X-ray energy band.

Moreover, XPoSat’s chosen energy range of 8-30 keV for polarisation measurements is complementary to IXPE’s energy range of 2-8 keV. Together, the satellites will provide a wide observation window in the energy range of 2-30 keV for polarimetric measurements associated with bright X-ray sources.

In the future, another X-ray polarimetry satellite may join IXPE and XPoSat for space-based X-ray polarisation studies. The X-ray Imaging Polarimetry Explorer (XIPE) concept proposed by the European Space Agency (ESA) has been in the works for at least a decade and may be launched sometime this decade.

India, therefore, is an early mover with the XPoSat mission. It provides an opportunity to build expertise in India in the area of X-ray polarimetry. For this purpose, the role of the Indian science community, especially early-career scientists, will be important to make the most of XPoSat and X-ray polarimetry in general.