Opinion Meet ISRO’s new X-ray eye in the sky

X-rays, like ordinary light, are electromagnetic waves. These are generated by movements of electric charges when the electric and magnetic fields in its vicinity are disturbed. Just as molecules bob up and down in water waves, the electric field keeps changing as X-ray moves in space. The “direction” of its variation depends on the motion of the electric charge responsible for the wave. For an up-down motion of the charge, for example, the electric field in X-ray waves also fluctuates vertically.

It happens in the case of ordinary light as well. But the light we see from a bulb comes from the random motion of molecules. The “direction” of the variation is consequently mixed up in this case. Ditto for sunlight. But sometimes these variations acquire directionality, and we describe the light as “polarised”. We use polarised sunglasses, for example, in bright sunlight. Its special material allows only vertically altering electric fields, and helps to decrease the glare from ground reflection, which consists of mostly horizontal polarisation. Reflection “polarises” the randomly varying sunlight into a single direction. Scattering of sunlight by air molecules also has a similar effect. When the Sun peeks above the horizon, these scatterings grace the sky with highly polarised light. It is no wonder that photographers routinely use polarisers for pictures involving the sky.

Although stars like the Sun mostly emit unpolarised light, it is not the case with some peculiar objects in space. There exist stars with stupendous magnetic fields, sometimes a thousand billion times of what we encounter on the Earth. Electrons in those objects spiral around the magnetic field lines and emit X-rays. Their gyrating motion endows the X-rays with some directionality. Suppose the magnetic field lines are mostly aligned in some direction, then the electrons would twirl in the perpendicular direction. This would make the resulting X-rays polarised in the same direction. Clearly, measuring the polarisation of X-rays would then enable astronomers to gauge the directions of magnetic fields in these objects.

Pulsars are examples of such exotic stars. Typically the size of a city, they can pack as much mass as the Sun. Pulsars often shine in X-rays, and can also boast of whopping magnetic fields. Then there are regions around black holes, where matter spirals inward in the shape of a disc before finally falling prey to the monster. The immensely hot gas emits X-rays, which although unpolarised like starlight, acquires polarisation after reflection and scattering from the gas in the disc, just as sunlight becomes polarised after reflection on ground. If only one could examine the polarisation of celestial X-rays, the nature of these objects would be clearer to astronomers.

This is precisely what POLIX is designed to probe. But it is easier said than done. It is difficult enough to collect X-rays from space, firstly because their high energy makes it impossible to focus with lenses, like ordinary light, and secondly because the Earth’s atmosphere absorbs most of it. Measuring the polarisation of abundant light is one thing, and the case of feeble radiation like cosmic X-rays is something else altogether. There have been attempts with balloon-borne and short-lived instruments in the past. It was only in December 2021 that the first such instrument (IXPE) was launched into space by NASA. The NASA instrument contains gas, whose atoms are stripped of electrons after interacting with incoming X-rays. Upon scrutiny, the direction of motion of these ejected electrons gives clues to the X-ray polarisation.

RRI’s Biswajit Paul, however, had a rather unique design in mind. Hailing from North Bengal, he had worked in the 1990s for his PhD at the Tata Institute of Fundamental Research (TIFR) at Mumbai, where he eventually became a member of its X-ray research group. The TIFR X-ray group has zealously cultivated X-ray astronomy for decades, having built several instruments for space X-ray astronomy, and no doubt inspired Biswajit to think big and think differently. With POLIX finally up in space, his passion for building something new has finally paid off. He has also been fortunate in being ably assisted by a dedicated group of scientists, and being given a free hand by RRI in forging the new device.

POLIX is shaped like a cubical cylinder. At its core lies a disc of beryllium. Detectors kept along the walls collect X-rays after their scattering from the metallic disc. The scheme works on the principle of polarisation after scattering. The lighter the scatterer atoms, the better the signal. Lithium would have been better than beryllium but lithium is difficult to handle, so one had to settle for beryllium. If the electric field in the X-ray varies in the north-south direction, for example, then the detectors stowed in the east-west directions would receive a signal, just like sunlight from the horizon becomes polarised in the overhead direction after scattering in the atmosphere. The other instrument (XSPECT) aboard XPoSat will study timing and spectral properties of X-ray emitting objects in space.

Incidentally, POLIX’s beryllium disc will let astronomers probe lower energy X-rays than what the NASA instrument is capable of. The Indian mission will complement NASA’s device and the information gathered by these two instruments together will help astronomers to decipher the nature of pulsars and black holes. All eyes are now set towards XPoSat, waiting for it to start scanning the cosmos.

The writer is an astrophysicist at the Raman Research Institute, Bangalore