India’s X-Ray Polarimetry Astronomy Endeavour

India is set to launch its first X-Ray Polarimeter Satellite (XPoSat), aiming to investigate the polarization of intense X-Ray sources. While space-based X-Ray astronomy has been established in India, focusing predominantly on imaging, time-domain studies, and spectroscopy, this upcoming mission marks a major value-addition. The astronomy community is particularly enthused about the prospect of a systematic exploration into the polarization of X-Rays emitted by astronomical sources. This research, supplementing traditional time and frequency domain studies, introduces a novel dimension to X-Ray astronomy, generating anticipation and excitement within the scientific community.

The XPoSat spacecraft is designated for observation from Low Earth Orbit (non-sun synchronous orbit of ~650 km altitude, low inclination of ~6 degree), carrying two scientific payloads. With these two payloads, the XPoSat mission is capable of simultaneous studies of temporal, spectral, and polarization features of the bright X-Ray sources. The mission objectives include (i) measurement of X-Ray polarization in the energy band of 8-30 keV emanated from X-Ray sources, (ii) long-term spectral and temporal studies of cosmic X-Ray sources in the energy band of 0.8-15 keV. The mission life is expected to be ~ 5 years. The payloads onboard XPoSat will observe the X-Ray sources during its transit through the Earth’s shadow, i.e., during the eclipse period.

At this juncture, in order to appreciate the importance of this mission, it is apt to take a retrospective look of the evolution of astronomical instrumentation and gradual unfurling of the mysteries in the cosmos.

It all started with the invention of optical telescope, more than four hundred years ago. Even our initial encounter with Astronomy, during school days, often involved optical telescopes, observing the Moon's features and planets in our solar system. Progressing to attaching cameras for celestial photography, our capability expanded to astronomical imaging. Such imaging telescopes allowed imaging of planets, natural satellites, and monitoring starlight fluctuations. Later, we delved into analyzing the frequency components in stellar light, thus making spectroscopy a vital tool in Astronomy. Integrating imaging and spectroscopy, instruments emerged capable of capturing celestial bodies at diverse wavelengths. Notable examples include captivating images of the Sun in visible, ultraviolet, and X-Ray wavelengths, each revealing a facet of the Sun's intricate story.

Apart from imaging, studying the fluctuations of light from a source, and spectroscopy, there emerged another tool for observational astronomy. It was ‘polarization’, which is regarded as one of the intrinsic properties of light. As every frequency band in astronomy has a story to tell about the processes that generate the radiation, the information on polarization provides a deeper insight to the processes, as well as the local anisotropies of the fields (electric/magnetic/gravitational).

To be precise, X-Ray polarization serves as a crucial diagnostic tool for examining the radiation mechanism and geometry of celestial sources. Analyzing X-Ray polarization signatures enables measurements of the mass and spin of accreting black holes, comprehension of the source's geometric arrangement and local properties, exploration of accretion flow, outflow, and jets, investigation into the nature of X-Ray scattering and reflection mediums, estimation of strong magnetic fields, and revelation of the radiation zone and particle acceleration processes in pulsars, among other applications. Each of these processes manifests its distinct signature within appropriate energy bands, depending on the involved energetics. The XPoSat mission by ISRO is specifically designed to investigate such X-Ray polarization signatures emanating from bright X-Ray sources.

The XPoSat Mission

The primary payload of XPoSat, POLIX (Polarimeter Instrument in X-rays), is designed to measure polarimetry parameters—specifically the degree and angle of polarization—in the medium X-ray energy range of 8-30 keV photons originating from astronomical sources. Complementing this, the XSPECT (X-ray Spectroscopy and Timing) payload will provide spectroscopic information within the energy range of 0.8-15 keV. The POLIX payload is developed by the Raman Research Institute (RRI), Bangalore, with support from the ISRO centres. The XSPECT payload is developed by the U R Rao Satellite Centre (URSC), ISRO.

The POLIX payload serves as an X-ray Polarimeter designed for astronomical observations within the energy band of 8-30 keV. The instrument comprises a collimator, a scatterer, and four X-ray proportional counter detectors surrounding the scatterer. The scatterer, constructed from low atomic mass material, induces anisotropic Thomson scattering of incoming polarized X-rays. The collimator plays a crucial role in restricting the field of view to 3 degrees by 3 degrees, ensuring that only one bright source is within the field of view for most observations. POLIX's primary objective is to observe bright astronomical sources across various categories during the planned 5-year lifetime of the XPoSat mission. Notably, POLIX stands out as the first payload in the medium X-ray energy band specifically dedicated to polarimetry measurements.

XSPECT is an X-ray Spectroscopy and Timing instrument, designed to offer fast timing and excellent spectroscopic resolution in soft X-rays (0.8-15 keV). XSPECT also monitors changes in line flux and profile, offering simultaneous, long-term temporal monitoring of soft X-ray emission. The instrument employs an array of Swept Charge Devices (SCDs) with an effective area exceeding 30 cm² at 6 keV and an impressive energy resolution of less than 200 eV at 6 keV. XSPECT employs passive collimators to reduce background by narrowing its field of view. This payload is anticipated to observe a variety of sources, including X-ray pulsars, black hole binaries, low-magnetic field neutron stars (NS), active galactic nuclei (AGNs), and magnetars.

Bringing Together the Indian Astronomy Community

On May 25, 2023, ISRO conducted a one-day XPoSat User Meet with the objective of gathering national experts to collaboratively explore the optimal utilization of XPoSat in advancing scientific understanding in Astronomy. The User Meet witnessed the involvement of approximately 20 institutes and universities from across the country. A total of around 150 participants engaged in the meeting, with nearly 100 attending in-person and others joining through the online platform. The event featured approximately 20 presentations, including contributions from the XPoSat project team and the broader scientific community nationwide. Scientific sessions delved into discussions on the significance of X-ray polarization measurements in Astronomy and the anticipated impact of XPoSat on various astronomical sources such as black holes, neutron stars X-ray binaries, isolated neutron stars, active galactic nuclei, ultra-luminous X-ray sources, among others. Additionally, discussions encompassed the modeling requirements necessary for interpreting XPoSat data. The meeting also provided an open forum for participants to contribute to discussions about the way forward, aiming to maximize the scientific outcomes derived from XPoSat.

Following the meeting, the Astronomy community of India expressed immense scientific prospects in the XPoSat mission. Eager to analyze XPoSat data, they emphasized the importance of engaging the student community. Furthermore, the community highlighted the significance of building expertise in X-Ray polarimetry in India, with the XPoSat mission serving as an appropriate starting point.

International Trend in Space-Based X-Ray Polarimetry

Internationally, space-based study of X-Ray polarization is gaining utmost importance. The Imaging X-ray Polarimetry Explorer (IXPE) mission, launched on Dec 09, 2021, represents NASA's inaugural space-based endeavor, focused on scrutinizing X-ray polarization across various celestial objects. IXPE is dedicated to unraveling the mysteries surrounding some of the universe's most extreme phenomena. These include the study of the remnants of supernova explosions, the particle streams emitted by feeding black holes, and other intriguing cosmic events.

In this context, the XPoSat mission would play a significant role. The XPoSat energy range of 8-30 keV for polarization measurement is complimentary to IXPE energy range of 2-8 keV. Therefore, XPoSat and IXPE spacecrafts will collectively probe different emission mechanisms and physics for bright X-ray sources. Thus, when IXPE and XPoSat will be operational in the same time-frame, their coordinated observation will provide a wide observation window in the energy range of 2-30 keV for polarimetric observations for bright X-ray sources.

The Physics Behind

At this point, let us have a close look at the basic physics of polarization of light, and how it works as a diagnostic tool in astronomy.

Polarization of light

Light is often described as an electromagnetic wave, which is a train of time-and-space-oscillating Electric and Magnetic fields that propagates in space in perfect synchronization of phase. The length of the light-wave-train (one may visualize them as arrows of light with finite length), which is also referred to as ‘coherence length’, is given by the product of the velocity of light and the time associated with its generation. Let us discuss more about a single light-wave-train.

If time is frozen, and somehow, the electric and magnetic field vectors are made visible, the time-frozen light-wave ‘train’ would have been visualized as a spatially-varying sinusoidal electric field in the x-direction (say), spatially-varying magnetic field in the y-direction, and the wave-propagation in the z-direction. It means that the x-direction would represent the direction of oscillation of the Electric field vector, and the y-direction would represent the direction of oscillation of the Magnetic field vector. The polarization of that light-wave-train is said to be along x-direction, as it is customary to refer the direction of the Electric field vector of a light-wave-train as its direction of polarization. Since in a given light-wave-train the electric field vectors are aligned in a definite plain (and the magnetic field vectors are at perpendicular to the plain of electric field vectors), it is completely (i.e. 100%) polarized. Thus, one of the parameters that describes polarization of light is the angle of the polarization (i.e. the electric field vector), with respect to a reference direction.

Polarization as a Diagnostic Tool

So far we have discussed about a single light-wave-train, where the angle of polarization is well-defined. In reality, a source of light emits numerous number of light-wave-trains, and there is, in general, no correlation between the angles of polarization of the individual light-wave-trains. Hence, if infinite number of such light-wave-trains, emitted from a source, is considered, statistically (integrating over time) the net polarization angle will tend to zero, and such a source of light is known as an unpolarized light source. Here comes another parameter that quantifies the degree of polarization, expressed in percentage. For a light source where the net polarization is zero, the degree of polarization is zero percent. Similarly, if a source of light has 70% degree of polarization, it means that, on an average, 70% of the light-wave-trains are vibrating in a specific direction, while the remaining 30% are oriented randomly. This alignment, or breaking of symmetry (0% polarization represents perfect directional symmetry) typically occurs due to interactions with matter or other physical processes that preferentially transmit light waves with a particular orientation.

Thus, if a light source, which emits numerous light-wave-trains, displays a significantly non-zero value of the net polarization (integrating over a finite time), it indicates that there must be one or more reasons that are statistically affecting the polarizations of the light-wave-trains thus causing an overall polarization bias. The reason behind that could be an anisotropic field, and/or any other factor that is capable of affecting the angle of polarization of the light-wave-trains.

The factors that contribute to the overall non-zero polarization is also associated with the origin of the light-waves. While there are several fundamental ways in which light is emitted from a source, one of them, as relevant to this discussion, is the light generated by the process of acceleration of charged particles. As a charge particle is made to accelerate, it emits trains of electromagnetic radiation (say, light-wave-trains). The acceleration may be due to any field, may it be electrical, magnetic, or even gravitational. In this context, let us consider a single charged particle. The instantaneous direction of the electric field vector due to a charged particle depends on its instantaneous velocity and acceleration. As time progresses, both the velocity and acceleration of the charge particle are changed, and thus the direction of the electric field vector associated with the emitted light-wave-train (i.e. the angle of polarization) is also altered. If the polarization of the light-wave-train emitted from a single charged particle is integrated over time, it displays an averaging effect. This is what happens with the light-wave-train generated by a single charged particle.

In reality, a highly energetic process involves numerous charged particles, which emits numerous light-wave-trains, where angles of polarization are independent of one another. In an isotropic configuration of field, the net polarization of the light-wave-trains tend to zero, while integrated over time. If found otherwise, which means non-zero value of the net polarization, it indicates an anisotropy in the geometry, velocity distribution of the charged particles, and/or the local conditions of the electric, magnetic or gravitational fields. Thus, study of the polarization of light plays a significant role of a diagnostic tool in Physics.

When we study the net polarization from a light source, we study its degree of polarization, and the angle of polarization.

Polarisation in Astronomy

In Astronomy, there are several processes that emit radiation. In general, we ask three questions on such radiations, viz. (i) Does the intensity of the radiation change with time, (ii) What are the frequencies contained in that radiation (spectroscopy), and (iii) Polarization (degree and angle of polarization). While the energy associated with an astronomical process that emits a radiation govern its frequency contents, the anisotropy in the local conditions of field and/or velocity of the charged particles govern the net polarization of the emitted radiation. Depending on the energetics involved in the astronomical process, the emission could be in visible, UV, X-Ray, or some other frequency band.

A few such examples of such sources include synchrotron radiation due to the gyration of charged particles in a magnetic field, scattering of light by electrons

In case of a group of charged particles gyrating about a magnetic field, the net polarization of the synchrotron radiation is orthogonal to the magnetic field. The degree of polarization of the synchrotron radiation thus emitted depends on the uniformity of the magnetic field. In the example of scattering, depending on the relative energy of the incident photon and the scatterer (electrons, say), there could be cases of Thomson scattering or Compton scattering. Thomson scattering occurs when the incident photons have much lower energy compared to the rest mass energy of the electrons, while Compton scattering, on the other hand, occurs at higher energies where the energy of the incident photons is comparable to or greater than the rest mass energy of the electrons. In either cases, the perturbation of the electrons by the incident photons causes the former to oscillate in a the direction of the electric field of the incident photon, which, in turn, causes emission of highly polarized radiation. In the context of Astronomy, this plays a vital role. As for example, the accretion disc in Black Hole X-Ray Binaries produces X-Rays, which are scattered and generate high degrees of polarization. The observed X-Ray polarization depends on the inclination of the disc, and the strong gravitational field bends the light, thereby rotating the plain of polarization. These phenomena help us to estimate the mass and spin of the black hole, as the spin of the black hole would distort the space-time continuum, affecting the plain of polarization of the X-Rays. In addition, Compton scattering occurs at the hot corona in the inner regions of the flow.

Magnetically confined jets in microquasars are also sources of high energy radiation with strong degree of polarization. The plain of polarization is governed by the jet axis / the local magnetic field. In addition to these, accreting, strongly polarized neutron stars, ultra-strong magnetic fields in magnetars, rotation powered pulsars can also produce strongly polarized X-Rays.

Epilogue

In the realm of astronomy, understanding the emission mechanisms from diverse sources is challenging due to the complex physical processes involved. While existing space-based observatories offer valuable spectroscopic and timing data, deciphering the exact nature of emissions from these sources remains a profound challenge for astronomers. The introduction of polarimetry measurements by India’s XPoSat mission, capturing the degree and angle of polarization, would add two crucial dimensions to our comprehension. XPoSat would serve as an space-based observation platform to study X-Ray polarization from bright cosmic sources, enhancing our ability to understand the intricate emission processes therefrom. The combination of polarimetric observations and spectroscopic measurements is anticipated to break the degeneracy inherent in various theoretical models of astronomical emission processes. This avenue of research is expected to be a primary focus for the Indian scientific community utilizing data from XPoSat. ISRO puts all efforts to synergize the wisdom of the X-Ray astronomers of the country to utilize the mission data, as well as to engage the student community to build their scientific career on space-based X-Ray Polarimetry.

XPoSat is anticipated to bring substantial benefits to the Astronomy community globally. Apart from its capability of timing and spectroscopy based observations, the insights derived from X-ray polarization measurements, especially on celestial objects like black holes, neutron stars, and active galactic nuclei, hold the potential to significantly improve our understanding of their physics. The extensive discussions on XPoSat mission at several forum over last few years have underscored the excitement within the community, showcasing a collective enthusiasm to analyze XPoSat data and engage the student community in these scientific pursuits. Additionally, the mission is poised to play a pivotal role in building expertise in X-Ray polarimetry in India, providing a foundation for future advancements and fostering a collaborative network within the Astronomy community.