XPoSat is a dedicated Indian polarimetry mission to study various dynamics of astronomical sources in extreme conditions. It works in medium energy band and long duration spectroscopic observation in soft energy X-ray band. The mission will help to understand the emission mechanism from a variety of X-ray sources.
The spacecraft will carry two scientific payloads in a low earth orbit with preference for a low inclination orbit. The primary payload POLIX (Polarimeter Instrument in X-rays) will measure the polarimetry parameters (degree and angle of polarization) of astronomical sources in medium X-ray energy of 8-30 keV photons. The XSPECT (X-ray Spectroscopy and Timing) payload will give spectroscopic information of soft X-rays in the energy range of 0.8-15 keV.
The spacecraft is planned to be launched in 2021 but due to Chinese Wuhan virus pandemic the launch may get delayed. XPOSAT will provide a service time of at least five years. The observatory will be placed in a circular low Earth orbit of 500 to 700 km.
Since the birth of X-ray astronomy in early 1960s, there has been tremendous improvement in the sensitivity of the X-ray astronomical observations. As a result, three of the four dimensions of X-ray astronomy viz. photometry, imaging and spectroscopy are very well developed and fully mature subjects. However, the fourth dimension, namely polarimetry, so far has been almost untouched observationally. There has been only one measurement of polarization in X-ray astronomy, which was carried out for Crab nebula about 30 years ago. The OSO-8 satellite carrying an X-ray polarimeter was launched by NASA in 1976 which measured ~19% polarization at 2.6 keV and 5.2 keV for the Crab nebula. The only other polarimeter ever launched onboard satellite Arial-5 and few earlier rocket and balloon-borne experiments did not result in successful measurement. The main reason for the lack of X-ray polarization measurements so far is their extremely photon hungry nature that severely limits the sensitivity of the instruments. This coupled with the limitations of the measurement techniques resulted in almost three decade void in X-ray polarization measurements.
Polarization is a very important property of radiation from astrophysical sources. It carries unique information regarding the emission mechanism, physical conditions as well as emission geometry at the origin. Polarization measurements in X-rays can provide unique opportunity to study the behavior of matter and radiation under extreme magnetic fields and extreme gravitational fields. Unfortunately, over past two decades, when X-ray astronomy witnessed multiple order of magnitude improvement in temporal, spatial and spectral sensitivities, there is no (or very little) progress in the field of polarization measurements of astrophysical X-rays.
Recently, a proposal has been submitted to Indian Space Research Organization (ISRO) for a dedicated small satellite based experiment to carry out X-ray polarization measurement, which aims to provide the first X-ray polarization measurements since 1976.
The X-ray polarization measurements are very important because they provide two independent parameters i.e. degree and angle of polarization to constrain the physical model for the X-ray source. Such measurements can provide unique opportunity to study the behavior of matter and radiation under extreme magnetic and gravitational fields. Prime targets for X-ray polarimetric observations are neutron stars (isolated neutron stars as well as neutron stars in binary systems) where the X-ray polarization measurements can provide direct information of the intensity and geometry of the magnetic field.
Polarimetric observations of accreting Galactic Black Hole systems are also very interesting because they provide unique opportunity to test some of the predictions of general relativity which are inaccessible by any other means. Another set of interesting targets for polarization observations are the cosmic acceleration sites such as supernovae remnants and jets in Active Galactic Nuclei (AGN) or micro-quasars. Polarization observations of these sites will provide direct information about the geometry of the shocked sites as well as the structure and intensity of magnetic fields therein.
The importance of X-ray polarimetric observations was realized for long time. There have been many reports on theoretical prediction of X-ray polarization from various types of X-ray sources even during early years of X-ray astronomy. Particularly in recent times, requirement of X-ray polarimetric observations has been strongly realized, mainly because, in various classes of X-ray sources, even the best spectroscopic or photometric observations cannot remove the degeneracy in the theoretical models. As a result, several groups worldwide have been attempting experiments which can provide the meaningful Xray polarimetric observations.
X-ray polarization measurements can give valuable insights about
The X-ray polarimeter (POLIX) is a scientific payload currently being built at Raman Research Institute to detect polarized X-rays from celestial sources. It is the only Thomson X-ray polarimeter currently being built worldwide and has an energy range of 5-30 keV. The instrument is designed, developed and tested at the Raman Research Institute
POLIX is poised to be the first dedicated X-ray polarimeter mission in the world and to open a new window in high energy astrophysics by measuring X-ray polarization in about 50 bright X-ray sources, ahead of the NASA and ESA space mission proposals for launching X-ray polarimeters. The primary payload POLIX (Polarimeter Instrument in X-rays) will measure the polarimetry parameters (degree and angle of polarization) of astronomical sources in the medium X-ray energy of 5-30 keV photons.
POLIX is a Thomson X-ray polarimeter for. The instrument consists of a collimator, a scatterer and a set proportional counters to detect the scattered X-rays. This instrument will provide unprecedented opportunity to measure X-ray polarisation in the medium energy range in a large number of sources of different classes with a minimum detectable linear polarisation degree of 2-3%. The prime objects for observation with this instrument are the X-ray bright accretion powered neutron stars, accreting black holes in different spectral states, rotation powered pulsars, magnetars, and active galactic nuclei. This instrument will be a bridge between the soft X-ray polarimeters and the Compton polarimeters.
Principle of operation
The instrument is based on anisotropic Thomson scattering of X-ray photons. X-rays from the source are made to undergo Thomson scattering and the intensity distribution of the scattered photons is measured as a function of azimuthal angle. Polarised X-rays will produce an azimuthal modulation in the count rate as opposed to uniform azimuthal distribution of count rate for unpolarised X-rays.
The mechanical configuration of the polarimeter consists of X-ray detectors, placed on all sides of a scattering element. X-rays from cosmic sources are allowed to fall on this scatterer through a collimator which restricts the field of view of the instrument. The total configuration will be rotated about the viewing axis.
In order to compensate for inaccuracy in satellite pointing and to attain constant effective area, a collimator with a flat topped response is required. To minimize the effect of photoelectric absorption of photons in the scattering element, a low atomic mass scatterer (lithium/beryllium) is preferred.
POLIX consists of four independent detectors, each with its own front end and processing electronics. Localization of the X-ray photon in the detectors is carried out by the method of charge division in a set of resistive anode wires connected in series. The analog electronics of POLIX include high voltage generation and control for operation of the detectors, amplification of the charge pulses, threshold comparisons and digitization of the housekeeping parameters.
Electronics Engineering Group of RRI has carried out the design and development of both the analog and digital electronics systems for this instrument, meeting stringent requirements of a typical space mission. EEG has developed in-house a complete electronics system for (i) detector operation, (ii) pulse processing and digitization, (iii) on-board data handling, (iv) housekeeping, and (v) control
The digital electronics consists of anti-coincidence logic, digitation of the pulse amplitudes, data generation in multiple modes and combining of data from the four detectors.RRI- EEG has successfully developed a digital signal processing system for POLIX which is more complex than schemes used in other space experiments based on proportional counters. The telecommand and telemetry interface of the POLIX payload with the satellite bus is developed jointly with the Space Astronomy Group of ISRO.
The POLIX detectors are designed and fabricated in collaboration with the MES and are wired and assembled in EEG. Each detector has about 400 wires of 25 and 50 micron diameters precisely wired and soldered onto a frame. EEG has developed expertise over several years, of making it reliable so that the large number of wires sustains satellite launch vibration and thermal cycling in space for several years
Rationale for choosing polarimeter design based on Thomson scattering
There are three basic techniques for measuring linear polarization of the X-ray photons, namely
Bragg reflection, Thomson scattering and photo-electron imaging. Bragg reflection is one of the oldest and clean techniques to measure polarization of X-rays. Both the X-ray polarimeters flown to space so far (onboard Arial-5 and OSO-8) were of this type. However, the major drawback of this technique is that it works only at discrete energies. This results in very low sensitivity. Photo-electron tracking is the latest technique for measuring polarization of X-rays. Very high resolution position sensitive X-ray detector required to image the photo-electron tracks (a few hundred microns in a gaseous medium) are becoming available only recently and have not been used so far in any space experiment. The main disadvantage of this technique, particularly in the context of the present ISRO announcement of opportunity is that they have very small collecting area. Therefore these detectors must be used with the X-ray focusing optics. This combination might provide the best sensitivity for X-ray polarization measurement. However, it requires a full fledge X-ray astronomy mission which is out of scope for the present opportunity. Therefore, adopted a pragmatic approach to limit the sensitivity goal and use the well established technique of X-ray polarization measurement based on Thomson scattering.
Though the most sensitive polarisation measurement devices are based on photoelectron track imaging, they require to be coupled with high throughput X-ray mirrors due to a small detector size. They also cover a softer energy range. The proposed scattering experiment is based on the well established technique of X-ray polarisation measurement using Thomson scattering which has moderate sensitivity over a relatively large bandwidth suited in the energy band of our interest. The experiment conﬁguration consists of a central low Z (Lithium, Lithium Hydride or Beryllium) scatterer surrounded by xenon ﬁlled X-ray proportional counters as X-ray detectors which collects the scattered X-ray photons. The instrument is rotated along the viewing axis leading to the measurement of the the azimuthal distribution of the scattered X-ray photons which gives information on polarisation. The sensitivity of this experiment is dependent on a) collecting area b) scattering and detection eﬃciency c) detector background and d) modulation factor of the instrument.
Progress of POLIX during the years
During 2018-19, significant progress has been made in making the Qualification Model of POLIX and fabrication of some of the Flight Model components of POLIX has been initiated. The MOU between RRI and ISRO for POLIX was revised and second phase of funding for POLIX was released to initiate the Flight Model of POLIX.
During 2019-20, significant progress has been made towards completion of the Qualification Model (QM) and Flight Model (FM) of POLIX.
XSPECT (X-ray Spectroscopy and Timing)
The XSPECT (X-ray Spectroscopy and Timing) payload will give spectroscopic information of soft X-rays. X-ray Polarimeter Satellite mission carrying Polarimetry in X-rays (POLIX) and X-ray Spectroscopy and Timing (XSPECT) experiments with their viewing angles aligned with each other. In the aftermath of India's first multi-wavelength astronomy mission, a small payload (XSPECT) is proposed to address complementary timing and spectroscopy at low-energy X-rays.
XSPECT provide unique opportunity to observe astrophysical sources for long durations to study their spectral and temporal variability in 0.8 to 15 keV X-ray band. The energy resolution of <200 eV @ 5.9keV and at -20°C and timing resolution of ~ 2 msec is planned.
The proposed detector achieves modest effective area without the use of focusing optics using the new large area Swept Charge Devices (SCDs; CCD-236) which are a variant of X-ray CCDs. SCDs permit fast readouts (10-100 kHz) and moderately good spectral resolution at the cost of a position sensitivity. These devices are unique in requiring very benign cooling requirement (uses only passive cooling) unlike traditional X-ray CCDs. XSPECT with a passive collimator of 1 deg × 1 deg field-of-view, is ideally suited to pursue soft x-ray timing studies, complementary to what LAXPC does at high energies on ASTROSAT while simultaneously providing good resolution spectrum in the 1-20 keV band. Key science objectives include understanding long-term behavior of x-ray sources through correlation of timing characteristics with spectral state changes and emission line variations. Alongside x-ray polarimeter, this can provide a near-complete system to address photon energy, timing and polarization simultaneously.
The instrument is under development and expected to be delivered in 2021.