S. V. Vadawale

Hard X-ray polarimetry with Astrosat-CZTI

X-ray polarimetry is largely an unexplored area of an otherwise mature field of X-ray astronomy. Except for a few early attempts during the 1970s, no dedicated X-ray polarimeter has been flown during the past four decades. On the other hand, the scientific value of X-ray polarization measurement has been well known for a long time, and there has been significant technical progress in developing sensitive X-ray polarimeters in recent years. But there are no approved dedicated X-ray polarimetric experiments to be flown in the near future, so it is important to explore the polarimetric capabilities of other existing or planned instruments and examine whether they can provide significant astrophysical polarization measurements. In this paper, we present experimental results to show that the CZTI instrument onboard the forthcoming Indian astronomy mission, Astrosat, will be able to provide sensitive measurements of X-ray polarization in the energy range of 100 300 keV. CZTI will be able to constrain any intrinsic polarization greater than 40% for bright X-ray sources (>500 mCrab) within a short exposure of 100 ks with a 3-sigma confidence level. We show that this seemingly “modest” sensitivity can play a very significant role in addressing long pending questions, such as the contribution of relativistic jets to hard X-rays in black hole binaries and X-ray emission mechanism and geometry in X-ray pulsars.

ASTROSAT CZT IMAGER OBSERVATIONS OF GRB 151006A: TIMING, SPECTROSCOPY, AND POLARIZATION STUDY

AstroSat is a multi-wavelength satellite launched on 2015 September 28. The CZT Imager of AstroSat on its very first day of operation detected a long duration gamma-ray burst (GRB) namely GRB 151006A. Using the off-axis imaging and spectral response of the instrument, we demonstrate that CZT Imager can localise this GRB correct to about a few degrees and it can provide, in conjunction with Swift, spectral parameters similar to that obtained from Fermi/GBM. Hence CZT Imager would be a useful addition to the currently operating GRB instruments (Swift and Fermi). Specifically, we argue that the CZT Imager will be most useful for the short hard GRBs by providing localisation for those detected by Fermi and spectral information for those detected only by Swift. We also provide preliminary results on a new exciting capability of this instrument: CZT Imager is able to identify Compton scattered events thereby providing polarisation information for bright GRBs. GRB 151006A, in spite of being relatively faint, shows hints of a polarisation signal at 100-300 keV (though at a low significance level). We point out that CZT Imager should provide significant time resolved polarisation measurements for GRBs that have fluence 3 times higher than that of GRB 151006A. We estimate that the number of such bright GRBs detectable by CZT Imager is 5 - 6 per year. CZT Imager can also act as a good hard X-ray monitoring device for possible electromagnetic counterparts of Gravitational Wave events.

A Tale of Two Transients: GW 170104 and GRB 170105A

We present multi-wavelength follow-up campaigns by the AstroSat CZTI and GROWTH collaborations in search of an electromagnetic counterpart to the gravitational wave event GW 170104. At the time of the GW 170104 trigger, the AstroSat CZTI field of view covered 50.3% of the sky localization. We do not detect any hard X-ray (>100 keV) signal at this time, and place an upper limit of ≈4.5 x 10^(-7) erg cm^(-2) s^(-1), for a 1 s timescale. Separately, the ATLAS survey reported a rapidly fading optical source dubbed ATLAS17aeu in the error circle of GW 170104. Our panchromatic investigation of ATLAS17aeu shows that it is the afterglow of an unrelated long, soft GRB 170105A, with only a fortuitous spatial coincidence with GW 170104. We then discuss the properties of this transient in the context of standard long GRB afterglow models.

Surprise in simplicity: an unusual spectral evolution of a single pulse GRB 151006A

We present a detailed analysis of GRB 151006A, the first gamma-ray burst (GRB) detected by AstroSat Cadmium-Zinc-Telluride Imager (CZTI). We study the long-term spectral evolution by exploiting the ...

Measurement of Low Energy Detection Efficiency of a Plastic Scintillator: Implications on the Lower Energy Limit and Sensitivity of a Hard X-Ray Focal Plane Compton Polarimeter

The polarization measurements in X-rays offer a unique opportunity for the study of physical processes under the extreme conditions prevalent at compact X-ray sources, including gravitation, magnetic field, and temperature. Unfortunately, there has been no real progress in observational X-ray polarimetry thus far. Although photoelectron tracking-based X-ray polarimeters provide realistic prospects of polarimetric observations, they are effective in the soft X-rays only. With the advent of hard X-ray optics, it has become possible to design sensitive X-ray polarimeters in hard X-rays based on Compton scattering. An important point that should be carefully considered for the Compton polarimeters is the lower energy threshold of the active scatterer, which typically consists of a plastic scintillator due to its lowest effective atomic number. Therefore, an accurate understanding of the plastic scintillators energy threshold is essential to make a realistic estimate of the energy range and sensitivity of any Compton polarimeter. In this context, we set up an experiment to investigate the plastic scintillators behavior for very low energy deposition events. The experiment involves the detection of Compton scattered photons from a long, thin, plastic scintillator (a similar configuration as the eventual Compton polarimeter) by a high resolution CdTe detector at different scattering angles. We find that it is possible to detect energy deposition well below 1 keV, though with decreasing efficiency. We present detailed semianalytical modeling of our experimental setup and discuss the results in the context of the energy range and sensitivity of the Compton polarimeter involving plastic scintillators.

A conceptual design of hard X-ray focal plane detector for simultaneous x-ray polarimetric, spectroscopic, and timing measurements

Importance of polarisation measurement of X-rays from celestial sources has been realized for long time. Such measurements can provide unique opportunity to study the behaviour of matter and radiation under extreme magnetic and gravitational fields. However sensitivity of the X-ray polarimeters has always been an issue and as a result no X-ray polarization measurement has been flown in last three decades. The situation is expected to change in near future with launch of GEMS, but these polarisation measurements will be limited to energies below 10KeV. On the other hand most of the X-ray sources are expected to have higher degree of polarisation at higher energies. With the advent of high energy focussing telescopes (e.g. NuSTAR, ASTRO-H), it is now possible to design a focal plane Compton polarimeter which can be sensitive upto 80KeV. However, X-ray polarisation measurement is extremely photon hungry. Therefore, a dedicated X-ray polarimeter always has lower sensitivity when compared to any other type of X-ray detector for equal collecting area and time. In this context, we explore a new design of hard X-ray focal plane detector which can provide simultaneous measurements of X-ray polarisation measurements along with high resolution X-ray spectroscopy as well as timing. This design employs a sandwich of a 0.5mm thick Si detector and 10mm thick plastic detector which is surrounded by a cylindrical array of scintillator detectors. Here we present results of detailed Geant4 simulations for estimating sensitivity of this configuration.

In-orbit performance AstroSat CZTI

Cadmium-Zinc-Telluride Imager (CZTI) is one of the five payloads on-board recently launched Indian astronomy satellite AstroSat. CZTI is primarily designed for simultaneous hard X-ray imaging and spectroscopy of celestial X-ray sources. It employs the technique of coded mask imaging for measuring spectra in the energy range of 20 - 150 keV. It was the first scientific payload of AstroSat to be switched on after one week of the launch and was made operational during the subsequent week. Here we present preliminary results from the performance verification phase observations and discuss the in-orbit performance of CZTI.

Line profile modelling for multi-pixel CZT detectors

Cadmium Zinc Telluride (CZT) detectors have been the mainstay for hard X-ray astronomy for its high quantum efficiency, fine energy resolution, near room temperature operation, and radiation hardness. In order to fully utilize the spectroscopic capabilities of CZT detectors, it is important to generate accurate response matrix, which in turn requires precise modelling of the line profiles for the CZT detectors. We have developed a numerical model taking into account the mobility and lifetime of the charge carriers and intrpixel charge sharing for the CZT detectors. This paper describes the details of the modelling along with the experimental measurements of mobility, lifetime and charge sharing fractions for the CZT detector modules of thickness of 5 mm and 2.5 mm pixel size procured from Orbotech Medical Solutions (same modules used in AstroSat-CZTI).

Development of a hard x-ray focal plane compton polarimeter: a compact polarimetric configuration with scintillators and Si photomultipliers

X-ray polarization measurement of cosmic sources provides two unique parameters namely degree and angle of polarization which can probe the emission mechanism and geometry at close vicinity of the compact objects. Specifically, the hard X-ray polarimetry is more rewarding because the sources are expected to be intrinsically highly polarized at higher energies. With the successful implementation of Hard X-ray optics in NuSTAR, it is now feasible to conceive Compton polarimeters as focal plane detectors. Such a configuration is likely to provide sensitive polarization measurements in hard X-rays with a broad energy band. We are developing a focal plane hard X-ray Compton polarimeter consisting of a plastic scintillator as active scatterer surrounded by a cylindrical array of CsI(Tl) scintillators. The scatterer is 5 mm diameter and 100 mm long plastic scintillator (BC404) viewed by normal PMT. The photons scattered by the plastic scatterer are collected by a cylindrical array of 16 CsI(Tl) scintillators (5 mm × 5 mm × 150 mm) which are read by Si Photomultiplier (SiPM). Use of the new generation SiPMs ensures the compactness of the instrument which is essential for the design of focal plane detectors. The expected sensitivity of such polarimetric configuration and complete characterization of the plastic scatterer, specially at lower energies have been discussed in [11, 13]. In this paper, we characterize the CsI(Tl) absorbers coupled to SiPM. We also present the experimental results from the fully assembled configuration of the Compton polarimeter.

A new technique for measuring the leakage current in Silicon Drift Detector based X-ray spectrometer—implications for on-board calibration

In this work, we report a new technique of measuring the leakage current in Silicon Drift Detectors (SDD) and propose to use this technique as a tool for on-board estimation of the radiation damage to the SDD employed in space-borne X-ray spectrometers. The leakage current of a silicon based detector varies with the detector operating temperature and increases with the radiation dose encountered by the detector in the space environment. The proposed technique to measure detector leakage current involves measurement of the reset frequency of the reset type charge sensitive pre-amplifier when the feedback capacitor is charged only due to the detector leakage current. Using this technique, the leakage current is measured for large samples of SDDs having two different active areas of 40 mm2 and 109 mm2 with 450 micron thick silicon. These measurements are carried out in the temperature range of -50°C to 20°C. At each step energy resolution is measured for all SDDs using Fe-55 X-ray source and shown that the energy resolution varies systematically with the leakage current irrespective of the difference among the detectors of the same as well as different sizes. Thus by measuring the leakage current on-board, it would be possible to estimate the time dependent performance degradation of the SDD based X-ray spectrometer. This can be particularly useful in case where large numbers of SDD are used.