arXiv subjects

Tanmoy Chattopadhyay

Publications and source records attributed to Tanmoy Chattopadhyay.

49 records · Page 3Linked to original sources

arXiv2021

Exploring Sub-MeV Sensitivity of AstroSat-CZTI for ON-axis Bright Sources

The Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat is designed for hard X-ray imaging and spectroscopy in the energy range of 20 - 100 keV. The CZT detectors are of 5 mm thickness and hence have good efficiency for Compton interactions beyond 100 keV. The polarisation analysis using CZTI relies on such Compton events and have been verified experimentally. The same Compton events can also be used to extend the spectroscopy up to 380 keV. Further, it has been observed that about 20% pixels of the CZTI detector plane have low gain, and they are excluded from the primary spectroscopy. If these pixels are included, then the spectroscopic capability of CZTI can be extended up to 500 keV and further up to 700 keV with a better gain calibration in the future. Here we explore the possibility of using the Compton events as well as the low gain pixels to extend the spectroscopic energy range of CZTI for ON-axis bright X-ray sources. We demonstrate this technique using Crab observations and explore its sensitivity.

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arXiv2021

Sub-MeV spectroscopy with AstroSat-CZT Imager for Gamma Ray Bursts

Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100 - 300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels (around 20% of the detector plane) after careful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature also allows single-pixel spectroscopy of the GRBs to the sub-MeV range which is otherwise limited to 150 keV. We also introduced a new noise rejection algorithm in the analysis ('Compton noise'). These new additions not only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by Fermi. This article describes the methodology of single, Compton event and veto spectroscopy in 100 - 600 keV for the GRBs detected in the first year of operation. CZTI in last five years has detected around 20 bright GRBs. The new methodologies, when applied on the spectral analysis for this large sample of GRBs, has the potential to improve the results significantly and help in better understanding the prompt emission mechanism.

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arXiv2020

Tiny-box: A tool for the versatile development and characterization of low noise fast X-ray imaging detectors

X-ray Charge Coupled Devices (CCDs) have been the workhorse for soft X-ray astronomical instruments for the past quarter century. They provide broad energy response, extremely low electronic read noise, and good energy resolution in soft X-rays. These properties, along with the large arrays and small pixel sizes available with modern-day CCDs, make them a potential candidate for next generation astronomical X-ray missions equipped with large collecting areas, high angular resolutions and wide fields of view, enabling observation of the faint, diffuse and high redshift X-ray universe. However, such high collecting area (about 30 times Chandra) requires these detectors to have an order of magnitude faster readout than current CCDs to avoid saturation and pile up effects. In this context, Stanford University and MIT have initiated the development of fast readout X-ray cameras. As a tool for this development, we have designed a fast readout, low noise electronics board (intended to work at a 5 Megapixel per second data rate) coupled with an STA Archon controller to readout a 512 x 512 CCD (from MIT Lincoln Laboratory). This versatile setup allows us to study a number of parameters and operation conditions including the option for digital shaping. In this paper, we describe the characterization test stand, the concept and development of the readout electronics, and simulation results. We also report the first measurements of read noise, energy resolution and other parameters from this set up. While this is very much a prototype, we plan to use larger, multi-node CCD devices in the future with dedicated ASIC readout systems to enable faster, parallel readout of the CCDs.

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arXiv2020

Spectro-polarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks

GRB 160325A is the only bright burst detected by AstroSat CZT Imager in its primary field of view to date. In this work, we present the spectral and polarimetric analysis of the prompt emission of the burst using AstroSat, Fermi and Niel Gehrels Swift observations. The prompt emission consists of two distinct emission episodes separated by a few seconds of quiescent/ mild activity period. The first emission episode shows a thermal component as well as a low polarisation fraction of $PF < 37\, \%$ at $1.5\, σ$ confidence level. On the other hand, the second emission episode shows a non-thermal spectrum and is found to be highly polarised with $PF > 43\, \%$ at $1.5 σ$ confidence level. We also study the afterglow properties of the jet using {\it Swift}/XRT data. The observed jet break suggests that the jet is pointed towards the observer and has an opening angle of $1.2^{\circ}$ for an assumed redshift, $z = 2$. With composite modelling of polarisation, spectrum of the prompt emission and the afterglow, we infer that the first episode of emission originates from the photosphere with localised dissipation happening below it, and the second from the optically thin region above the photosphere. The photospheric emission is generated mainly by inverse Compton scattering, whereas the emission in the optically thin region is produced by the synchrotron process. The low radiation efficiency of the burst suggests that the outflow remains baryonic dominated throughout the burst duration with only a subdominant Poynting flux component, and the kinetic energy of the jet is likely dissipated via internal shocks which evolves from an optically thick to optically thin environment within the jet.

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arXiv2019

Prompt emission polarimetry of Gamma Ray Bursts with ASTROSAT CZT-Imager

X-ray and Gamma-ray polarization measurements of the prompt emission of Gamma-ray bursts (GRBs) are believed to be extremely important for testing various models of GRBs. So far, the available measurements of hard X-ray polarization of GRB prompt emission have not significantly constrained the GRB models, particularly because of the difficulty of measuring polarization in these bands. The CZT Imager (CZTI) onboard {\em AstroSat} is primarily an X-ray spectroscopic instrument that also works as a wide angle GRB monitor due to the transparency of its support structure above 100 keV. It also has experimentally verified polarization measurement capability in the 100 $-$ 300 keV energy range and thus provides a unique opportunity to attempt spectro-polarimetric studies of GRBs. Here we present the polarization data for the brightest 11 GRBs detected by CZTI during its first year of operation. Among these, 5 GRBs show polarization signatures with $\gtrapprox$3$σ$, and 1 GRB shows $\>$2$σ$ detection significance. We place upper limits for the remaining 5 GRBs. We provide details of the various tests performed to validate our polarization measurements. While it is difficult yet to discriminate between various emission models with the current sample alone, the large number of polarization measurements CZTI expects to gather in its minimum lifetime of five years should help to significantly improve our understanding of the prompt emission.

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arXiv2019

Time varying polarized gamma-rays from GRB 160821A: evidence for ordered magnetic fields

GRB 160821A is the third most energetic gamma ray burst observed by the {\it Fermi} gamma-ray space telescope. Based on the observations made by Cadmium Zinc Telluride Imager (CZTI) on board {\it AstroSat}, here we report the most conclusive evidence to date of (i) high linear polarization ($66^{+26}_{-27} \%$; $5.3 σ$ detection), and (ii) variation of its polarization angle with time happening twice during the rise and decay phase of the burst at $3.5 σ$ and $3.1 σ$ detections respectively. All confidence levels are reported for two parameters of interest. These observations strongly suggest synchrotron radiation produced in magnetic field lines which are highly ordered on angular scales of $1/Γ$, where $Γ$ is the Lorentz factor of the outflow.

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arXiv2019

Magnetars as Astrophysical Laboratories of Extreme Quantum Electrodynamics: The Case for a Compton Telescope

A next generation of Compton and pair telescopes that improve MeV-band detection sensitivity by more than a decade beyond current instrumental capabilities will open up new insights into a variety of astrophysical source classes. Among these are magnetars, the most highly magnetic of the neutron star zoo, which will serve as a prime science target for a new mission surveying the MeV window. This paper outlines the core questions pertaining to magnetars that can be addressed by such a technology. These range from global magnetar geometry and population trends, to incisive probes of hard X-ray emission locales, to providing cosmic laboratories for spectral and polarimetric testing of exotic predictions of QED, principally the prediction of the splitting of photons and magnetic pair creation. Such fundamental physics cannot yet be discerned in terrestrial experiments. State of the art modeling of the persistent hard X-ray tail emission in magnetars is presented to outline the case for powerful diagnostics using Compton polarimeters. The case highlights an inter-disciplinary opportunity to seed discovery at the interface between astronomy and physics.

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arXiv2019

AstroSat-CZTI detection of variable prompt emission polarization in GRB 171010A

We present spectro-polarimetric analysis of \thisgrb\ using data from \asat, \fermi, and \swift, to provide insights into the physical mechanisms of the prompt radiation and the jet geometry. Prompt emission from \thisgrb\ was very bright (fluence $>10^{-4}$~ergs~cm$^{-2}$) and had a complex structure composed of the superimposition of several pulses. The energy spectra deviate from the typical Band function to show a low energy peak $\sim 15$~keV --- which we interpret as a power-law with two breaks, with a synchrotron origin. Alternately, the prompt spectra can also be interpreted as Comptonized emission, or a blackbody combined with a Band function. Time-resolved analysis confirms the presence of the low energy component, while the peak energy is found to be confined in the range of 100--200~keV. Afterglow emission detected by \fermi-LAT is typical of an external shock model, and we constrain the initial Lorentz factor using the peak time of the emission. \swift-XRT measurements of the afterglow show an indication for a jet break, allowing us to constrain the jet opening angle to $>$ 6$\degr$. Detection of a large number of Compton scattered events by \asat-CZTI provides an opportunity to study hard X-ray polarization of the prompt emission. We find that the burst has high, time-variable polarization, with the emission {\bf have higher polarization} at energies above the peak energy. We discuss all observations in the context of GRB models and polarization arising due to {\bf due to physical or geometric effects:} synchrotron emission from multiple shocks with ordered or random magnetic fields, Poynting flux dominated jet undergoing abrupt magnetic dissipation, sub-photospheric dissipation, a jet consisting of fragmented fireballs, and the Comptonization model.

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arXiv2019

X-ray Hybrid CMOS Detectors: Recent Development and Characterization Progress

X-ray Hybrid CMOS Detectors (HCDs) have advantages over X-ray CCDs due to their higher readout rate abilities, flexible readout, inherent radiation hardness, and low power, which make them more suitable for the next generation large area X-ray telescope missions. The Penn State high energy astronomy laboratory has been working on the development and characterization of HCDs in collaboration with Teledyne Imaging Sensors (TIS). We characterized an H2RG detector with a Cryo-SIDECAR readout and controller, and we find an improved energy resolution of ~2.7 % at 5.9 keV and read noise of ~6.5 e-. This detector was successfully flown on NASA's first water recovery sounding rocket flight on April 4th, 2018. We have also been developing several new HCDs with potential applications for future X-ray astronomy missions. We are characterizing the performance of small-pixel HCDs (12.5 μm pitch), which are important for the development of a next-generation high-resolution imager with HCDs. We also characterized a 64 x 64 pixel prototype Speedster-EXD detector that uses comparators in each pixel to read out only those pixels having detectable signal, thereby providing an order of magnitude improvement in the effective readout rate. HCDs can also be utilized as a large FOV instrument to study the prompt and afterglow emissions of GRBs and detect black hole transients. In this context, we are characterizing a Lobster-HCD system for future CubeSat experiments. This paper briefly presents these new developments and experimental results.

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arXiv2018

Recent X-ray hybrid CMOS detector developments and measurements

The Penn State X-ray detector lab, in collaboration with Teledyne Imaging Sensors (TIS), have progressed their efforts to improve soft X-ray Hybrid CMOS detector (HCD) technology on multiple fronts. Having newly acquired a Teledyne cryogenic SIDECAR ASIC for use with HxRG devices, measurements were performed with an H2RG HCD and the cooled SIDECAR. We report new energy resolution and read noise measurements, which show a significant improvement over room temperature SIDECAR operation. Further, in order to meet the demands of future high-throughput and high spatial resolution X-ray observatories, detectors with fast readout and small pixel sizes are being developed. We report on characteristics of new X-ray HCDs with 12.5 micron pitch that include in-pixel CDS circuitry and crosstalk-eliminating CTIA amplifiers. In addition, PSU and TIS are developing a new large-scale array Speedster-EXD device. The original 64 x 64 pixel Speedster-EXD prototype used comparators in each pixel to enable event driven readout with order of magnitude higher effective readout rates, which will now be implemented in a 550 x 550 pixel device. Finally, the detector lab is involved in a sounding rocket mission that is slated to fly in 2018 with an off-plane reflection grating array and an H2RG X-ray HCD. We report on the planned detector configuration for this mission, which will increase the NASA technology readiness level of X-ray HCDs to TRL 9.

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arXiv2018

BlackCAT CubeSat: A Soft X-ray Sky Monitor, Transient Finder, and Burst Detector for High-energy and Multimessenger Astrophysics

Here we present the conceptual design of a wide field imager onboard a 6U class CubeSat platform for the study of GRB prompt and afterglow emission and detection of electromagnetic counterparts of gravitational waves in soft X-rays. The planned instrument configuration consists of an array of X-ray Hybrid CMOS detectors (HCD), chosen for their soft-X-ray response, flexible and rapid readout rate, and low power, which makes these detectors well suited for detecting bright transients on a CubeSat platform. The wide field imager is realized by a 2D coded mask. We will give an overview of the instrument design and the scientific requirements of the proposed mission.

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arXiv2018

Violation of synchrotron line of death by the highly polarized $GRB~160802A$

$GRB~160802A$ is one of the brightest gamma-ray bursts (GRBs) observed with $Fermi$ Gamma-ray Burst Monitor (GBM) in the energy range of $10~-~1000$ keV, while at the same time it is surprisingly faint at energies $\gtrsim2$ MeV. An observation with $AstroSat$/CZT Imager (CZTI) also provides the polarisation which helps in constraining different prompt emission models using the novel joint spectra-polarimetric data. We analyze the $Fermi$/GBM data, and find two main bursting episodes that are clearly separated in time, one particularly faint in higher energies and having certain differences in their spectra. The spectrum in general shows a hard-to-soft evolution in both the episodes. Only the later part of the first episode shows intensity tracking behaviour corresponding to multiple pulses. The photon index of the spectrum is hard, and in over 90 per cent cases, cross even the slow cooling limit ($α=-2/3$) of an optically thin synchrotron shock model (SSM). Though such hard values are generally associated with a sub-dominant thermal emission, such a component is not statistically required in our analysis. In addition, the measured polarisation in 100--300\,keV is too high, $π=85\pm29\%$, to be accommodated in such a scenario. Jitter radiation, which allows a much harder index up to $α=+0.5$, in principle can produce high polarisation but only beyond the spectral peak, which in our case lies close to $200~-~300$ keV during the time when most of the polarisation signal is obtained. The spectro-polarimetric data seems to be consistent with a subphotospheric dissipation process occurring within a narrow jet with a sharp drop in emissivity beyond the jet edge, and viewed along its boundary.

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arXiv2016

In-orbit performance of 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.

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