VARUN BHALERAO
Articles written in Journal of Astrophysics and Astronomy
Volume 42 All articles Published: 1 July 2021 Article ID 0064 SCIENCE RESULTS
Using collimated CZTI as all-sky X-ray detector based on Earth occultation technique
AKSHAT SINGHAL RAHUL SRINIVASAN VARUN BHALERAO DIPANKAR BHATTACHARYA A. R. RAO SANTOSH VADAWALE
All-sky monitors can measure the fluxes of astrophysical sources by measuring the changes in observed counts as the source is occulted by the Earth. Such measurements have typically been carried out by all-sky monitors like
Volume 42 All articles Published: 3 July 2021 Article ID 0067 SCIENCE RESULTS
Exploring sub-MeV sensitivity of AstroSat–CZTI for ON-axis bright sources
ABHAY KUMAR TANMOY CHATTOPADHYAY SANTOSH V. VADAWALE A. R. RAO SOUMYA GUPTA N. P. S. MITHUN VARUN BHALERAO DIPANKAR BHATTACHARYA
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 CZTIrelies 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.
Volume 42 All articles Published: 3 July 2021 Article ID 0069 SCIENCE RESULTS
SOURAV PALIT AKASH ANUMARLAPUDI VARUN BHALERAO
A considerable fraction of incident high energy photons from astrophysical transients such as Gamma Ray Bursts (GRBs) is Compton scattered by the Earth’s atmosphere. These photons, sometimes referred to as the ‘‘reflection component’’, contribute to the signal detected by space-borne X-ray/c-rayinstruments. The effectiveness and reliability of source parameters such as position, flux, spectra and polarization, inferred by these instruments are therefore highly dependent on the accurate estimation of this scattered component. Current missions use dedicated response matrices to account for these effects. However, these databases are not readily adaptable for other missions, including many upcoming transient search and gravitational wave high-energy electromagnetic counter part detectors. Furthermore, possible systematiceffects in these complex simulations have not been thoroughly examined and verified in literature. We are in the process of investigation of the effect with a detailed Monte Carlo simulations in GEANT4 for a Low Earth Orbit (LEO) X-ray detector. Here, we discuss the outcome of our simulation in form of AtmosphericResponse Matrix (ARM) and its implications of any systematic errors in the determination of source spectral characteristics. We intend to apply our results in data processing and analysis for AstroSat-CZTI observation of such sources in near future. Our simulation output and source codes will be made publicly available for use by the large number of upcoming high energy transient missions, as well as for scrutiny and systematic comparisons with other missions.
Volume 42 All articles Published: 21 July 2021 Article ID 0082 SCIENCE RESULTS
Sub-MeV spectroscopy with AstroSat-CZT imager for gamma ray bursts
TANMOY CHATTOPADHYAY SOUMYA GUPTA VIDUSHI SHARMA SHABNAM IYYANI AJAY RATHEESH N. P. S. MITHUN E. AARTHY SOURAV PALIT ABHAY KUMAR SANTOSH V. VADAWALE A. R. RAO VARUN BHALERAO DIPANKAR BHATTACHARYA
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 ($\sim$20% of the detector plane) aftercareful 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 additionsnot only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by
Volume 42 All articles Published: 2 August 2021 Article ID 0093 SCIENCE RESULTS
The AstroSat mass model: Imaging and flux studies of off-axis sources with CZTI
SUJAY MATE TANMOY CHATTOPADHYAY VARUN BHALERAO E. AARTHY ARVIND BALASUBRAMANIAN DIPANKAR BHATTACHARYA SOUMYA GUPTA KRISHNAN KUTTY N. P. S. MITHUN SOURAV PALIT A. R. RAO DIVITA SARAOGI SANTOSH VADAWALE AJAY VIBHUTE
The Cadmium Zinc Telluride Imager (CZTI) on AstroSat is a hard X-ray coded-aperture mask instrument with a primary field-of-view of $4.6^{\circ} \times 4.6^{\circ}$ (FWHM).The instrument collimators become increasinglytransparent at energies above $\sim$100 keV, making CZTI sensitive to radiation from the entire sky. While this has enabled CZTI to detect a large number of off-axis transient sources, calculating the source flux or spectrum requires knowledge of the direction and energy dependent attenuation of the radiation incident upon the detector. Here, we present a GEANT4-based mass model of CZTI and AstroSat that can be used to simulate the satellite response to the incident radiation, and to calculate an effective ‘‘response file’’ for converting the source counts into fluxes and spectra. We provide details of the geometry and interaction physics, and validate the model by comparing the simulations of imaging and flux studies with observations. Spectroscopic validation of the massmodel is discussed in a companion paper, Chattopadhyay
Volume 43 All articles Published: 24 August 2022 Article ID 0053 TRANSIENTS
Gravitational waves and electromagnetic transients
AKSHAT SINGHAL SOURAV PALIT SUMAN BALA GAURAV WARATKAR HARSH KUMAR VARUN BHALERAO
The advanced gravitational wave (GW) detector network has started routine detection of signals from merging compact binaries. Data indicate that in a fair fraction of these sources, at least one component was a neutron star, bringing with it the possibility of electromagnetic (EM) radiation. So far, a confirmed link between EM and GW radiation has been established for only one source, GW170817. Joint analysis ofbroadband multi-wavelength data and the GW signal have yielded rich information spanning fields as varied as jet physics, cosmology and nucleosynthesis. Here, we discuss the importance of such joint observations, as well as current and near-future efforts to discover and study more EM counterparts to GW sources.
Volume 43 All articles Published: 6 December 2022 Article ID 0091
K. G. ANUSREE DIPANKAR BHATTACHARYA VARUN BHALERAO AKASH ANUMARLAPUDI
The Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat, an open detector above $\sim$100 keV, is a promising tool for the investigation of hard X-ray characteristics of $\gamma$-ray pulsars. A custom algorithm has been developed to detect pulsars from long integration ($\sim$years) of archival data, as reported by us earlier. Here, we extend this method in the analysis to include additional $\sim$20% of the CZTI pixels that were earlier ignored due to their lower gain values. Recent efforts have provided better and more secure calibration of these pixels, demonstrating their higher thresholds and extended energy range up to $\sim$1 MeV. Here, we use the additional information provided by these pixels, enabling the construction of pulse profiles over a larger energy range. We compare the profiles of the Crab pulsar at different sub-bands and show that the behavior is consistent with theextended energy coverage. As detailed spectroscopy over this full band remains difficult due to the limited count rate, we construct hardness ratios which, together with AstroSat mass model simulations, are able to constrain the power-law index of the radiation spectrum. We present our results for the phase-resolved spectrum of PSRJ0534+2200 and for the total pulsed emission of PSR J1513$-$5908. The recovered photon indices are found to be accurate within $\sim$20%.
Volume 44, 2023
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