arXiv subjects

Sourav Palit

Publications and source records attributed to Sourav Palit.

14 recordsLinked to original sources

arXiv2026

Numerical study of multiple solar flare induced modulation of Very Low Frequency (VLF) diurnal profile

Earth's ionosphere is a perpetual detector of ionizing radiation received from celestial objects, particularly the Sun. Solar ionizing radiation in the form of extreme ultraviolet (EUV) and X-rays during both quiet and active phase of the Sun, and charged particles associated with a solar wind imprint their ionization signatures on the ionosphere. Although due to the bipolar nature of the geomagnetic field, the events, such as the solar coronal mass ejections (CMEs) and associated solar wind enhancement, usually disturb the polar ionosphere only, the UV and X-rays from the solar flares produce sudden ionospheric disturbances (SIDs) in low-mid-latitude part of the earth's ionosphere. Such ionospheric disturbances are studied with the help of the influence they exert on radio waves propagating through earth-ionosphere waveguide. For the lower part of the ionosphere, called the D region, prominent modification in electron-ion density during solar flares can be observed via deviation in earth bound Very Low Frequency (VLF) wave signal from its ambient diurnal profile. In earlier work, successful model of the deviation in VLF amplitude due to different classes of solar flares was formulated. There, calculation of rate of ionization with Monte Carlo simulation and ion-chemistry evaluation of plasma density enhancement followed by a radio propagation simulation was used. Presently, we attempt to numerically reconstruct the modulation in VLF signal from its diurnal pattern produced by multiple solar flares occurring over a single day. Successful reconstruction of the VLF signal modulation for such a complex flaring scenario points to the accuracy of our understanding of the ionization effect due to solar activity on the lower ionosphere, and strengthen our claim to use earth's ionosphere as a high energy space transient event detector.

astro-ph.SR↗

arXiv2024

Development of a Compton Imager Setup

Hard X-ray photons with energies in the range of hundreds of keV typically undergo Compton scattering when they are incident on a detector. In this process, an incident photon deposits a fraction of its energy at the point of incidence and continues onwards with a change in direction that depends on the amount of energy deposited. By using a pair of detectors to detect the point of incidence and the direction of the scattered photon, we can calculate the scattering direction and angle. The position of a source in the sky can be reconstructed using many Compton photon pairs from a source. We demonstrate this principle in the laboratory by using a pair of Cadmium Zinc Telluride (CZT) detectors sensitive in the energy range of 20-200 keV, similar to those used in $\textit{AstroSat}$/CZT Imager (CZTI). The laboratory setup consists of the two detectors placed perpendicular to each other in a lead-lined box. The detectors are read out by a custom-programmed Xilinx PYNQ-Z2 FPGA board, and data are then transferred to a personal computer (PC)}. There are two key updates from CZTI: the detectors are read concurrently rather than serially, and the time resolution has been improved from $20~\mu$s to $7.5~\mu$s. We irradiated the detectors with a collimated $^{133}\mathrm{Ba}$ source and identified Compton scattering events for the 356 keV line. We run a Compton reconstruction algorithm to correctly infer the location of the source in the detector frame, with a location-dependent angular response measure of $16\deg-30\deg$. This comprises a successful technology demonstration for a Compton imaging camera in the hard X-ray regime. We present the details of our setup, the data acquisition process, and software algorithms, and showcase our results. We also quantify the limitations of this setup and discuss ways of improving the performance in future experiments.

astro-ph.IM↗

arXiv2024

Science with the Daksha High Energy Transients Mission

We present the science case for the proposed Daksha high energy transients mission. Daksha will comprise of two satellites covering the entire sky from 1~keV to $>1$~MeV. The primary objectives of the mission are to discover and characterize electromagnetic counterparts to gravitational wave source; and to study Gamma Ray Bursts (GRBs). Daksha is a versatile all-sky monitor that can address a wide variety of science cases. With its broadband spectral response, high sensitivity, and continuous all-sky coverage, it will discover fainter and rarer sources than any other existing or proposed mission. Daksha can make key strides in GRB research with polarization studies, prompt soft spectroscopy, and fine time-resolved spectral studies. Daksha will provide continuous monitoring of X-ray pulsars. It will detect magnetar outbursts and high energy counterparts to Fast Radio Bursts. Using Earth occultation to measure source fluxes, the two satellites together will obtain daily flux measurements of bright hard X-ray sources including active galactic nuclei, X-ray binaries, and slow transients like Novae. Correlation studies between the two satellites can be used to probe primordial black holes through lensing. Daksha will have a set of detectors continuously pointing towards the Sun, providing excellent hard X-ray monitoring data. Closer to home, the high sensitivity and time resolution of Daksha can be leveraged for the characterization of Terrestrial Gamma-ray Flashes.

astro-ph.HE↗

arXiv2024

Daksha: On Alert for High Energy Transients

We present Daksha, a proposed high energy transients mission for the study of electromagnetic counterparts of gravitational wave sources, and gamma ray bursts. Daksha will comprise of two satellites in low earth equatorial orbits, on opposite sides of earth. Each satellite will carry three types of detectors to cover the entire sky in an energy range from 1 keV to >1 MeV. Any transients detected on-board will be announced publicly within minutes of discovery. All photon data will be downloaded in ground station passes to obtain source positions, spectra, and light curves. In addition, Daksha will address a wide range of science cases including monitoring X-ray pulsars, studies of magnetars, solar flares, searches for fast radio burst counterparts, routine monitoring of bright persistent high energy sources, terrestrial gamma-ray flashes, and probing primordial black hole abundances through lensing. In this paper, we discuss the technical capabilities of Daksha, while the detailed science case is discussed in a separate paper.

astro-ph.IM↗

arXiv2023

Prospects of measuring Gamma-ray Burst Polarisation with the Daksha mission

The proposed Daksha mission comprises of a pair of highly sensitive space telescopes for detecting and characterising high-energy transients such as electromagnetic counterparts of gravitational wave events and gamma-ray bursts (GRBs). Along with spectral and timing analysis, Daksha can also undertake polarisation studies of these transients, providing data crucial for understanding the source geometry and physical processes governing high-energy emission. Each Daksha satellite will have 340 pixelated Cadmium Zinc Telluride (CZT) detectors arranged in a quasi-hemispherical configuration without any field-of-view collimation (open detectors). These CZT detectors are good polarimeters in the energy range 100 -- 400 keV, and their ability to measure polarisation has been successfully demonstrated by the Cadmium Zinc Telluride Imager (CZTI) onboard AstroSat. Here we demonstrate the hard X-ray polarisation measurement capabilities of Daksha and estimate the polarisation measurement sensitivity (in terms of the Minimum Detectable Polarisation: MDP) using extensive simulations. We find that Daksha will have MDP of~$30\%$ for a fluence threshold of $10^{-4}$ erg cm$^2$ (in 10 -- 1000 keV). We estimate that with this sensitivity, if GRBs are highly polarised, Daksha can measure the polarisation of about five GRBs per year.

astro-ph.IM↗

arXiv2022

Gravitational Waves and Electromagnetic Transients

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 of broadband multiwavelength 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.

astro-ph.HE↗

arXiv2021

Revisiting the Earth's atmospheric scattering of X-ray/$γ$-rays and its effect on space observation: Implication for GRB spectral analysis

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/$γ$-ray instruments. 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 counterpart detectors. Furthermore, possible systematic effects in these complex simulations have not been thoroughly examined and verified in the literature. We are in the process of investigation of the effect with 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 the Atmospheric Response 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 a large number of upcoming high energy transient missions, as well as for scrutiny and systematic comparisons with other missions

astro-ph.HE↗

arXiv2021

The AstroSat Mass Model: Imaging and Flux studies of off-axis sources with CZTI

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 x 4.6 degrees (FWHM). The instrument collimators become increasingly transparent 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 mass model is discussed in a companion paper, Chattopadhyay 2021.

astro-ph.IM↗

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.

astro-ph.HE↗

arXiv2020

Surface and oceanic habitability of Trappist-1 planets under the impact of flares

The discovery of potentially habitable planets around the ultracool dwarf star Trappist-1 naturally poses the question: could Trappist-1 planets be home to life? These planets orbit very close to the host star and are most susceptible to the UV radiation emitted by the intense and frequent flares of Trappist-1. Here we calculate the UV spectra (100 - 450 nm) of a superflare observed on Trappist-1 with the K2 mission. We couple radiative transfer models to this spectra to estimate the UV surface flux on planets in the habitable zone of Trappist-1 (planets $e$, $f$, and $g$), assuming atmospheric scenarios based on a pre-biotic and an oxygenic atmosphere. We quantify the impact of the UV radiation on living organisms on the surface and on a hypothetical planet ocean. Finally, we find that for non-oxygenic planets, UV resistant lifeforms would survive on the surface of planets f and g. Nevertheless, more fragile organisms (i.e. \textit{E. coli}) could be protected from the hazardous UV effects at ocean depths greater than 8m. If the planets have an ozone layer, any lifeforms studied here would survive in the HZ planets.

astro-ph.EP↗

arXiv2015

Inverse problem in Ionospheric Science: Prediction of solar soft-X-ray spectrum from Very Low Frequency Radiosonde results

X-rays and gamma-rays from astronomical sources such as solar flares are mostly absorbed by the Earth's atmosphere. Resulting electron-ion production rate as a function of height depends on the intensity and wavelength of the injected spectrum and therefore the effects vary from one source to another. In other words, the ion density vs. altitude profile has the imprint of the incident photon spectrum. In this paper, we investigate whether we can invert the problem uniquely by deconvolution of the VLF amplitude signal to obtain the details of the injected spectrum. We find that it is possible to do this up to a certain accuracy. Our method is useful to carry out a similar exercise to infer the spectra of more energetic events such as the Gamma Ray Bursts (GRBs), Soft Gamma Ray Repeaters (SGRs) etc. by probing even the lower part of the atmosphere. We thus show that to certain extent, the Earth's atmosphere could be used as a gigantic detector of relatively strong events.

astro-ph.EP↗

arXiv2013

A New Paradigm in Space Based Experiments Using Rubber Balloons

Indian Centre for Space Physics is engaged in long duration balloon borne experiments with typical payloads less than ~ 3kg. Low cost rubber balloons are used. In a double balloon system, the booster balloon lifts the orbiter balloon to its cruising altitude where data is taken for a long time. Here we present results of muon detections and recent solar activities, including the light curves and flare spectra in the 20-100keV range. We not only show that we have successfully obtained several flares and there spectra at different altitudes, we also found that the high energy X-ray flux of strong flares at altitudes of 10-13 km (the flight altitude of commercial planes) could be more than the contribution due to cosmic rays.

astro-ph.IM↗

arXiv2009

X-ray experiments for Space applications in intermediate energy range

X-ray experiments in the intermediate energy range (1-50 keV) are carried out at the Indian Centre for Space Physics (ICSP), Kolkata for space application. The purpose is to carry out developmental studies of space instruments to observe energetic phenomena from compact objects (black hole and compact stars) and active stars and their testing and evaluation. The testing/evaluation setup primarily consists of an X-ray generator, various X-ray imaging masks, an X-ray imager (CMOS) and an X-ray spectrometer (Si-PIN photo-diode). The X-ray generator (Mo target) operates in 1-50 kV anode voltage, and 1-30 mA beam current. A 45 feet long shielded collimator is used to collimate the beam which leads to the detector chamber having a 30 arc-sec angular diameter. Two types of imaging masks are used - conventional Coded Aperture Masks (CAM) and Tungsten Fresnel half-period zone-plates (ZPs) having angular resolutions of a few tens of arc-sec. The Moire fringe pattern produced by the composite shadows of two ZPs is inverse Fourier transformed to obtain the X-ray source distribution. CAMs are advantageous but the resolution obtained is limited by their smallest pixel size. Our setup has been extensively used in testing and evaluation of RT-2 payloads which have been launched recentlya (January 30, 2009). More experiments for improving imaging techniques are being designed and tested.

astro-ph.IM↗

arXiv2009

Fresnel Zone Plate Telescopes as high resolution imaging devices

Combination of Fresnel Zone Plates (FZP) can make excellent telescopes for imaging in X-rays. We present the results of our experiments with such telescopes with an X-ray source kept at a distance of 45 feet. We compare the patterns obtained from experiments with those obtained by our Monte-Carlo simulations. In simulations, we allow the sources to be at finite distances (diverging beam) as well as at infinite distances (parallel beam) and show that the resolution is worsened when the source is nearby. We also present simulated results for the observation of the galactic center and show that the sources may be reconstructed with accuracy. We compare the performance of such a telescope with other X-ray imaging devices used in space-astronomy. The Zone Plate based instrument has been sent for the first time in a recently launched KORONAS-FOTON satellite.

astro-ph.IM↗