arXiv subjects

Debdutta Paul

Publications and source records attributed to Debdutta Paul.

4 recordsLinked to original sources

arXiv2021

Characterisation of Cosmic Ray Induced Noise Events in AstroSat-CZT Imager

The Cadmium Zinc Telluride (CZT) Imager onboard AstroSat, consists of pixelated CZT detectors, which are sensitive to hard X-rays above 20 keV. The individual pixels are triggered by ionising events occurring in them, and the detectors operate in a self-triggered mode, recording each event separately with information about its time of incidence, detector co-ordinates, and channel that scales with the amount of ionisation. The detectors are sensitive not only to photons from astrophysical sources of interest, but also prone to a number of other events like background X-rays, cosmic rays, and noise in detectors or the electronics. In this work a detailed analysis of the effect of cosmic rays on the detectors is made and it is found that cosmic rays can trigger multiple events which are closely packed in time (called 'bunches'). Higher energy cosmic rays, however, can also generate delayed emissions, a signature previously seen in the PICsIT detector on-board INTEGRAL. An algorithm to automatically detect them based on their spatial clustering properties is presented. Residual noise events are examined using examples of Gamma Ray Bursts as target sources.

astro-ph.IM↗

arXiv2018

Binary neutron star merger rate via the luminosity function of short gamma-ray bursts

The luminosity function of short Gamma Ray Bursts (GRBs) is modelled by using the available catalogue data of all short GRBs (sGRBs) detected till October, 2017. The luminosities are estimated via the `pseudo-redshifts' obtained from the `Yonetoku correlation', assuming a standard delay distribution between the cosmic star formation rate and the production rate of their progenitors. While the simple powerlaw is ruled out to high confidence, the data is fit well both by exponential cutoff powerlaw and broken powerlaw models. Using the derived parameters of these models along with conservative values in the jet opening angles seen from afterglow observations, the true rate of short GRBs are derived. Assuming a short GRB is produced from each binary neutron star merger (BNSM), the rate of gravitational wave (GW) detections from these mergers are derived for the past, present and future configurations of the GW detector networks. Stringent lower limits of $1.87 \pyr$ for the aLIGO-VIRGO, and $3.11 \pyr$ for the upcoming aLIGO-VIRGO-KAGRA-LIGO/India configurations are thus derived for the BNSM rate at $68 \%$ confidence. The BNSM rates calculated from this work and that independently inferred from the observation of the only confirmed BNSM observed till date, are shown to have a mild tension; however the scenario that all BNSMs produce sGRBs cannot be ruled out.

astro-ph.HE↗

arXiv2017

Modelling the luminosity function of long Gamma Ray Bursts using SWIFT and FERMI

I have used a sample of long Gamma Ray Bursts (GRBs) common to both \emph{Swift} and \emph{Fermi} to re-derive the parameters of the Yonetoku correlation. This allowed me to self-consistently estimate pseudo redshifts of all the bursts with unknown redshifts. This is the first time such a large sample of GRBs from these two instruments are used, both individually and in conjunction, to model the long GRB luminosity function. The GRB formation rate is modelled as the product of the cosmic star formation rate and a GRB formation efficiency for a given stellar mass. An exponential cut-off powerlaw luminosity function fits the data reasonably well, with $ν= 0.6$ and $ L_b = 5.4 \times10^{52} \, \rm{erg.s^{-1}},$ and does not require a cosmological evolution. In the case of a broken powerlaw, it is required to incorporate a sharp evolution of the break given by $L_{b}\sim0.3\times10^{52}\left(1+z\right)^{2.90} \, \rm{erg.s^{-1}},$ and the GRB formation efficiency (degenerate up to a beaming factor of GRBs) decreases with redshift as $\propto\left(1+z\right)^{-0.80}.$ However it is not possible to distinguish between the two models. The derived models are then used as templates to predict the distribution of GRBs detectable by CZTI on board \emph{AstroSat}, as a function of redshift and luminosity. This demonstrates that via a quick localization and redshift measurement of even a few CZTI GRBs, \emph{AstroSat} will help in improving the statistics of GRBs both typical and peculiar.

astro-ph.HE↗

arXiv2016

Anisotropy of partially self-absorbed jets and the jet of Cyg X-1

We study the angular dependence of the flux from partially synchrotron self-absorbed conical jets (proposed by Blandford \& K{ö}nigl). We consider the jet viewed from either a side or close to on axis, and in the latter case, either from the jet top or bottom. We derive analytical formulae for the flux in each of these cases, and find the exact solution for an arbitrary angle numerically. We find that the maximum of the emission occurs when the jet is viewed from top on-axis, which is contrast to a previous result, which found the maximum at some intermediate angle and null emission on-axis. We then calculate the ratio of the jet-to-counterjet emission for this model, which depends on the viewing angle and the index of power-law electrons. We apply our results to the black-hole binary Cyg X-1. Given the jet-to-counterjet flux ratio of $\gtrsim$50 found observationally and the current estimates of the inclination, we find the jet velocity to be $\gtrsim 0.8c$. We also point out that when the projection effect is taken into account, the radio observations imply the jet half-opening angle of $\lesssim 1^{\rm o}$, a half of the value given before. When combined with the existing estimates of $Γ_{\rm j}$, the jet half-opening angle is low, $\ll 1/Γ_{\rm j}$, and much lower than values observed in blazars, unless $Γ_{\rm j}$ is much higher than currently estimated.

astro-ph.HE↗