arXiv subjects

T. Mondal

Publications and source records attributed to T. Mondal.

4 recordsLinked to original sources

arXiv2025

Probing TeV Afterglow Emission of GRB~221009A with Gaussian Structured jet in Wind-driven medium

Recent detections of very high energy (VHE; GeV-TeV) photons from gamma-ray burst (GRB) afterglows, most notably the extreme event GRB 221009A, require refined models that include realistic jet structures and complex circumburst environments. The jet's angular structure is crucial for shaping afterglow emission. Our recent work demonstrates that Gaussian jets, with their smooth angular decline, naturally produce early bright peaks for on-axis observers and delayed, softer, dimmer peaks at higher inclinations. The gradual decline suppresses excessive lateral expansion, unlike the sharp edge in top-hat jets, making Gaussian jets a compelling alternative to both top-hat and other structured-jet models. Here we implement a Gaussian structured-jet model to explain TeV afterglows from adiabatic forward shocks propagating in a wind-driven medium. We show that the TeV peak time and flux depend sensitively on jet geometry, kinetic energy, wind density, and on microphysical parameter ratios that scale the SSC component. We identify the afterglow parameter space that is favourable for detecting sub-TeV photons with the Cherenkov Telescope Array (CTA), finding that only about ten per cent of simulated TeV events exceed CTA sensitivity in a wind medium. These detections arise from near core-aligned views, with high kinetic energy and wind density, moderate initial Lorentz factor and downstream magnetic field, and a relatively large fraction of energy in nonthermal electrons. Applying this model to GRB 221009A, we perform multi-band fits including wind-modified dynamics, Klein-Nishina effects, and EBL attenuation, and find that a mildly off-axis geometry reproduces the observed X-ray and GeV-TeV light curves.

astro-ph.HE↗

arXiv2025

Multi Messenger Study of GRB 221009A with VHE Gamma-ray and Neutrino Afterglow from a Gaussian Structured Jet

Recent detections of very-high-energy (VHE; $\gtrsim 100~{\rm GeV}$) emission from GRB afterglows, most notably the unprecedented brightness of GRB~221009A observed by LHAASO, reveal components beyond the standard electron synchrotron model. The multi-TeV photons motivate synchrotron self-Compton and possible hadronic contributions, while the non-detection of coincident neutrinos by IceCube/KM3NeT/GRAND200k constrains the microphysical parameters, jet kinetic energy, and ambient-medium density. We model the VHE afterglow of GRB~221009A with an external forward shock from a Gaussian structured jet in a uniform-density medium. This angular structure reproduces the extreme TeV output at an off-axis angle but without demanding large energies as in a top-hat jet. We compute the corresponding $p\gamma$ neutrino flux in the PeV-EeV range and derive a time-integrated upper limit using the effective areas of IceCube-Gen2 and GRAND200k. This provides insight into the contribution of individual GRBs to neutrino events. The predicted neutrino flux for GRB~221009A, using parameters inferred from the multi-wavelength spectral energy distribution, lies below the sensitivities of these detectors. Even our correlation analysis, optimized for neutrino searches with the upcoming GRAND200k, indicates that the expected number of events from this GRB is of order $\sim 0.1$ under a highly optimistic microphysical parameter regime. We also compare neutrino-flux variations from on-axis and off-axis viewing geometries and find an approximately order of magnitude difference in the signal. Thus, our study concludes that a brighter burst closer than GRB~221009A would be crucial for neutrino detection by upcoming telescopes. Future GRB detections by the Cherenkov Telescope Array will provide important constraints on their geometry, radiation mechanisms, and possible associated neutrino signals.

astro-ph.HE↗

arXiv2024

Likelihood and Deep Learning Analysis of the electron neutrino event sample at Intermediate Water Cherenkov Detector (IWCD) of the Hyper-Kamiokande experiment

Hyper-Kamiokande (Hyper-K) is a next-generation long baseline neutrino experiment. One of its primary physics goals is to measure neutrino oscillation parameters precisely, including the Dirac CP violating phase. As conventional $\nu_{\mu}$ beam generates from the J-PARC neutrino baseline contains only 1.5$\%$ of $\nu_{e}$ interaction of total, it is challenging to measure $\nu_{e}/\bar{\nu}_{e}$ scattering cross-section on nuclei. To reduce these systematic uncertainties, IWCD will be built to study neutrino interaction rates with higher precision. Simulated data comprise $\nu_{e}CC0\pi$ as the main signal with NC$\pi^{0}$ and $\nu_{\mu}CC$ are major background events. To reduce the backgrounds initially, a log-likelihood-based reconstruction algorithm to select candidate events was used. However, this method sometimes struggles to distinguish $\pi^{0}$ events properly from electron-like events. Thus, a Machine Learning-based framework has been developed and implemented to enhance the purity and efficiency of $\nu_{e}$ events.

hep-ex↗

arXiv2024

Follow-up of Neutron Star Mergers with CTA and Prospects for Joint Detection with Gravitational-Wave Detectors

The joint gravitational wave (GW) and electromagnetic observations of the binary neutron star (BNS) merger GW170817 marked a giant leap in multi-messenger astrophysics. The extensive observation campaign of the associated Gamma-Ray Burst (GRB) and its afterglow has strengthened the hypothesis associating GRBs with BNS mergers and provided insights on mass ejection, particularly the relativistic outflow launched in BNS mergers. In this paper, we investigate the joint detection probabilities of BNS mergers by GW detectors and the upcoming ground-based very-high-energy (VHE) $\gamma$-ray instrument, the Cherenkov Telescope Array (CTA). Using an empirical relation that constrains the distance-inclination angle plane, we simulated BNS mergers detectable in the O5 run of the LIGO/Virgo/Kagra (LVK) network with $300$~Mpc BNS horizon. Assuming Gaussian structured jets and ignoring large sky localization challenges of GW detectors, we estimated VHE afterglow detection probability by CTA. We have explored the afterglow parameter space to identify conditions favourable for detecting synchrotron self-Compton emission by CTA. Our study reveals that events viewed at angles $\lesssim3$ times the jet core angle are detectable by CTA when the initial bulk Lorentz factor at the jet axis ranges between 100 and 800. We find high kinetic energy ($E_k>10^{50}$ erg), ambient density ($n_0>10^{-1}$ $cm^{-3}$), and energy content in non-thermal electrons significantly enhance the likelihood of CTA detection within 300 Mpc. The joint detection rate varies significantly with afterglow parameter distributions, ranging from $0.003$ to $0.5$ per year.

astro-ph.HE↗