arXiv subjects

Ankur Ghosh

Publications and source records attributed to Ankur Ghosh.

12 recordsLinked to original sources

arXiv2026

Investigating peculiar prompt emission properties of the multi-Peaked GRB 250129A

We present a high-energy spectral analysis of GRB 250129A, which was triggered by the Swift-BAT. The burst exhibits a complex, multi-peaked temporal structure characterized by two distinct emission episodes, with the main peak occurring approximately 180 seconds after the BAT trigger. The time-integrated spectral analysis in the 15 - 150 keV energy range indicates that a broken power-law (BPL) model provides the best fit, signifying a non thermal origin of the prompt emission. A time resolved spectral analysis, performed using the Bayesian block technique, shows that the intervals around the main emission peak are well described by the BPL model, while the fits for low count intervals remain less constrained. An evident intensity tracking behavior is observed between the flux and the spectral peak energy (Ep). Furthermore, both the Amati relation and hardness - intensity correlation suggest that GRB~250129A occupies an intermediate regime, acting as a bridge between long and ultra long GRBs.

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arXiv2025

GRB 230204B: GIT Discovery of a Fast Fading Afterglow Associated with an Energetic GRB from a Massive-Star Progenitor

We present a comprehensive multi-wavelength study of a bright gamma-ray burst GRB 230204B, analyzing both prompt and afterglow emissions. This GRB is highly energetic, with an isotropic equivalent energy emission $E_{\mathrm{iso}} \sim 2.2 \times 10^{54}\ \mathrm{erg}$, released during the prompt emission. The GROWTH-India Telescope discovered a bright afterglow ($m_r = 15.55$) that faded rapidly ($\propto t^{-1.82}$). The prompt emission shows strong thermal photospheric emission, along with a non-thermal high-energy component. We explore the evolution of these components and find them to be consistent with theoretical expectations. Afterglow modeling reveals an energetic jet $E_{tot} \gtrsim 10^{52}\ \mathrm{erg}$ expanding into a wind-type medium viewed nearly on-axis, suggesting a massive star progenitor with strong winds. We also explore correlations between the prompt emission and afterglow that may help to understand the complete picture of GRB progenitors.

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arXiv2025

Dissecting the radiation mechanism of short GRB~160821B through multi-wavelength modelling

GRB~160821B is the only short GRB detected to date at very high energy (VHE, $\gtrsim 100$ GeV). At a redshift $z=0.161$, it was detected by MAGIC telescopes approximately four hours since the trigger. VHE dataset was complied with the datasets of other wavelengths in between the timescale of 1.7 to 4 hours to construct the broadband spectral energy distribution (SED). In previous studies of GRB~160821B, synchrotron and external Compton (EC) model could explain the VHE emission better than the synchrotron and synchrotron self-Compton (SSC) model. Although, these fits were mostly eyeballing data without any optimisation. Our model includes the combination of synchrotron, SSC, and EC models with Markov Chain Monte Carlo (MCMC) techniques. Our analysis reveals that the EC contribution is negligible in comparison with the SSC and our model explains the VHE data well for the wind medium. We found that GRB~160821B is the least energetic VHE GRB and it occurred in high density wind medium which is quiet unusual for a short GRB. But like other long-duration VHE GRBs, GRB~160821B occurred in a poorly magnetised medium. As there is no statistical study on afterglow modelling of short GRB sample, we compare the inferred properties of GRB~160821B with other VHE GRBs. It stands out distinctively in the $E_{k, \rm iso}$ - $\epsilon_B$ parameter space and lies outside the 3-$\sigma$ region of the correlation. In future, more VHE detections of short GRBs, in the CTA era, will provide crucial insights into the emission sites, radiation mechanisms, and particle acceleration, as well as their connection to long GRBs.

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arXiv2025

Deciphering the Physical Origin of GRB 240825A: A Long GRB Lacking a Bright Supernova

We present a comprehensive multiwavelength analysis of GRB 240825A, a bright gamma-ray burst (GRB) detected by Fermi and Swift, with a prompt duration ($T_{\rm 90}$ ~ 4 sec in 50-300 keV) near the boundary separating short and long GRBs, prompting a detailed investigation into its classification and progenitor. Using classical prompt metrics (duration, minimum variability timescale (MVT), lag, and spectral hardness) and modern classification techniques (machine-learning (ML) based t-SNE, support vector machine, energy-hardness-duration, and $\epsilon \equiv E_{\gamma,\mathrm{iso},52} / E_{p,z,2}^{5/3}$), we find GRB 240825A exhibits hybrid characteristics. The short MVT (13.830 $\pm$ 1.574 ms), rest-frame duration, and ML-based classification indicate a merger-like or ambiguous nature, while its energetics and position on the Amati relation favor a collapsar origin. We conducted deep optical and NIR photometric and spectroscopic late-time search for an associated supernova (SN)/kilonova (KN) and the host galaxy using 10.4 m GTC and 8.4 m binocular LBT telescopes. No bright SN (like SN 1998bw) is detected down to stringent limits (e.g., $m_r > 26.1$ mag at 17.59 days), despite a redshift of $z$ = 0.659 measured from GTC spectroscopy. Host galaxy SED modeling with Prospector indicates a massive, dusty, and star-forming galaxy-typical of collapsar GRB hosts, though with low sSFR and large offset. We compare these findings with hybrid events like GRB 211211A, GRB 230307A, GRB 200826A, including SNe-GRBs, and conclude that GRB 240825A likely originated from a massive star collapse, with the associated supernova obscured by a dusty host environment or low luminosity SN with absolute magnitude M$_{V}$ fainter than -18.0. This study emphasizes the need for multiwavelength follow-up and a multi-layered classification to determine GRB progenitors.

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arXiv2025

Modelling multiwavelength afterglows of the VHE-GRB population

The recent detection of very high energy (VHE, $\gtrsim$ 100 GeV) $\gamma$-ray emission from gamma-ray bursts (GRBs) has provided new insights into afterglow physics. Understanding the temporal and spectral evolution of VHE GRBs requires detailed modelling of multiwavelength observations spanning radio to VHE $\gamma$ rays. Previous studies interpreted afterglow of VHE GRBs using a range of frameworks, including single- and multi-zone jet configurations, synchrotron radiation from forward and reverse shocks, synchrotron self-Compton (SSC) processes, as well as hadronic emission processes. We have modeled five long-duration VHE GRBs - GRB 180720B, GRB 190114C, GRB 190829A, GRB 201216C and GRB 221009A; using the NAIMA code and modifications to it. The results from our analysis indicate that SSC is the dominant VHE emission mechanism, with negligible contribution from external Compton. Most VHE GRBs are well described by the forward shock model in a spherical jet configuration, where constant density interstellar medium is preferred over wind medium. Additionally, we find that VHE GRBs tend to occur in environments with lower magnetic fields and higher ambient medium densities. Interestingly, VHE GRBs lie at the edge of the $3\sigma$ region of the $E_{\rm k,iso}$ - $\epsilon_B$ correlation observed in other energetic GRBs. Our model slightly over predicts the radio fluxes, indicating that a more complicated modelling might be required in some cases. These findings provide crucial constraints on VHE GRB emission sites and mechanisms and serve as a benchmark for future observations and theoretical studies in the era of CTA and next-generation $\gamma$-ray observatories.

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arXiv2024

Modeling the late time merger ejecta emission in short Gamma Ray Bursts

The short Gamma Ray Bursts (GRBs) are the aftermath of the merger of binary compact objects (neutron star -- neutron star or neutron star -- black hole systems). With the simultaneous detection of Gravitational Wave (GW) signal from GW 170817 and GRB 170817A, the much-hypothesized connection between GWs and short GRBs has been proved beyond doubt. The resultant product of the merger could be a millisecond magnetar or a black hole depending upon the binary masses and their equation of state. In the case of a magnetar central engine, fraction of the rotational energy deposited to the emerging ejecta produces late time synchrotron radio emission from the interaction with the ambient medium. In this paper, we present an analysis of a sample of short GRBs located at a redshift of $z \leq 0.16$ which were observed at the late time to search for the emission from merger ejecta. Our sample consists of 7 short GRBs which have radio upper limits available from VLA and ATCA observations. We generate the model lightcurves using the standard magnetar model incorporating the relativistic correction. Using the model lightcurves and upper limits we constrain the number density of the ambient medium to be $10^{-5} - 10^{-3} cm^{-3}$ for rotational energy of the magnetar $E_{rot} \sim 5\times10^{51}$ erg. Variation of ejecta mass does not play a significant role in constraining the number density.

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arXiv2024

Prompt emission properties of GRB~200613

We study the prompt emission properties of the long duration GRB~200613A using \textit{Fermi}-Gamma-Ray Burst Monitor (GBM) and Large Area Telescope (LAT) data. The prompt emission light curve of GRB~200613A reveals a strong peak emission up to $\sim$ 50 s after the burst accompanied by an extended emission up to $\sim$ 470 s similar to that seen in ultra-long GRB light curves. The time-integrated spectroscopy shows that the Band function best fits the main emission episode, and the extended emission follows the power-law behaviour because of poor count rates. Due to its high isotropic energy and low peak energy, GRB~200613A lies at the extreme end in both the $E_{\rm p}$--$E_{\rm iso}$ and $E_{\rm p}$--$T_{90}$ plots. In addition to the GBM detection, the \textit{Fermi}-LAT detected the highest energetic photons of 7.56 GeV after 6.2 ks since burst, which lies beyond the maximum synchrotron energy range.

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arXiv2023

Insights into the properties of GRBs with TeV emission

This study investigates the environments and characteristics of Gamma-Ray Bursts (GRBs) exhibiting very high energy (VHE) emission. Recent detections of VHE emission, up to TeV energies, challenge synchrotron-only emission models and particle acceleration concepts in GRBs. Until now, only a handful of GRBs have been detected in the VHE range. We compare the number densities of the circumburst medium of VHE-detected GRBs to check if the environment impacts the VHE emission. This shows that these GRBs have environments similar to the larger population of GRBs. We employ machine learning algorithms to create two-dimensional embeddings of GRB prompt emission light curves from the {\it Swift}-BAT catalog. VHE-detected GRBs are located across the map, indicating that VHE emission does not favour any particular cluster. These findings indicate that VHE-detected GRBs do not show any peculiar characteristics other than the observational detection of VHE photons. Future detections will increase the sample size required for a rigorous understanding of the origin of VHE emission in GRBs.

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arXiv2022

Search for merger ejecta emission from late time radio observations of short GRBs using GMRT

Short gamma-ray bursts (GRBs) are the aftermath of compact binary mergers involving neutron stars. If the merger remnant is a millisecond magnetar instead of a black hole, a significant proportion of the rotational energy deposited to the emerging ejecta can produce a late-time radio brightening from its interaction with the ambient medium. Detection of this late-time radio emission from short GRBs can have profound implications for understanding the physics of the progenitor. We report the radio observations of five short GRBs - 050709, 061210, 100625A, 140903A, and 160821B using the Giant Metrewave Radio Telescope (GMRT) at 1250, 610, and 325 MHz frequencies after $\sim$ $2 - 11$ years from the time of the burst. The GMRT observations at low frequencies are particularly important to detect the signature of merger ejecta emission at the peak. These observations are the most delayed searches associated with some of these GRBs for any late-time low-frequency emission. We find no evidence for such an emission. We find that none of these GRBs are consistent with maximally rotating magnetar with a rotational energy of $\sim 10^{53}\, {\rm ergs}$. However, magnetars with lower rotational energies cannot be completely ruled out. Despite the non detection, our study underscores the power of radio observations in the search for magnetar signatures associated with short GRBs. However, only future radio observatories may have the capabilities to either detect these signatures or put more stringent constraints on the model.

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arXiv2022

The long-active afterglow of GRB 210204A: Detection of the most delayed flares in a Gamma-Ray Burst

We present results from extensive broadband follow-up of GRB 210204A over the period of thirty days. We detect optical flares in the afterglow at 7.6 x 10^5 s and 1.1 x 10^6 s after the burst: the most delayed flaring ever detected in a GRB afterglow. At the source redshift of 0.876, the rest-frame delay is 5.8 x 10^5 s (6.71 d). We investigate possible causes for this flaring and conclude that the most likely cause is a refreshed shock in the jet. The prompt emission of the GRB is within the range of typical long bursts: it shows three disjoint emission episodes, which all follow the typical GRB correlations. This suggests that GRB 210204A might not have any special properties that caused late-time flaring, and the lack of such detections for other afterglows might be resulting from the paucity of late-time observations. Systematic late-time follow-up of a larger sample of GRBs can shed more light on such afterglow behaviour. Further analysis of the GRB 210204A shows that the late time bump in the light curve is highly unlikely due to underlying SNe at redshift (z) = 0.876 and is more likely due to the late time flaring activity. The cause of this variability is not clearly quantifiable due to the lack of multi-band data at late time constraints by the bad weather conditions. The flare of GRB 210204A is the latest flare detected to date.

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arXiv2022

GRB 210217A: A short or a long GRB?

Gamma-ray bursts are traditionally classified as short and long bursts based on their $T_{\rm 90}$ value (the time interval during which an instrument observes $5\%$ to $95\%$ of gamma-ray/hard X-ray fluence). However, $T_{\rm 90}$ is dependent on the detector sensitivity and the energy range in which the instrument operates. As a result, different instruments provide different values of $T_{\rm 90}$ for a burst. GRB 210217A is detected with different duration by {\it Swift} and {\it Fermi}. It is classified as a long/soft GRB by {\it Swift}-BAT with a $T_{\rm 90}$ value of 3.76 sec. On the other hand, the sub-threshold detection by {\it Fermi}-GBM classified GRB 210217A as a short/hard burst with a duration of 1.024 sec. We present the multi-wavelength analysis of GRB 210217A (lying in the overlapping regime of long and short GRBs) to identify its actual class using multi-wavelength data. We utilized the $T_{\rm 90}$-hardness ratio, $T_{\rm 90}$-\Ep, and $T_{\rm 90}$-$t_{\rm mvts}$ distributions of the GRBs to find the probability of GRB 210217A being a short GRB. Further, we estimated the photometric redshift of the burst by fitting the joint XRT/UVOT SED and place the burst in the Amati plane. We found that GRB 210217A is an ambiguous burst showing properties of both short and long class of GRBs.

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arXiv2021

Revealing nature of GRB 210205A, ZTF21aaeyldq (AT2021any), and follow-up observations with the 4K$\times$4K CCD Imager+3.6m DOT

Optical follow-up observations of optical afterglows of gamma-ray bursts are crucial to probe the geometry of outflows, emission mechanisms, energetics, and burst environments. We performed the follow-up observations of GRB 210205A and ZTF21aaeyldq (AT2021any) using the 3.6m Devasthal Optical Telescope (DOT) around one day after the burst to deeper limits due to the longitudinal advantage of the place. This paper presents our analysis of the two objects using data from other collaborative facilities, i.e., 2.2m Calar Alto Astronomical Observatory (CAHA) and other archival data. Our analysis suggests that GRB 210205A is a potential dark burst once compared with the X-ray afterglow data. Also, comparing results with other known and well-studied dark GRBs samples indicate that the reason for the optical darkness of GRB 210205A could either be intrinsic faintness or a high redshift event. Based on our analysis, we also found that ZTF21aaeyldq is the third known orphan afterglow with a measured redshift except for ZTF20aajnksq (AT2020blt) and ZTF19abvizsw (AT2019pim). The multiwavelength afterglow modelling of ZTF21aaeyldq using the afterglowpy package demands a forward shock model for an ISM-like ambient medium with a rather wider jet opening angle. We determine circumburst density of $n_{0}$ = 0.87 cm$^{-3}$, kinetic energy $E_{k}$ = 3.80 $\times 10^{52}$ erg and the afterglow modelling also indicates that ZTF21aaeyldq is observed on-axis ($θ_{obs} < θ_{core}$) and a gamma-ray counterpart was missed by GRBs satellites. Our results emphasize that the 3.6m DOT has a unique capability for deep follow-up observations of similar and other new transients for deeper observations as a part of time-domain astronomy in the future.

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