arXiv subjects

Sushmita Agarwal

Publications and source records attributed to Sushmita Agarwal.

7 recordsLinked to original sources

arXiv2026

Constraining Orbital Eccentricity of a Supermassive Black Hole Binary Candidate PKS 2131-0211

A detailed analysis of the decades-long radio light curve of blazar PKS 2131-021 showed epochs of sinusoidal variations in the radio flux density time-series as detailed in O'Neill et al. (2022). The observed sinusoidal flux modulation arises naturally from relativistic Doppler boosting of the jet when the jet-emitting supermassive black hole (SMBH) orbits its companion. For SMBHs in circular orbits, this scenario yields sinusoidal light curves, offering a simple kinematic explanation for the observed variability in PKS 2131-021. We present an approach that incorporates the effects of orbital eccentricity into the Kinematic Orbital model for PKS 2131-021, using the Keplerian parametric solution to describe the SMBH binary orbit. Using the available radio light curve data, we demonstrate that the proposed SMBH binary likely possesses a residual orbital eccentricity, which we constrain through detailed Bayesian parameter estimation studies to be 0.053 \pm 0.015 with a Bayes factor of 3.15 over the circular model. However, when the analysis accounts for the presence of red noise in the data using a Damped Random Walk (DRW) process, the circular model is preferred, giving an eccentricity upper limit of e < 0.15. Nevertheless, our efforts reveal that the Circular+DRW model is strongly favored. This model consistently recovers a coherent periodic signal across all datasets, with the orbital period remaining well-defined even when accounting for broader uncertainties. This analysis incorporated archival observations from the Haystack Observatory, the University of Michigan Radio Astronomy Observatory (UMRAO), and the Owens Valley Radio Observatory (OVRO), spanning the period from 1975 to 2021, compiled by O'Neill et al. (2022).

astro-ph.HE↗

arXiv2025

Hunting Very High-Energy ($>$100 GeV) Emitting High-Synchrotron Peaked Blazars

Very-high energy (VHE; $>$100 GeV) $γ$-ray emission originates via some of the most extreme particle acceleration processes in the universe. Considering beamed active galactic nuclei, i.e., blazars, only a small fraction, mainly high synchrotron peak BL Lacs, have been detected in the VHE band with the ground-based Cherenkov telescopes. We utilized $\sim$16 years of Fermi-Large Area Telescope (LAT) observations in the 0.1$-$2 TeV energy range to systematically search for potential VHE emitters in a sample of high synchrotron peaked ($ν^{\rm peak}_{\rm syn}>10^{15}$ Hz) BL Lac sources. We identified, for the first time, 92 VHE emitting blazars at $\geq 5σ$ confidence level. A significant VHE emission was also detected from 52 sources previously reported as VHE blazars. Comparing with the general blazar population, these VHE emitting blazars are found to be located at low redshifts (mean $z=0.2 \pm 0.1$) and exhibit bright synchrotron emission ($\log F^{\rm peak}_{\rm syn}=-11.2 \pm 0.4$, in erg cm$^{-2}$ s$^{-1}$). We also investigated the coincidence of VHE photon arrivals with the source activity states and found that Fermi-LAT has detected VHE photons during both quiescent and elevated activity epochs. These VHE emitting blazars represent promising targets for current and next-generation ground-based Cherenkov telescopes, and provide powerful laboratories for probing particle acceleration in relativistic jets, testing multi-messenger connections, and constraining extragalactic background light models.

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arXiv2025

Broadband Modelling of GRB 230812B Afterglow: Implications for VHE $\gamma$-ray Detection with IACTs

A significant fraction of the energy from the $\gamma$-ray burst (GRB) jets, after powering the keV-MeV emission, forms an ultra-relativistic shock propagating into the circumburst medium. The particles in the medium accelerate through the shock and produce afterglow emission. Recently, a few GRB afterglows have been observed in TeV $\gamma$-rays by Cherenkov Telescopes. This provides access to broadband spectra of GRB afterglows containing rich information about the microphysics of relativistic shocks and the profile of the circumburst medium. Since the transition from synchrotron to inverse Compton regime in afterglow spectra occurs between hard X-rays and the very-high-energy (VHE) $\gamma$-rays, detection in one of these bands is required to identify the two spectral components. The early afterglow data in the hard X-rays, along with the GeV emission, could accurately constrain the spectral shape and help in capturing the spectral turnover to distinguish the two components. We present the multiwavelength spectral and temporal study, focused on the keV-VHE domain, of GRB 230812B, one of the brightest GRBs detected by Fermi Gamma Ray Burst Monitor (GBM), along with the detection of a 72 GeV photon in Large Area Telescope (LAT) during the early afterglow phase. Through a detailed modelling of the emission within the afterglow external forward shock in a wind-like scenario, we predict optical to high-energy observations up to ~1 day. We emphasize the importance of following up poorly localised GRBs by demonstrating that even without prompt sub-degree localisation, such as in GRB 230812B, it is possible to recover the emission using imaging atmospheric Cherenkov telescopes, thanks to their relatively wider field of view. Moreover, we show that the low energy threshold of Large-Sized Telescope is essential in discovering the VHE component at much higher redshifts, typical of long GRBs.

astro-ph.HE↗

arXiv2025

Constraining $γ$-ray dissipation site in gravitationally lensed quasar -- PKS 1830$-$211

Variable $γ$-ray flares upto minute timescales reflect extreme particle acceleration sites. However, for high-redshift blazars, the detection of such rapid variations remains limited by current telescope sensitivities. Gravitationally lensed blazars serve as powerful tools to probe $γ$-ray production zones in distant sources, with time delays between lensed signals providing crucial insights into the spatial distribution of emission regions relative to the lens's mass-weighted center. We have utilized 15 years of Fermi-LAT $γ$-ray data from direction of PKS 1830$-$211 to understand the origin of flaring high-energy production zone at varying flux states. To efficiently estimate the (lensed) time delay, we used a machine learning-based tool - the Gaussian Process regression algorithm, in addition to - Autocorrelation function and Double power spectrum. We found a consistent time delay across all flaring activity states, indicating a similar location for the $γ$-ray emission zone, possibly within the radio core. The estimated time delay of approximately 20 days for the five flaring epochs was significantly shorter than previously estimated radio delays. This suggests that the $γ$-ray emission zone is closer to the central engine, in contrast to the radio emission zone, which is expected to be much farther away. A linear relationship between lag and magnification has been observed in the identified source and echo flares. Our results suggest that the $γ$-ray emission zone originates from similar regions away from the site of radio dissipation.

astro-ph.HE↗

arXiv2025

Imprint of "Local Opacity" Effect in Gamma-Ray Spectrum of Blazar Jet

Relativistic jets from accreting supermassive black holes at cosmological distances can be powerful emitters of $γ$-rays. However, the precise mechanisms and locations responsible for the dissipation of energy within these jets, leading to observable $γ$-ray radiation, remain elusive. We detect evidence for an intrinsic absorption feature in the $γ$-ray spectrum at energies exceeding $10\,$GeV, presumably due to the photon-photon pair production of $γ$-rays with low ionization lines at the outer edge of Broad-line region (BLR), during the high-flux state of the flat-spectrum radio quasar PKS 1424$-$418. The feature can be discriminated from the turnover at higher energies resulting from $γ$-ray absorption in the extragalactic background light. It is absent in the low-flux states supporting the interpretation that powerful dissipation events within or at the edge of the BLR evolve into fainter $γ$-ray emitting zones outside the BLR, possibly associated with the moving VLBI radio knots. The inferred location of $γ$-ray emission zone is consistent with the observed variability time scale of the brightest flare, provided that the flare is attributed to external Compton scattering with BLR photons.

astro-ph.HE↗

arXiv2022

A numerical study on the role of instabilities on multi-wavelength emission signatures of blazar jets

Blazars, a class of active galaxies whose jets are relativistic and collimated flows of plasma directed along the line of sight and are prone to a slew of magneto-hydrodynamic (MHD) instabilities. We aim to study the interplay of radiation and particle acceleration processes in regulating the multi-band emission and variability signatures from blazars. In particular, the goal is to decipher the impact of shocks arising due to MHD instabilities in driving the longterm variable emission signatures from blazars. In this regard, we have performed RMHD simulations of a representative section of blazar jet. The jet is evolved using a hybrid Eulerian-Lagrangian framework to account for radiative losses due to synchrotron process and particle acceleration due to shocks. Additionally, we have incorporated and validated radiative losses due to the external Compton (EC) process that are relevant for blazars. We have further compared the effects of different radiation mechanisms through numerical simulation of 2D slab jet as a validation test. Finally, we have carried out a parametric study to quantify the effect of magnetic fields and external radiation field characteristics by performing 3D simulations of a plasma column. The synthetic light curves and spectral energy distribution (SEDs) are analysed to qualitatively understand the impact of instability driven shocks. We observe that shocks produced with the evolution of instabilities give rise to flaring signatures in the high energy band. The impact of such shocks is also evident from the instantaneous flattening of the synchrotron component of the SEDs. At later stages, we observe the transition in X-ray emission from the synchrotron process to that dominated by EC. The inclusion of the EC process also gives rise to gamma-ray emission and shows signatures of mild Compton dominance as typically seen in Low Synchrotron Peaked blazars.

astro-ph.HE↗

arXiv2022

Long-term optical and $γ-$ray variability of the blazar PKS~1222+216

The $γ-$ray emission from flat-spectrum radio quasars (FSRQs) is thought to be dominated by the inverse Compton scattering of the external sources of photon fields, e.g., accretion disk, broad-line region (BLR), and torus. FSRQs show strong optical emission lines and hence can be a useful probe of the variability in BLR output, which is the reprocessed disk emission. We study the connection between the optical continuum, H$γ$ line, and $γ-$ray emissions from the FSRQ PKS~1222+216, using long-term ($\sim$2011-2018) optical spectroscopic data from Steward Observatory and $γ-$ray observations from $Fermi$-LAT. We measured the continuum ($F_{C,opt}$) and H$γ$ ($F_{Hγ}$) fluxes by performing a systematic analysis of the 6029-6452 Å optical spectra. We observed stronger variability in $F_{C,opt}$ than $F_{Hγ}$, an inverse correlation between H$γ$ equivalent width and $F_{C,opt}$, and a redder-when-brighter trend. Using discrete cross-correlation analysis, we found a positive correlation (DCF$\sim$0.5) between $F_{γ-ray>100MeV}$ and $F_{C,opt}$ (6024-6092 Å) light curves with time-lag consistent with zero at 2$σ$ level. We found no correlation between $F_{γ-ray>100MeV}$ and $F_{Hγ}$ light curves, probably dismissing the disk contribution to the optical and $γ$-ray variability. The observed strong variability in the $Fermi$-LAT flux and $F_{γ-ray>100MeV}-F_{C,opt}$ correlation could be due to the changes in the particle acceleration at various epochs. We derived the optical-to-$γ$-ray spectral energy distributions (SEDs) during the $γ$-ray flaring and quiescent epochs that show a dominant disk component with no variability. Our study suggests that the $γ$-ray emission zone is likely located at the edge of the BLR or in the radiation field of the torus.

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