arXiv subjects

Subir Bhattacharyya

Publications and source records attributed to Subir Bhattacharyya.

At least 19 recordsLinked to original sources

arXiv2026

Understanding Pulsar Wind Nebulae with the SKA

Produced by the interaction between the ``pulsar wind'' powered by the rotational energy of a neutron star and its surroundings, the study of pulsar wind nebulae (PWNe) provides vital insight into the physics of neutron star magnetospheres and ultra-relativistic outflows. Spatially-resolved studies of the continuum and polarized radio emission of these sources are vital for understanding the production of $e^\pm$ in the magnetospheres of neutron stars, the acceleration of these particles (and potentially baryons) to $\gtrsim10^{15}~{\rm eV}$ energies, and their propagation within the PWN and in the surrounding interstellar medium. The significant improvements in sensitivity, dynamic range, timing capabilities offered by the Square Kilometer Array have the potential to greatly improve our understanding of the origin of some of the highest energy particles produced in the Milky Way.

astro-ph.HE↗

arXiv2026

The broadband spectral energy distribution of candidate neutrino blazars

Blazars, the jet dominated class of AGN comprising flat spectrum radio quasars (FSRQs) and BL Lac objects (BL Lacs) are now increasingly identified as potential sources of high energy neutrinos. Such neutrino blazars are ideal targets to investigate the high energy emission processes and to understand their role as neutrino sources. We report results on four candidate neutrino blazars, PKS 0446+112, TXS 0506+056, PKS 1424$-$418 and PKS 1502+106. We carried out $γ$-ray spectral and timing analysis on three time periods that comprise a quiescent epoch, an epoch that corresponds to neutrino detection and a flaring epoch. We also carried out modeling of the broadband pectral energy distribution (SED) on those three epochs. We found that the $γ$-ray spectra of the BL Lac TXS 0506+056 can be adequately described by a power-law, while the spectra of the other three FSRQs require a log-parabola model. On shorter timescales, we observed flux variability with doubling/halving timescales of 4.70 hrs, 9.24 hrs, 30.76 hrs and 15.42 hrs for PKS 0446+112, TXS 0506+056, PKS 1424$-$418 and PKS 1502+106, respectively. The SEDs of most of the epochs for the sources are well explained by a leptonic scenario. However, the quiescent epoch of PKS 1502+106 and the neutrino-emission epoch of PKS 0446+112 required an additional hadronic component to reproduce the observed SEDs. Our analysis reveals a complex interplay of leptonic and hadronic processes. While certain neutrino-associated epochs align with a leptonic model, others necessitate a hadronic component to explain the emission features.

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arXiv2026

Particle acceleration to PeV energies in Pulsar Wind Nebula: a two zone model

\textit{PeVatrons} are the extreme galactic accelerators capable of producing PeV particles. Recent observation of Large High Altitude Air Shower Observatory have detected UHE photons ($\geq$ 100 TeV) from 43 galactic sources. Detection of UHE photons demands the presence of at least PeV particles in the acceleration site. Although the exact nature of most of the sources are still unknown, a large fraction of these sources have spatial association with pulsar wind nebula. In this work we investigate the acceleration mechanism in pulsar wind nebula by following a magnetohydrodynamics approach. Current study relates the MHD flow solution in immediate downstream with the particle spectrum and spectral energy distribution of photons. Our study shows that MHD description in the PWN environment reduces the parameter space and most of the parameters can be constrained in terms of a single parameter, \textit{the magnetization parameter} $σ$ only. Considering the effect of $σ$, we show that in low $σ$ environment pulsar wind nebula can produce PeV particles. We have also investigate the role of turbulence in the nebular region in acceleration of particle to PeV energy. Current study shows that both low $σ$ environment and turbulence environment is favorable for acceleration of particles up to PeV energy. We have also tested our model in two different LHAASO detected PeVatron 1LHAASO J1848-000u and 1LHAASO J1929+1846u.

astro-ph.HE↗

arXiv2025

Understanding Pulsar Wind Nebulae with the SKA

Produced by the interaction between the ``pulsar wind'' powered by the rotational energy of a neutron star and its surroundings, the study of pulsar wind nebulae (PWNe) provides vital insight into the physics of neutron star magnetospheres and ultra-relativistic outflows. Spatially-resolved studies of the continuum and polarized radio emission of these sources are vital for understanding the production of $e^\pm$ in the magnetospheres of neutron stars, the acceleration of these particles to $\gtrsim10^{15}~{\rm eV}$ energies, and the propagation of these particles within the PWN as well as the surrounding interstellar medium. The significant improvements in sensitivity, dynamic range, timing capabilities offered by the Square Kilometer Array have the potential to significantly improve our understanding of the origin of some of the highest energy particles produced in the Milky Way.

astro-ph.HE↗

arXiv2025

Variability study of classical supergiant X-ray binary 4U 1907+09 using NuSTAR

We investigate the X-ray variability of the supergiant X-ray binary 4U 1907+09 using the new NuSTAR observation of 2024. The source had a relatively stable flux level during previous NuSTAR observations, but the flux varied significantly during the current one. The light curve exhibits dips (off-state) and flares (on-state). The phase-coherent timing analysis during the on-state yields a pulse period of $443.99(4)~\mathrm{s}$, showing the pulsar's continued spin-down. The pulse profiles show an asymmetric double-peaked structure with a phase separation of 0.47 between the two peaks. A cyclotron resonance scattering feature (CRSF) is also detected at $\sim 17.6~\mathrm{keV}$, along with its harmonic at $\sim 38~\mathrm{keV}$, persisting across all flux states. Flux-resolved spectroscopy reveals that the CRSF remains constant despite a 25-fold change in flux. The spectral parameters like photon index and e-fold energy are out of phase with the pulse shape, whereas cutoff energy is in phase with the pulse shape. The source's luminosity during the on-state is $2.85 \times 10^{35}~\mathrm{erg~s^{-1}}$, consistent with a "pencil" beam radiation pattern expected at this flux level from a collisionless gas-mediated shock. These results offer further insights into the accretion dynamics and magnetic field geometry of this system.

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arXiv2025

A White Paper on The Multi-Messenger Science Landscape in India

The multi-messenger science using different observational windows to the Universe such as Gravitational Waves (GWs), Electromagnetic Waves (EMs), Cosmic Rays (CRs), and Neutrinos offer an opportunity to study from the scale of a neutron star to cosmological scales over a large cosmic time. At the smallest scales, we can explore the structure of the neutron star and the different energetics involved in the transition of a pre-merger neutron star to a post-merger neutron star. This will open up a window to study the properties of matter in extreme conditions and a guaranteed discovery space. On the other hand, at the largest cosmological scales, multi-messenger observations allow us to study the long-standing problems in physical cosmology related to the Hubble constant, dark matter, and dark energy by mapping the expansion history of the Universe using GW sources. Moreover, the multi-messenger studies of astrophysical systems such as white dwarfs, neutron stars, and black holes of different masses, all the way up to a high redshift Universe, will bring insightful understanding into the physical processes associated with them that are inaccessible otherwise. This white paper discusses the key cases in the domain of multi-messenger astronomy and the role of observatories in India which can explore uncharted territories and open discovery spaces in different branches of physics ranging from nuclear physics to astrophysics.

astro-ph.HE↗

arXiv2025

Signature of hadronic emission in gamma-ray spectrum of B2 1308+326

The Flat Spectrum Radio Quasar (FSRQ) B2\,1308+326 was in its highest $γ$-ray flaring state during 60260-60310\,MJD. During this period, the source was detected in very high energy (VHE) by the large-sized telescope (LST-1). We conducted a detailed broadband spectral study of this source using the simultaneous data available in optical/UV, X-ray, and $γ$-ray bands. For the broadband spectral study, we select two gamma-ray high flux states (59750-59800\,MJD, 60260-60310\,MJD) and one low flux state (59250-59320\,MJD). During the epochs, 59750-59800\,MJD (high flux state) and 59250-59320\,MJD (low flux state), the broadband spectral energy distribution (SED) is well fitted using one zone leptonic emission model involving synchrotron, synchrotron self Compton (SSC) and external Compton (EC) processes. However, the flaring state (60260-60310\,MJD) during which the source showed VHE emission requires an additional component. We show that the inclusion of the photo-meson process can successfully explain this excess $γ$-ray emission. Further the estimated parameters, also suggest the source is transparent to VHE gamma-rays against pair production process.

astro-ph.HE↗

arXiv2024

uGMRT observation of the unidentified PeVatron candidate LHAASO J2108+5157

Recent observations by the Large High Altitude Air Shower Observatory (LHAASO) detected Ultra High Energy (UHE) photons in the range 100 TeV to 1.4 PeV from twelve sources including Crab nebula. The detection of these photons demands the presence of at least PeV energy particle in the source. It is important to understand particle acceleration and radiation emission processes in such source. One of those twelve sources, LHAASO J2108+5157 does not show any association or counterparts at any other wavelength. In search of counterpart, we surveyed the region with Giant Metrewave Radio Telescope (GMRT) at 650 MHz frequency. GMRT observation revel radio emission from an extended source within the PSF of LHAASO which shows disk-jet morphology. Considering the spatial association and extent of the source, it is plausible that particle acceleration to PeV energies originates from this source.

astro-ph.HE↗

arXiv2024

Multi-wavelength spectral modelling of the candidate neutrino blazar PKS 0735+178

The BL Lac object PKS 0735+178 was in its historic $γ$-ray brightness state during December 2021. This period also coincides with the detection of a neutrino event IC211208A, which was localized close to the vicinity of PKS 0735+178. We carried out detailed $γ$-ray timing and spectral analysis of the source in three epochs (a) quiescent state ($E_{1}$), (b) moderate activity state ($E_{2}$) and (c) high activity state ($E_{3}$) coincident with the epoch of neutrino detection. During the epoch of neutrino detection ($E_{3}$), we found the largest variability amplitude of 95%. The $γ$-ray spectra corresponding to these three epochs are well fit by the power law model and the source is found to show spectral variations with a softer when brighter trend. In the epoch $E_{3}$, we found the shortest flux doubling/halving time of 5.75 hrs. Even though the spectral energy distribution in the moderate activity state and in the high activity state could be modeled by the one-zone leptonic emission model, the spectral energy distribution in the quiescent state required an additional component of radiation over and above the leptonic component. Here we show that a photo-meson process was needed to explain the excess $γ$-ray emission in the hundreds of GeV which could not be accounted for by the synchrotron self-Compton process.

astro-ph.HE↗

arXiv2023

Understanding the Very High Energy γ-ray excess in nearby blazars using leptonic model

The availability of simultaneous X-ray and Very High Energy (VHE) observations of blazars helps to identify the plausible radiative contributors to the VHE emission. Under leptonic scenario, the VHE emission from BL Lacs are attributed to the synchrotron self Compton (SSC) emission. However, many BL Lacerate (BL Lacs) have shown significant hardening at VHE after correction for the Extra Galactic Background Light (EBL) attenuation. We study the spectral hardening of two nearby BL Lac objects, Mkn 421 and Mkn 501 having most number of simultaneous X-ray and VHE observations available among all the blazars. These BL Lacs are relatively close and the effect of EBL attenuation is relatively minimal/negligible. We study the scatter plot between the X-ray spectral indices and intrinsic VHE indices to identify the plausible origin of the VHE emission. For Mkn 501, the VHE spectral indices are steeper than X-ray spectra, suggesting the scattering process happening at extreme Klein-Nishina regime. On the other hand, for Mkn 421, the VHE spectra is remarkably harder than the X-ray spectra, which suggests an additional emission mechanism other than the SSC process. We show this hard VHE spectrum of Mkn 421 can be explained by considering the inverse Compton (IC) emission from a broken power law electron distribution with Maxwellian pileup. The possibility of the hadronic contribution at VHE γ-rays is also explored by modeling the hard spectrum under photomeson process.

astro-ph.HE↗

arXiv2021

Multiwavelength monitoring of NGC 1275 over a decade: Evidence of a shift in synchrotron peak frequency and long-term multi-band flux increase

We carried out a detailed study of the temporal and broadband spectral behaviour of one of the brightest misaligned active galaxies in gamma-rays, NGC 1275 utilising 11 years of Fermi, and available Swift and AstroSat observations. Based on the cumulative flux distribution of the gamma-ray lightcurve, we identified four distinct activity states and noticed an increase in the baseline flux during the first three states. Similar nature of the increase in the average flux was also noticed in X-ray and UV bands. A large flaring activity in gamma-rays was noticed in the fourth state. The source was observed twice by AstroSat for shorter intervals (~days) during the longer observing periods (~years) state 3 and 4. During AstroSat observing periods, the source gamma-ray flux was higher than the average flux observed during longer duration states. The increase in the average baseline flux from state 1 to state 3 can be explained considering a corresponding increase of jet particle normalisation. The inverse Comptonisation of synchrotron photons explained the average X-ray and gamma-ray emission by jet electrons during the first three longer duration states. However, during the shorter duration AstroSat observing periods, a shift of the synchrotron peak frequency was noticed, and the synchrotron emission of jet electrons well explained the observed X-ray flux.

astro-ph.HE↗

arXiv2017

Sensitivity estimate of the MACE gamma ray telescope

The MACE (Major Atmospheric Cherenkov Experiment) is a 21 m diameter gamma-ray telescope which is presently being installed at Hanle in Ladakh, India (32^0 46^' 46^" N, 78^0 58^' 35^" E) at an altitude of 4270 m a.s.l. Once operational, it will become the highest altitude very high energy (VHE) gamma-ray telescope in the world based on Imaging Atmospheric Cherenkov Technique (IACT). In the present work, we discuss the sensitivity estimate of the MACE telescope by using a substantially large Monte Carlo simulation database at 5^0 zenith angle. The sensitivity of MACE telescope is estimated by carrying out the gamma-hadron segregation using the Random Forest method. It is estimated that the MACE telescope will have an analysis energy threshold of 38 GeV for image intensities above 50 photoelectrons. The integral sensitivity for point like sources with Crab Nebula-like spectrum above 38 GeV is ~2.7% of Crab Nebula flux at 5 sigma statistical significance level in 50 hrs of observation.

astro-ph.IM↗

arXiv2015

The QPO states of 4U 1630-47

Among the transient black hole binary systems, 4U 1630-47 is one of the most active sources exhibiting outbursts every few hundred days, with every outburst lasting typically around hundred days. During the 2002-2004 outburst the appearance of quasi-periodic oscillations (QPOs) coincide with the onset of anomalous state. There are two distinct QPO states: namely the single QPO state with one QPO and the twin QPO state with two QPOs not related harmonically. The spectral features of this state are corroborated by a previous outburst in 1998. The evolution of the inner disc temperature and the inner disc radius suggest the possible onset of geometrically thicker, slim disc as a possible explanation of the energy spectral features. The other possibilities involving the Comptonizing cloud also exist that may explain the anomalous state. The two different QPO states exhibit different spectral features, and we provide a possible empirical physical scenario from the observation of the evolution of the spectral features.

astro-ph.HE↗

arXiv2014

Unveiling the Nature of an X-ray flare from 3XMM J014528.9+610729: A candidate spiral galaxy

We report an X-ray flare from 3XMM J014528.9+610729, serendipitously detected during the observation of the open star cluster NGC 663. The colour-colour space technique using optical and infrared data reveals the X-ray source as a candidate spiral galaxy. The flare shows fast rise and exponential decay shape with a ratio of the peak and the quiescent count rates of $\sim$60 and duration of $\sim$5.4 ks. The spectrum during the flaring state is well fitted with a combination of thermal ({\sc Apec}) model with a plasma temperature of $\rm{1.3\pm0.1}$ keV and non-thermal ({\sc Power-law}) model with power-law index of $\rm{1.9\pm0.2}$. However, no firm conclusion can be made for the spectrum during the quiescent state. The temporal behavior, plasma temperature and spectral evolution during flare suggest that the flare from 3XMM J014528.9+610729 can not be associated with tidal disruption events.

astro-ph.HE↗

arXiv2014

The XMM-Newton view of the radio loud narrow line Seyfert1 galaxy PMN J0948+0022

We analysed the archival XMM-Newton data of the radio loud narrow line Seyfert1 galaxy PMN J0948+0022 in the energy range 0.3--10.0 keV. The X-ray data reveal that the spectrum in the 0.3--10.0 keV energy band is not a simple power-law as previously described in the literature. Instead it consists of a power-law with soft excess below 2.5 keV. The X-ray spectrum was fitted with four different models and it was shown the soft excess component of the spectrum in the 0.3--2.5 keV energy range could be described reasonably well within the framework of the thermal Comptonization model as well as relativistically blurred reflection model. The power-law component required to fit the spectrum beyond 2.5 keV was found to be rather hard compared to the ones observed in other unobscured active galactic nuclei. It is also shown that the Swift/XRT spectrum from the source could not reveal the soft excess component due to poor statistics. The fractional variability estimated from XMM-Newton data indicates the presence of independently varying components in the spectrum above and below 1 keV.

astro-ph.HE↗

arXiv2011

Time evolution of probability density function of gamma ray burst (GRB) - a possible indication of turbulence origin of GRB

Gamma ray burst (GRB) time series is a non-stationary time series with all its statistical properties varying with time. Considering that each GRB is a different manifestation of the same stochastic process we studied the time dependent as well as time averaged probability density function (\emph{pdf}) characterizing the underlying stochastic process. The \emph{pdf}s are fitted with Gaussian distribution function and it has been argued that the Gaussian \emph{pdf}s possibly indicate the turbulence origin of GRB. The spectral and temporal evolution of GRBs are also studied through the evolution of spectral forms, color-color diagrams and hysteresis loops. The results do not contradict the turbulence interpretation of GRB.

astro-ph.HE↗

arXiv2011

Low frequency radio spectrum of LS 5039 during periastron and apastron passages

We have recently studied LS 5039, a gamma-ray binary, with Giant Meterwave Radio Telescope (GMRT) during its periastron and apastron passage. The results presented here show that the spectra are inverted at the low frequency and the flux densities do not differ significantly for two different orbital phases. Assuming that the free-free absorption of radio in stellar wind environment is responsible for the optically thick radio emission we calculated the free-free absorption optical depth and constrained the height of the radio emitting region from the orbital plane. The height is found to be around 1.6 AU for a spherical stellar wind geometry. This estimate may change if the stellar wind is focussed or the radio absorption is due to synchrotron self-absorption.

astro-ph.HE↗

arXiv2008

Low frequency radio spectrum and spectral turnover of LS 5039

LS 5039, a possible black hole x-ray binary, was recently observed with Giant Meterwave Radio Telescope. The observed spectrum presented here shows that the spectrum is inverted at the low frequency. When combined with the archival data with orbital phase similar to the present observations, it shows a clear indication of a spectral turnover. The combined data are fitted with a broken power-law and the break frequency signifies a possible spectral turnover of the spectrum around 964 MHz. Truly simultaneous observations in radio wavelength covering a wide range of frequencies are required to fix the spectrum and the spectral turn over which will play a crucial role in developing a deeper understanding of the radio emitting jet in LS 5039.

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