arXiv subjects

Agnibha De Sarkar

Publications and source records attributed to Agnibha De Sarkar.

12 recordsLinked to original sources

arXiv2026

Temporal evolution of the periodic GeV signal from 4FGL J1913.2+0512 and analysis of the SS 433 / W50 lobes

SS 433 is a microquasar whose relativistic jets precess every ~162 days, providing a laboratory for jet-interstellar medium interactions. We present a comprehensive analysis of 16 years of Fermi Large Area Telescope data (August 2008-September 2024) of the SS 433/W50 field, using events in the 0.3-300 GeV range and employing pulsar gating to mitigate contamination from the bright nearby pulsar PSR J1907+0602. We detect the GeV source 4FGL J1913.2+0512 (TS = 45, where TS denotes the likelihood-ratio Test Statistic) with a power-law spectrum (photon index 2.61 +- 0.08) and confirm a GeV excess at the western lobe (TS = 17). The eastern lobe of SS 433 is hinted at with lower significance. One additional GeV excess, Fermi J1909.6+0552 (TS = 20; TS = 28 over 0.1-300 GeV), located outside the SS 433 / W50 system, is revealed after gating. Exposure-corrected Lomb-Scargle periodograms and precessional phase-folded light curves show a ~162-day modulation in 4FGL J1913.2+0512. This periodicity is prominent during the first 10 years of the mission (2008-2018) but disappears thereafter, with the phase-folded flux concentrated in precessional phases 0.0-0.5. Over the full 16-year dataset, the modulation remains detectable but with reduced significance, consistent with dilution by the later non-modulated epoch. These results indicate that the efficiency and/or geometry of gamma-ray production in the SS 433 environment evolves on multi-year timescales.

astro-ph.HE↗

arXiv2026

HESS J1832$-$085: evidence for a new gamma-ray binary candidate

The Galactic plane survey conducted by the High Energy Stereoscopic System (H.E.S.S.) has revealed numerous teraelectronvolt (TeV) sources, many of which remain unidentified. HESS~J1832$-$085 is a point-like TeV source lacking a confirmed multiwavelength (MWL) counterpart. In this paper, we present evidence that HESS~J1832$-$085 is likely a gamma-ray binary. We aim to investigate the nature of HESS~J1832$-$085 using {\it Fermi}-LAT and X-ray data, complemented by broadband radiative modeling, to assess its classification as a potential gamma-ray binary. We analyzed $\sim$17.3~yr of {\it Fermi}-LAT data between 0.1 and 500~GeV to establish the gigaelectronvolt (GeV) counterpart of HESS~J1832$-$085, including performing spectral, spatial, and periodicity analyses. Archival X-ray observations were examined to search for a counterpart and to characterize its spectrum and potential variability. The broadband emission was interpreted using models commonly applied to gamma-ray binaries. We detect a point-like GeV gamma-ray source spatially consistent with HESS~J1832$-$085, with spectral properties compatible with known gamma-ray binaries. No significant GeV periodic modulation is detected. A potential X-ray counterpart is identified in archival X-ray data, exhibiting a hard, absorbed spectrum and moderate variability. The broadband spectral energy distribution is reproduced by the adopted binary radiative model. Our results indicate that HESS~J1832$-$085 is likely a gamma-ray binary candidate, motivating dedicated MWL follow-up observations to confirm the source nature.

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arXiv2026

Investigating the emission signatures of pulsar halo candidate HESS J1813-126

Extended gamma-ray sources surrounding middle-aged pulsars, primarily observed at teraelectronvolt energies, have been interpreted as pulsar halos, where relativistic $e^\pm$ diffuse into the interstellar medium and produce inverse-Compton (IC) emission. HESS J1813-126, associated with the energetic, radio-quiet gamma-ray pulsar PSR J1813-1246, has been suggested as a candidate pulsar halo, though its nature remains uncertain. We interpreted the high-energy emission of PSR J1813-1246 using the synchro-curvature (SC) radiation model and tested whether the gamma-ray spectral energy distribution (SED) of HESS J1813-126 can be explained as a pulsar halo powered by PSR J1813-1246. We explain the X-ray and gamma-ray SEDs of the pulsar using the SC framework. We further computed the transport and losses of $e^\pm$ injected by the pulsar through time-dependent diffusion-loss equations, exploring various common pulsar halo transport models. The resulting IC emission was compared with \textit{Fermi}-LAT, H.E.S.S., HAWC, and LHAASO data. We present predictions for the surface brightness profiles (SBPs) and the aperture-dependent emission for the different transport models, providing key diagnostics for assessing the observability of HESS J1813-126 with current and future instruments. The SC framework successfully reproduces the emission of PSR J1813-1246. The SED of HESS J1813-126 can be consistently reproduced within different pulsar halo frameworks, albeit with distinct predictions across different transport models. The corresponding SBP predictions and aperture-dependent emission offer testable signatures for future imaging atmospheric Cherenkov telescopes, which will be crucial for discriminating between the transport models. We further examined the link between the pulsar central engine and its extended halo by comparing the pair multiplicities in the magnetospheric and halo regions.

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arXiv2025

On the orbital variability of gamma-ray emission in $γ^2$ Velorum

Colliding wind binaries (CWBs) are promising sources of high-energy gamma-ray emission driven by shock acceleration of particles at wind interaction zones. The nearby CWB system $γ^2$ Velorum (WR 11), composed of a Wolf-Rayet (WR) and an O-star, has been recently associated with GeV gamma-ray emission observed by Fermi-LAT, including evidence of orbital variability. This offers a valuable opportunity to test models of phase-dependent hadronic emission and absorption in CWBs. We aim to explain both the spectral energy distribution (SED) and orbital variability of gamma-ray emission from $γ^2$ Velorum using a physically motivated phase-dependent hadronic model. We consider the injection of accelerated relativistic protons based on the WR wind's kinetic energy intercepted at the wind collision region (WCR), and calculate the resulting phase-dependent hadronic gamma-ray emission assuming a proton conversion efficiency $η_p$ and accounting for energy-dependent diffusion, advection, conical shock interception and the evolution of the effective acceleration volume, assumed to scale with the WCR, with the orbital phase. Gamma-ray emission from hadronic interactions is attenuated by $γ$ - $γ$ absorption, calculated via full angular integration over both stellar photon fields. Our model successfully reproduces the observed SED and is consistent with the apastron-to-periastron flux ratio, resulting in a dip in emission at periastron passage and an increase during apastron. Our findings support the conclusion that the observed orbital modulation is primarily driven by geometric variations of the WCR. This underscores the significant influence of evolving orbital geometry on the high-energy gamma-ray light curves of $γ^2$ Velorum.

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arXiv2023

Dissecting the emission from LHAASO J0341+5258: implications for future multi-wavelength observations

The Large High Altitude Air Shower Observatory (LHAASO) has detected multiple ultra-high energy (UHE; E$_γ\ge$ 100 TeV) gamma-ray sources in the Milky Way Galaxy, which are associated with Galactic ``PeVatrons'' that accelerate particles up to PeV (= 10$^{15}$ eV) energies. Although supernova remnants (SNRs) and pulsar wind nebulae (PWNe), as source classes, are considered the leading candidates, further theoretical and observational efforts are needed to find conclusive proof to confirm the nature of these PeVatrons. This work aims to provide a phenomenological model to account for the emission observed from the direction of LHAASO J0341+5258, an unidentified UHE gamma-ray source observed by LHAASO. 15 years of Fermi-LAT data was analyzed to find the high energy (HE; 100 MeV $\le$ E$_γ$ $\le$ 100 GeV) GeV gamma-ray counterpart of LHAASO J0341+5258, in the 4FGL-DR3 catalog. We have explained the spectrum of the closest 4FGL source, 4FGL J0340.4+5302, by a synchro-curvature emission formalism typically used in the case of GeV pulsars. Escape-limited hadronic interaction between protons accelerated in an old, now invisible SNR and cold protons inside associated molecular clouds (MCs) and leptonic emission from a putative TeV halo was explored to explain the multi-wavelength (MWL) spectral energy distribution (SED) observed from the LHAASO source region. We have further discussed possible observational avenues that can be explored in the near future and predicted the outcome of those observational efforts from the model explored in this paper.

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arXiv2023

Lepto-hadronic interpretation of 2021 RS Ophiuchi nova outburst

Very high energy (VHE; 100 GeV $<$ E $\leq$ 100 TeV) and high energy (HE; 100 MeV $<$ E $\leq$ 100 GeV) gamma-rays were observed from the symbiotic recurrent nova RS Ophiuchi (RS Oph) during its outburst in August 2021, by various observatories such as High Energy Stereoscopic System (H.E.S.S.), Major Atmospheric Gamma Imaging Cherenkov (MAGIC), and {\it Fermi}-Large Area Telescope (LAT). The models explored so far tend to favor a hadronic scenario of particle acceleration over an alternative leptonic scenario. This paper explores a time-dependent lepto-hadronic scenario to explain the emission from the RS Oph source region. We have used simultaneous low frequency radio data observed by various observatories, along with the data provided by H.E.S.S., MAGIC, and \textit{Fermi}-LAT, to explain the multi-wavelength (MWL) spectral energy distributions (SEDs) corresponding to 4 days after the outburst. Our results show that a lepto-hadronic interpretation of the source not only explains the observed HE-VHE gamma-ray data but the corresponding model synchrotron component is also consistent with the first 4 days of low radio frequency data, indicating the presence of non-thermal radio emission at the initial stage of nova outburst. We have also calculated the expected neutrino flux from the source region and discussed the possibility of detecting neutrinos.

astro-ph.HE↗

arXiv2023

Supernova connection of unidentified ultra high energy gamma-ray source LHAASO J2108+5157

We present a simple phenomenological model of hadronic interaction between protons accelerated in an old supernova remnant (SNR) and cold protons situated within the associated molecular clouds (MCs). The accelerated protons from the old SNR escaped the SNR shock front, and got injected into the MCs at an earlier time, producing ultra high energy gamma-rays and neutrinos through inelastic proton-proton interaction. We also take into account the acceleration and subsequent escape of electrons from the SNR shock front. The escaped electrons produce gamma-rays through various radiative cooling mechanisms, after getting injected into the MCs. We use the model discussed in this letter to explain the multiwavelength (MWL) spectral energy distribution (SED) of unidentified Galactic ultra high energy gamma-ray source LHAASO J2108+5157. We also discuss the feasibility of applying this model in other cases as well. Future observations can test the viability of the model discussed in this letter, which will in turn confirm that the SNRs can, in fact, accelerate particles up to PeV energies.

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arXiv2022

LHAASO J2226+6057 as a pulsar wind nebula

The Large High Altitude Air Shower Observatory has reported the detection of cosmic-ray sources in Milky Way that can accelerate particles up to PeV (= 10$^{15}$ eV) energies. These sources, so called ``PeVatrons'', are mostly unidentified. Several classes of sources, such as supernova remnants, pulsar wind nebula, or young stellar clusters can potentially be the counterparts of these PeVatrons. The aim of this work is to study a pulsar wind nebula interpretation of one of these PeVatrons, LHAASO J2226+6057, which has a relatively well covered multi-frequency spectrum. We have performed a leptonic, time-dependent modeling of the pulsar wind nebula (PWN) associated with PSR J2229+6114 considering a time-energy-dependent diffusion-loss equation. Injection, energy losses, as well as escape of particles were considered to balance the time-dependent lepton population. We have also included the dynamics of the PWN and the associated supernova remnant (SNR) and their interaction via the reverse shock to study the reverberation phase of the system. We have considered different values of braking index ($n$) and true age ($t_{age}$) for the fitting of the multi-wavelength (MWL) spectral energy distribution (SED) of LHAASO J2226+6057. The best-fit PWN model parameters and their 1$σ$ confidence intervals were evaluated. We have also demonstrated the impact of reverberation on the MWL SED with increasing time. Additionally, we have discussed the resultant large radius and low magnetic field associated with the PWN in question, as caveats for the possible physical connection of the pulsar as the origin of this high energy source.

astro-ph.HE↗

arXiv2022

Exploring the hadronic origin of LHAASO J1908+0621

Recent observations by the Large High Altitude Air Shower Observatory (LHAASO) have paved the way for the observational detection of PeVatrons in the Milky Way Galaxy, thus revolutionizing the field of $γ$-ray astrophysics. In this paper, we study one such detected source, LHAASO J1908+0621, and explore the origin of multi-TeV $γ$-ray emission from this source. A middle-aged radio supernova remnant SNR G40.5-0.5 and a GeV pulsar PSR J1907+0602 are co-spatial with LHAASO J1908+0621. Dense molecular clouds are also found to be associated with SNR G40.5-0.5. We explain the multi-TeV $γ$-ray emission observed from the direction of LHAASO J1908+0621, by the hadronic interaction between accelerated protons that escaped from the SNR shock front and cold protons present inside the dense molecular clouds, and the leptonic emission from the pulsar wind nebula (PWN) associated with the pulsar J1907+0602. Moreover, we explain lower energy $γ$-ray emission by considering the radiative cooling of the electrons that escaped from SNR G40.5-0.5. Finally, the combined lepto-hadronic scenario was used to explain the multi-wavelength spectral energy distribution (SED) of LHAASO J1908+0621. Although not yet significant, an ICECUBE hotspot of neutrino emission is spatially associated with LHAASO J1908+0621, indicating a possible hadronic contribution. In this paper, we show that if a hadronic component is present in LHAASO J1908+0621, then the second generation ICECUBE observatory will detect neutrino from this source.

astro-ph.HE↗

arXiv2022

Galactic molecular clouds as sources of secondary positrons

Secondary positrons produced inside Galactic Molecular Clouds (GMCs) can significantly contribute to the observed positron spectrum on Earth. Multi-wavelength data of GMCs are particularly useful in building this model. A very recent survey implemented the optical/IR dust extinction measurements to trace 567 GMCs within 4 kpc of Earth, residing in the Galactic plane. We use the updated catalog of GMCs reported in recent papers, distributed in the Galactic plane, to find the secondary positrons produced in them in interactions of cosmic rays with molecular hydrogen. Moreover, by analyzing the \textit{Fermi}-LAT data, new GMCs have been discovered near the Galactic plane. We also include some of these GMCs closest to the Earth, where cosmic ray interactions produce secondaries. It has been speculated earlier that cosmic rays may be reaccelerated in some GMCs. We select 7 GMCs out of 567 GMCs recently reported, within 4 kpc of Earth, where reacceleration due to magnetized turbulence is assumed. We include a hardened component of secondary positrons produced from the interaction of reaccelerated CRs in those 7 GMCs. We use publicly available code \texttt{DRAGON} for our simulation setup to study CR propagation in the Galaxy and show that the observed positron spectrum can be well explained in the energy range of 1 to 1000 GeV by our self-consistent model.

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arXiv2022

Possible TeV $γ$-ray binary origin of HESS J1828-099

The High Energy Stereoscopic System (H.E.S.S.) observatory has carried a deep survey of the Galactic plane, in the course of which the existence of a significant number of ($\sim$ 78) TeV $γ$-ray sources was confirmed, many of which remain unidentified. HESS J1828-099 is a point-like (Gaussian stand. dev. $<$ 0.07$^{\circ}$) unidentified source among the 17 confirmed point-like sources in the H.E.S.S. Galactic Plane Survey (HGPS) catalog. This source is also unique because it does not seem to have any apparent association with any object detected at other wavelengths. We investigate the nature and association of HESS J1828-099 with multi-wavelength observational data. A high mass X-Ray binary (HMXB) - comprising of pulsar XTE J1829-098 and a companion Be star - has been observed earlier in the X-ray and infrared bands, 14$'$ away from HESS J1828-099. With 12 years of $\textit{Fermi}$-LAT $γ$-ray data, we explore the possibility of 4FGL J1830.2-1005 being the GeV counterpart of HESS J1828-099. Within the RXTE confidence region, a steep spectrum ($α_{radio}$ = - 0.746 $\pm$ 0.284), plausible counterpart is detected in data from existing radio frequency surveys. In this letter, we probe for the first time using multi-wavelength data, whether HESS J1828-099, 4FGL J1830.2-1005 and the HMXB system have a common origin. Our study indicates that HESS J1828-099 might be a TeV high mass $γ$-ray binary source.

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arXiv2020

Positron Excess from Cosmic Ray Interactions in Galactic Molecular Clouds

The recently observed data by AMS-02 clearly confirms that the positron flux rises with energy and shows a peak near a few hundred GeV. This rising positron flux cannot be explained by interactions of cosmic rays with interstellar hydrogen gas. In this paper, our goal is to study whether secondary production due to cosmic ray interactions in nearby Galactic Molecular Clouds (GMCs) can contribute significantly to the observed positron spectrum on Earth. Due to the progress in multi-wavelength astronomy, many new GMCs have been discovered in our Galaxy recently. Using large scale CO survey, 1064 GMCs were detected in the Galaxy, which reside in the Galactic plane. Very recent survey implemented the optical/IR dust extinction measurements, to trace 567 GMCs within 4 kpc of Earth, also residing in the Galactic plane. We use the updated list of GMCs reported in recent papers, which are distributed in the Galactic plane, to find the secondary positrons produced in them in interactions of cosmic rays with molecular hydrogen. Moreover, by analysing the \textit{Fermi}-LAT data, new GMCs have been discovered near the Galactic plane. We also include some of these GMCs closest to the Earth where cosmic ray interactions are producing secondaries. It has been speculated earlier that cosmic rays may be reaccelerated in some GMCs. We select 7 GMCs out of 567 GMCs recently reported, within 4 kpc of Earth, where reacceleration due to magnetized turbulence is assumed. We include a hardened component of secondary positrons, produced from interaction of reaccelerated CRs in those 7 GMCs. We use publicly available code \textbf{DRAGON} for our simulation setup to study CR propagation in the Galaxy and show that the observed positron spectrum can be well explained in the energy range of 1 to 1000 GeV by our self consistent model.

astro-ph.HE↗