arXiv subjects

Athira M Bharathan

Publications and source records attributed to Athira M Bharathan.

4 recordsLinked to original sources

arXiv2026

Multi-wavelength Behaviour and Lepto-hadronic Modeling of PKS 0215+015 Around the Epoch of IceCube-220225A

The detection of high-energy astrophysical neutrinos has opened a new avenue for identifying cosmic particle accelerators. Blazars, active galactic nuclei (AGN) with relativistic jets oriented close to the observer's line of sight, are promising sites of hadronic interactions capable of producing PeV-scale neutrinos. We investigate the flat-spectrum radio quasar (FSRQ) PKS~0215+015 ($z \simeq 1.715$), positionally consistent with the \textit{IceCube-220225A} neutrino event of estimated energy $\sim$154~TeV. We combine \textit{Fermi}-LAT $γ$-ray data with Swift-XRT and Swift-UVOT observations to characterise the source around the neutrino epoch. PKS~0215+015 exhibited pronounced $γ$-ray, X-ray and optical activity in early 2022, consistent with a disturbance propagating along the jet. We construct broadband spectral energy distributions (SEDs) for the flaring interval and model them using leptonic and lepto-hadronic frameworks. The SED is well reproduced by a leptonic model comprising synchrotron, synchrotron self-Compton and external Compton emission, indicating that electron processes dominate. A co-accelerated proton population remains radiatively subdominant but requires a proton kinetic luminosity roughly two orders of magnitude above equipartition with the magnetic field. The predicted neutrino flux is well below that implied by a single \textit{IceCube-220225A}-like detection. Independent analysis of short-term $γ$-ray variability yields an emission-region size consistent with the SED fit, supporting the model's physical self-consistency. We discuss the temporal and energetic plausibility of the association between PKS~0215+015 and \textit{IceCube-220225A} and implications for future multimessenger studies of high-redshift FSRQs.

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.

astro-ph.HE↗

arXiv2025

Clues on the X-ray emission mechanism of blazars PKS 2155$-$304 and 3C 454.3 through polarization studies

X-ray polarization measurable with the imaging X-ray Polarimetry Explorer (\textit{IXPE}) could constrain the long-debated leptonic versus hadronic origin of the high-energy component in the broadband spectral energy distribution (SED) of blazars. We report \textit{IXPE} results and SED modeling of PKS 2155$-$304 and 3C 454.3, a high- and low-synchrotron-peaked blazar. For PKS 2155$-$304, model-independent analysis gives polarization angle $Ψ_X$ = (130$\pm$2.5) deg and polarization degree $Π_X$ = (20.9$\pm$1.8)\% in the 2$-$8 keV band, in agreement with spectro-polarimetric analysis. We found $Π_X$ varies with time and shows indications of energy dependence, suggesting stratified emission regions. For 3C 454.3, no X-ray polarization is detected in the June 2023 observation, analyzed here for the first time. The detection in PKS 2155$-$304 and non-detection in 3C 454.3 are consistent with X-ray emission from synchrotron and inverse Compton processes, respectively. Dividing the dataset into finer time bins allows a more granular view of polarization variability. We modeled the broadband SEDs using quasi-simultaneous optical, UV, and X-ray data from {\it Swift}, {\it AstroSat}, and $γ$-rays from {\it Fermi}. In PKS 2155$-$304, X-rays lie in the high-energy tail of the synchrotron component, while in 3C 454.3 they lie in the rising part of the inverse Compton component. Our SED modeling with X-ray polarization favors a leptonic scenario for PKS 2155$-$304. These results support a structured jet model where X-ray emission originates from a compact acceleration zone near the shock front, while lower-energy optical emission comes from a broader turbulent region.

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↗