arXiv subjects

Sarira Sahu

Publications and source records attributed to Sarira Sahu.

At least 19 recordsLinked to original sources

arXiv2026

Rapid Variability and Broadband Spectral Modeling in the Flaring Activity of BL Lacertae

We report a multi-wavelength study of two flaring episodes of the blazar BL Lacertae during MJD 60500-60800 (9 July 2024 - 5 May 2025). The source reached a daily-averaged $\gamma$-ray flux of $(1.03 \pm 0.05) \times 10^{-5} \, \mathrm{ph \, cm^{-2} \, s^{-1}}$ ($E > 100$ MeV) on MJD 60588 (5 October 2024). Using orbit-binned data from the Large Area Telescope (LAT) onboard the \textit{Fermi Gamma-ray Space Telescope}, we identify a minimum flux halving timescale of $\tau = 1.33 \pm 0.29$ hr. This constrains the upper limit on the $\gamma$-ray emitting region size to $R \le 2.0 \times 10^{15}$ cm, as well as its distance from the central supermassive black hole to $R_\mathrm{H} \le 5.9 \times 10^{16}$ cm, assuming a Doppler factor of $\delta = 14.8$ derived from the spectral energy distribution (SED) modeling. We find tentative evidence for sub-minute $\gamma$-ray variability with a minimum doubling time of $0.7 \pm 0.2$ min ($p$-value = 0.03). This may originate from an extremely compact region with a size of $R \le 1.8 \times 10^{13}$ cm, suggesting that the emission arises from magnetohydrodynamic substructures, such as plasmoids within a magnetic reconnection zone. Spectral analysis reveals a significant ``softer-when-brighter'' trend ($r = 0.96, p = 4.5 \times 10^{-4}$) during the minute-scale flare peaks, indicating a complex interplay between particle acceleration and radiative cooling. The SED is reproduced using a one-zone leptonic model, in which synchrotron self-Compton (SSC) and external Compton (EC) scattering effectively account for the high-energy emissions. The reduced magnetic field strengths and hard electron injection spectral indices observed during the flaring states suggest enhanced particle acceleration efficiency, possibly associated with relativistic magnetic reconnection.

astro-ph.HE↗

arXiv2026

Temporary EHBL-like behavior of Markarian 501 during July 2014 VHE flaring

Markarian 501, a BL Lac object well-known as a high energy gamma-ray source, has exhibited several epochs of very high energy (VHE) gamma-ray flaring events when its synchrotron peak frequency shifted above $10^{17}$ Hz, a signature of extreme behavior. From July 16 to July 31, 2014 such flaring events were observed for 15 days by various telescopes. On July 19 (MJD 56857.98), the X-ray outburst from the source was at its highest and on the same day an intriguing narrow peak-like feature around 3 TeV was observed by the MAGIC telescopes, a feature inconsistent with standard interpretations. Using the well-known two-zone photohadronic model, we study these VHE gamma-ray spectra on a day-by-day basis and offer explanation. Our two-zone photohadronic scenario shows that, on MJD 56857.98, the peak-like feature appears at a cutoff energy of $E^c_γ=3.18$ TeV. Below this energy the VHE spectrum increases slowly and is in high emission state. However, for $E^c_γ\, > 3.18$ TeV, the spectrum falls faster, resulting in a mild peak-like feature, not prominent enough as claimed by the MAGIC collaboration.

astro-ph.HE↗

arXiv2026

Fermion scattering in a Bose-Einstein condensate

In a recent work, we considered the propagation of fermions in the background of a scalar Bose-Einstein (BE) condensate. Using some illustrative Yukawa-type coupling models between the fermions and the scalar fields, we determined the dispersion relations of the fermions and the scalar modes in various models. To complement that work, here we consider the corresponding fermion spinors and propagators, which are required for the calculation of rates of processes involving the fermions, as well as the thermal, and/or higher order, corrections to such rates. We obtain and present here concise formulas which are useful for those applications. As an application and for illustrative purposes we specifically calculate the rate for a generic fermion (which we denote by $χ$) with the fermions in the BE background, using commonly used models for the fermion interactions. Due to the fact that the background fermion dispersion relations are helicity dependent, the kinematics have some unique and non-standard features. For example, at a particular value of the momentum one of the fermion modes has zero group velocity, which leads to a singularity in the scattering rate of the type of the Van Hove singularity in the density of states of some condensed matter systems. The results of the framework presented here, besides their merit in their own right, can be useful in specific contexts and applications, such as cosmic-ray electron cooling through dark matter-electron scattering, and similar ones involving neutrino and/or electron propagation in a scalar Dark Matter background.

hep-ph↗

arXiv2025

Plasmon damping in a charged Bose-Einstein condensate model

In this work we consider the calculation of the imaginary part of the dispersion relations of the propagating modes in a model of a charged scalar Bose-Einstein (BE) condensate, as well as the contribution to the imaginary part of the longitudinal component of the photon polarization tensor and the dielectric constant. In that model, two modes correspond to the transverse photon polarizations, while the other two modes are combinations of the longitudinal photon and the massive scalar field, which we denote as the $(\pm)$ modes. The dispersion relations of the transverse modes have the usual form for transverse photons in a plasma, and we do not consider here any further. In a previous work we determined the real part of the dispersion relations of the $(\pm)$ modes, as well as the real part of the longitudinal component of of polarization tensor and dielectric constant, which have some unique features. In the appropriate limit, those results reproduce the results obtained for the dielectric constant and dispersion relation in non-relativistic models of the BE condensation of charged scalars. Here we determine, in the same model, their imaginary part. The results can be useful in physical contexts involving the electrodynamics of a charged scalar BE condensate, and can serve as benchmark results for further exploration.

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arXiv2025

Constraining the redshift of PG 1553+113 using the photohadronic model

Between 2005 and 2015, the BL Lacertae object PG 1553+113 exhibited multiple very high-energy (VHE; > 100 GeV) gamma-ray flares, which were detected by the Cherenkov telescopes, High Energy Stereoscopic System (HESS), Major Atmospheric Gamma Imaging Cherenkov (MAGIC), and the Very Energetic Radiation Imaging Telescope Array System (VERITAS). Despite the uncertainty surrounding its redshift (z), various studies have sought to estimate this value. In this study, seventeen independently observed VHE gamma-ray spectra of PG 1553+113 are analyzed using four distinct extragalactic background light (EBL) models alongside the photohadronic framework. A global $χ^2$ fit is applied to all observational data to determine the best-fitting redshift for each EBL model. Additionally, confidence level (CL) intervals for the redshift are calculated across all EBL models. The findings demonstrate that the photohadronic framework effectively describes all observed spectra. Among the EBL models, the lowest total $χ^2$ value of 58.82 (with 58 degrees of freedom) was obtained using the model from Saldana-Lopez et al. (2021), while the highest value of 75.67 was associated with the model from Dominguez et al. (2011). The 95 per cent CL intervals for the statistical error of the redshift of PG 1553+113 are 0.500 < z < 0.537 for the Saldana-Lopez et al. (2021) model and 0.491 < z < 0.527 for the Dominguez et al. (2011) model. These results highlight the consistency of the photohadronic approach in interpreting the observed VHE gamma-ray spectra.

astro-ph.HE↗

arXiv2024

Photon propagation in a charged Bose-Einstein condensate

We consider the propagation of photons in a model of a charged scalar Bose-Einstein (BE) condensate. We determine the dispersion relations of the collective modes, as well as the photon polarization tensor and the dielectric constant in the model. Two modes correspond to the transverse photon polarizations, with dispersion relations of the usual form for transverse photons in a plasma. The other two modes, denoted as the $(\pm)$ modes, are combinations of the longitudinal photon and the massive scalar field. Their dispersion relations behave very differently as functions of momentum. The $(+)$ mode dispersion relation increases steadily and remains greater than the momentum as the momentum increases. The dispersion relation of the $(-)$ mode decreases in a given momentum range, with the group velocity being negative in that range, while in another range it increases steadily but remains smaller than the momentum, akin to the situation in a medium with an index of refraction greater than 1. We consider the non-relativistic limit of the $(\pm)$ dispersion relations and discuss some aspects of the results. We also determine the wavefunctions of the $(\pm)$ modes, which are useful to obtain the corrections to the dispersion relations, e.g., imaginary parts due to the damping effects and/or the effects of scattering, due to the interactions with the excitations of the system. The results can be useful in various physical contexts that have been considered in the literature involving the electrodynamics of a charged scalar BE condensate.

hep-ph↗

arXiv2024

Redshift constrain of BL Lac PKS 1424+240

In the period between 2009 and 2015, several very high-energy (VHE $> 100$ GeV) gamma-ray flaring events from the BL Lac object PKS 1424+240 were observed by the Cerenkov telescopes VERITAS and MAGIC. It had uncertain redshift ($z$) and using spectroscopical measurement, Paiano et al. (2017) found it to be $z=0.604$. Using four different extragalactic background light (EBL) models and the photohadronic model, nine independently observed VHE gamma-ray spectra of PKS 1424+240 are analyzed and a global $χ^2$ fit is performed on all observations to estimate the best-fit value for the redshift for each EBL model. Confidence levels (CL) intervals for the redshift are also estimated using all the EBL models. This method is tested by comparing our analysis with the observed value. It is shown that the photohadronic scenario provides an excellent description of all the observed spectra. It is found that the EBL model of Domínguez et al. (2011) is the one that provides the most restrictive limits on the redshift of PKS 1424+240, but in our analysis, $z=0.604$ lies within the $3σ$ CL interval of the EBL model of Saldana-Lopez et al. (2021).

astro-ph.HE↗

arXiv2024

TeV afterglow from GRB 221009A: photohadronic origin?

Gamma-ray burst (GRB), GRB 221009A, a long-duration GRB, was observed simultaneously by the Water Cherenkov Detector Array (WCDA) and the Kilometer Squared Array (KM2A) of the Large High Altitude Air Shower Observatory (LHAASO) during the prompt emission and the afterglow periods. Characteristic multi-TeV photons up to 13 TeV were observed in the afterglow phase. The observed very high-energy (VHE) gamma-ray spectra by WCDA and KM2A during different time intervals and in different energy ranges can be explained very well in the context of the photohadronic model with the inclusion of extragalactic background light models. In the photohadronic scenario, interaction of high-energy protons with the synchrotron self-Compton (SSC) photons in the forward shock region of the jet is assumed to be the source of these VHE photons. The observed VHE spectra from the afterglow of GRB 221009A are similar to the VHE gamma-ray spectra observed from the temporary extreme high-energy peaked BL Lac (EHBL), 1ES 2344+514 {\it only} during the 11th and the 12th of August, 2016. Such spectra are new and have been observed for the first time in a GRB.

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arXiv2023

A photohadronic interpretation of the H.E.S.S. afterglow observations from GRB 221009A

The High Energy Stereoscopic System (H.E.S.S.) started observing the extremely powerful long-duration gamma-ray burst, GRB 221009A, after 53 hours of the triggering event. The H.E.S.S. collaboration carried out observations on the 11, 12, and 17, of October 2022 under poor atmospheric conditions, without detecting significant very high-energy photons from the source and computed the upper limits of the fluxes for the different nights. We study these flux upper limits by using the photohadronic model and show that the interaction of high-energy protons with the synchrotron seed photons in the forward shock region of the GRB jet exhibits behavior compatible with the upper limits computed by the H.E.S.S. collaboration.

astro-ph.HE↗

arXiv2023

Model for the propagation of fermions in a Bose-Einstein condensate

We consider the dispersion relations of fermions that propagate in the background of a scalar Bose-Einstein condensate. Some illustrative examples are discussed using simple Yukawa-type coupling models between the fermions and the scalar fields. The dispersion relations are determined explicitly in those cases, to the lowest order. The method also allows to determine the corrections to the dispersion relations due to the interactions with the excitations of the Bose-Einstein condensate. Possible applications of the results to the case of neutrinos are indicated.

hep-ph↗

arXiv2023

Constraining the redshift of BL Lac VER J0521+211

Observation of several very high energy (VHE) flaring events of the BL Lac object VER J0521+211 were reported by the VERITAS and MAGIC collaborations between 2009 and 2014. The redshift of this source is uncertain and several analysis have derived different limits for it. In the framework of the photohadronic model, and using three different extragalactic background light (EBL) models, we analyze seven independent VHE spectra of VER J0521+211 and determine the limiting values on its redshift. It is observed that the photohadronic scenario provides excellent fits to the reported observations. It is further observed that the photohadronic scenario, along with the EBL model of Dominguez et al., puts the most restrictive limits on the redshift z of VER J0521+211: 0.29 <= z <= 0.31 from the confidence level (CL) intervals at 2 sigma, or a more conservative 0.28 <= z <= 0.33, at 3 sigma.

astro-ph.HE↗

arXiv2022

Deciphering the ~18 TeV photons from GRB 221009A

On 9 October, 2022, an extremely powerful gamma-ray burst, GRB 221009A, was detected by several instruments. Despite being obstructed by the Milky Way galaxy, its afterglow outburst outshone all other GRBs seen before. LHAASO detected several thousands very high energy photons extending up to 18 TeV. Detection of such energetic photons are unexpected due to the large opacity of the Universe. It is possible that in the afterglow epoch the intrinsic very high energy photon flux from the source might have increased manifolds, which could compensate the attenuation by pair-production with the extragalactic background light. We propose such a scenario and show that very high energy photons can be observed on the Earth from the interaction of very high energy protons with the seed synchrotron photons in the external forward shock region of the GRB jet.

astro-ph.HE↗

arXiv2022

Very high energy emission mechanism in the extreme blazar PGC 2402248

Extreme high-frequency peaked BL Lacs (EHBLs) are characterized by a synchrotron peak frequency exceeding $10^{17}$ Hz and a second peak that can be in the energy range of few GeVs to several TeVs. The MAGIC telescopes detected multi-TeV gamma-rays on April 19, 2018 for the first time from the EHBL PGC 2402248 which was simultaneously observed in multiwavelength by several other instruments. The broad band spectral energy distribution of the source is conventionally modelled using the leptonic and the hadronic models. Due to the success of the photohadronic model in interpreting the enigmatic very high-energy (VHE) flaring events from many high-energy blazars, we extend this model to explain the VHE events from PGC 2402248 observed by MAGIC telescopes and compare our results with other models. We conclude that the photohadronic fits are comparable and even fare better than most other models. Furthermore, we show that the spectrum is not hard and is in a low emission state. The estimated bulk Lorentz factor for this flaring event is found to be $\lesssim 34$.

astro-ph.HE↗

arXiv2022

Photohadronic interpretation of the different incarnation of 1ES 2344+514

Since its discovery in 1995, the high-energy peaked blazar 1ES 2344+514 has undergone several episodes of GeV-TeV flaring and has been observed in the multiwavelength by several other telescopes. The observed X-ray spectrum of 1996 and the flaring event of 2016 establish that 1ES 2344+514 has a temporary EHBL-like behavior. Such behavior has also been observed in several nearby high-energy peaked blazars. We use the photohadronic model to account for the GeV-TeV flaring observed events of 1995 and 2007. Also, a recently proposed two-zone photohadronic model, which is successful in explaining the multi-TeV flaring events of many transient EHBL-like source, is employed to explain the GeV-TeV flaring spectra of MJD 57611 and MJD 57612. We find that the zone-2 parameters of the two-zone photohadronic model play a central role in explaining these spectra. Probably this is an indication of a new type of transient EHBL-like source. We find that our fits to the observed spectra are comparable or better than the other leptonic and hadronic models employed in the literature to address the same issue.

astro-ph.HE↗

arXiv2022

Neutrino effective potential and damping in a fermion and scalar background in the resonance region

We consider the propagation of a neutrino or an antineutrino in a medium composed of fermions $f$ and scalars $ϕ$ interacting via a Yukawa-type coupling of the form $\bar fνϕ$, for neutrino energies at which the processes like $ν+ ϕ\leftrightarrow f$ or $ν+ \bar f \leftrightarrow \barϕ$, and the corresponding ones for the antineutrino, are kinematically accessible. The relevant energy values are around $|m^2_ϕ- m^2_f|/2m_ϕ$ or $|m^2_ϕ- m^2_f|/2m_f$, where $m_ϕ$ and $m_f$ are the masses of $ϕ$ and $f$, respectively. We refer to either one of these regions as a resonance energy range. Near these points, the one-loop formula for the neutrino self-energy has a singularity. From a technical point of view, that feature is indicative that the self-energy acquires an imaginary part, which is associated with damping effects and cannot be neglected, while the integral formula for the real part must be evaluated using the principal value of the integral. We carry out the calculations explicitly for some cases that allow us to give analytic results. Writing the dispersion relation in the form $ω= κ+ V_{\text{eff}} - iγ/2$, we give the explicit formula for $V_{\text{eff}}$ and $γ$ for the cases considered. When the neutrino energy is either much larger or much smaller than the resonance energy, $V_{\text{eff}}$ reduces to the effective potential that has been already determined in the literature in the high or low momentum regime, respectively. The virtue of the formula we give for $V_{\text{eff}}$ is that it is valid also in the \emph{resonance energy range}, which is outside the two limits mentioned. As a guide to possible applications we give the relevant formulas for $V_{\text{eff}}$ and $γ$, and consider the solution to the oscillation equations including the damping term, in a simple two-generation case.

hep-ph↗

arXiv2022

Very High-energy Afterglow Emission of GRB 190829A: Evidence for Its Hadronic Origin?

The detection of multi-TeV gamma-rays from the afterglow phase of GRB 190829A by High Energy Stereoscopic System (H.E.S.S.) telescope is an addition to the already existing list of two more GRBs observed in the very high energy (VHE) gamma-rays in recent years. Jets of blazars and GRBs have many similarities and the photohadronic model is very successful in explaining the VHE gamma-ray spectra from the high energy blazars. Recently, the photohadronic model has been successfully applied to study the sub-TeV gamma-rays from the afterglow phases of GRB 180720B and GRB 190114C. We employed this model again to explain the VHE spectra observed for the two consecutive nights from GRB 190829A. We show that the spectra of GRB 190829A can be due to the interactions of high energy protons with the synchrotron self-Compton photons in the forward shock region of the GRB jet, similar to the low emission state of the VHE flaring events of high energy blazars. We speculate that, if in future, it is possible to observe the VHE gamma-ray spectra from nearby GRBs in their afterglow phases, then some of them could only be explained by employing two different spectral indices. If confirmed, such VHE spectra could be interpreted as a result of the interactions of the high energy protons with the photons, both from the synchrotron background and the synchrotron self-Compton background in the forward shock region.

astro-ph.HE↗

arXiv2022

Neutrino effective potential in a fermion and scalar background

We consider the neutrino self-energy in a background composed of a scalar particle and a fermion using a simple model for the coupling of the form $λ\bar f_Rν_Lϕ$. The results are useful in the context of Dark Matter-neutrino interaction models in which the scalar and/or fermion constitute the dark-matter. The corresponding formulas for models in which the scalar particle couples to two neutrinos via a coupling of the form $λ^{(ννϕ)}\barν^c_Rν_Lϕ$ are then obtained as a special case. In the presence of these interactions there can be new contributions to the neutrino effective potential and index of refraction in the context of neutrino collective oscillations in a supernova and in the Early Universe hot plasma before neutrino decoupling. The formulas obtained here can be used to estimate those effects and/or put limits on the model parameters based on the contribution that such interactions can have in those contexts. A particular feature of the results is that the contribution to the neutrino self-energy or effective potential in a neutrino background due to the $ννϕ$ coupling is proportional to the antineutrino-neutrino asymmetry $(n_{\barν} - n_ν)$, in contrast to the standard $Z$ contribution which is proportional to $(n_ν - n_{\barν})$. Therefore the two contributions tend to cancel. If the cancellation is significant, it is conceivable that the $O(1/m^4_ϕ)$ terms give the dominant contribution. Alternatively, a limit can be set by requiring that the contribution of the $ννϕ$ interaction to the neutrino effective potential does not cancel the standard contribution in an appreciable way.

hep-ph↗

arXiv2021

Neutrino decoherence in an electron and nucleon background

We consider the decoherence effects in the propagation of active neutrinos due to the non-forward neutrino scattering processes in a matter background composed of electrons and nucleons. We calculate the contribution to the imaginary part of the neutrino self-energy arising from such processes. Since the initial neutrino state is depleted but does not actually disappear (the initial neutrino transitions into a neutrino of a different flavor but does not decay) those processes should be associated with decoherence effects that cannot be described in terms of the coherent evolution of the state vector. Based on the formalism developed in our previous work for treating the non-forward scattering processes using the notion of the stochastic evolution of the state, we identify the jump operators, as used in the context of the master or Lindblad equation, in terms of the results of the the calculation of the non-forward neutrino scattering contribution to the imaginary part of the neutrino self-energy. As a guide to estimating the decoherence effects in situations of practical interest we give explicit formulas for the decoherence terms for different background conditions, and point out some of the salient features in particular the neutrino energy dependence. To establish contact wih previous works in which the decoherence terms are treated as phenomenological parameters, we consider the solution to the evolution equation in the two-generation case. We give formulas that are useful for estimating the effects of the decoherence terms under various conditions and environments, including the typical conditions applicable to long baseline experiments, where matter effects are important. In those contexts the effects appear to be small, and indicative that if significant decoherence effects were to be found they would be due to non-standard contributions to the decoherence terms.

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