arXiv subjects

Subhash Rajpoot

Publications and source records attributed to Subhash Rajpoot.

At least 19 recordsLinked to original sources

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 $\chi^2$ 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 $\chi^2$ 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↗

arXiv2025

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 $E^c_{\gamma}=3.18$ TeV. Below this energy the VHE spectrum increases slowly and is in high emission state. However, for $E^c_{\gamma}\, > 3.18$ TeV, the spectrum falls faster, resulting in a mild peak-like feature, not prominent enough as claimed by the MAGIC collaboration.

astro-ph.HE↗

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 $\chi^2$ 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 Dom\'inguez 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 $3\sigma$ 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.

astro-ph.HE↗

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

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

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

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

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

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↗

arXiv2021

Extreme HBL-like behavior of Markarian 421 and its two-zone photohadronic interpretation

Markarian 421 is the nearest high-energy peaked blazar and also is the first extragalactic source to be detected in multi-TeV $γ$-rays. It has been observed in multiwavelength for exceptionally long period of time with dense monitoring and several major outbursts have been detected from this source. In March 2010, the source was in a high state of activity and was observed in multiwavelength by various telescopes for 13 consecutive days. During this period the position of the synchrotron peak was found to be above $10^{17}$ Hz and also the position of the second peak was shifted towards higher energy, a signature of extreme HBL-like behavior. We observed that the standard photohadronic model is inadequate to explain the observed spectra. However, a recently proposed two-zone photohadronic model explains very well the GeV-TeV flaring events observed by both MAGIC and VERITAS telescopes. From the observation of the highest energy $γ$-ray event on MJD 55266 we also estimated the minimum bulk Lorentz factor.

astro-ph.HE↗

arXiv2020

A two-zone photohadronic interpretation of the EHBL-like behavior of the 2016 multi-TeV flares of 1ES 1959+650

The high-energy-peaked blazar 1ES 1959+650 is a well-known and well studied nearby blazar that has undergone several episodes of multi-TeV flaring. In 2002 for the first time an orphan TeV flare was observed from this blazar. During a multiwavelength campaign between 29th April to 21st November 2016, MAGIC telescopes observed multi-TeV flarings during the nights of 13th, 14th June and 1st July 2016 when the position of the synchrotron peak was found to be above $10^{17}$ Hz. Also observed was that the second peak of the spectral energy distribution shifted towards higher energy, and exhibiting extreme HBL-like behavior. The photohadronic model which is very successful in explaning the multi-TeV flaring from many high energy blazars including 1ES 1959+650 is applied to study the flaring events of 2016. It is observed that the photohadronic model is unable to explain the observed spectra. Here, we use a two-zone photohadronic model to explain the observed spectra. We clearly demonstrate that the low energy regime (zone-1) of the spectra corresponds to the standard flaring events of the high energy blazar and the high energy regime (zone-2) of the spectra are solely due to the extreme nature of the flaring events. Our two-zone photohadronic model explains very well the multi-TeV flaring events observed by MAGIC telescopes.

astro-ph.HE↗

arXiv2020

A two-zone photohadronic scenario for EHBL-like behavior of Mrk 501

Major outbursts have been observed from the well-known high-energy peaked blazar Markarian 501 since its discovery in 1996. Especially, two episodes of very high energy gamma-ray flaring events during May-July 2005 and June 2012 are of special significance, when the source exhibited extreme HBL-like behavior. The successful standard photohadronic model seems inadequate to explain these extraneous behaviors. We propose a two-zone photohadronic scenario to overcome this problem. In this picture, the low energy regime (zone-1) of the spectrum follows the standard photohadronic interpretation, while the high energy regime (zone-2) of the spectrum is new, with a spectral index $δ_2\geq 3.1$, which is solely due to the extreme nature of the flaring event. We also estimate the bulk Lorentz factor corresponding to these extreme flaring events. By analyzing many flaring events before and after these extreme events we argue that the extreme HBL-like events are transient and may repeat in future.

astro-ph.HE↗

arXiv2017

Massive Interacting Yang-Mills Multiplets Nine and Five Dimensions

We present interacting massive N=1 vector multiplet (VM) in nine dimensions (9D). Due to the identically-vanishing mass-term m(\Barλλ) \equiv 0 for (symplectic) pseudo-Majorana gaugino in 9D, we employ unconventional technique to give masses to fermions. In 9D, we consider the gauge group G for the VM (A_μ^I, λ^I , φ^I) (I = 1, 2, \cdots, dim G), where G is the Yang-Mills gauge group, and the gaugino λ^I is a pseudo-Majorana spinor. We break G by shifting the scalar φ^I, so that the gaugino λ^I as well as its super-partner gauge boson A_μ^I will get the same mass. The scalar φ^I plays the role of a Nambu-Goldstone boson absorbed into the longitudinal components of A_μ^I$, making the latter massive as a super-Proca-Stueckelberg mechanism. We also show that a similar method can be also applied to N=2 VMs in 5D.

hep-th↗

arXiv2017

On Supersymmetric Geometric Flows and $\mathcal{R}^2$ Inflation From Scale Invariant Supergravity

Models of geometric flows pertaining to $\mathcal{R}^2$ scale invariant (super) gravity theories coupled to conformally invariant matter fields are investigated. Related to this work are supersymmetric scalar manifolds that are isomorphic to the Kählerian spaces $\mathcal{M}_n=SU(1,1+k)/U(1)\times SU(1+k)$ as generalizations of the non-supersymmetric analogs with $SO(1,1+k)/SO(1+k)$ manifolds. For curved superspaces with geometric evolution of physical objects, a complete supersymmetric theory has to be elaborated on nonholonomic (super) manifolds and bundles determined by non-integrable superdistributions with additional constraints on (super) field dynamics and geometric evolution equations. We also consider generalizations of Perelman's functionals using such nonholonomic variables which result in the decoupling of geometric flow equations and Ricci soliton equations with supergravity modifications of the $R^2$ gravity theory. As such, it is possible to construct exact non-homogeneous and locally anisotropic cosmological solutions for various types of (super) gravity theories modelled as modified Ricci soliton configurations. Such solutions are defined by employing the general ansatz encompassing coefficients of generic off-diagonal metrics and generalized connections that depend generically on all spacetime coordinates. We consider nonholonomic constraints resulting in diagonal homogeneous configurations encoding contributions from possible nonlinear parametric geometric evolution scenarios, off-diagonal interactions and anisotropic polarization/ modification of physical constants. We also analyze the conditions under which such modified mimetic type theories and solutions reproduce the Starobinsky inflationary models in the double scalar approach.

hep-th↗

arXiv2017

Cosmological Attractors and Anisotropies in Two Measure Theories, Effective EYMH systems, and Off--Diagonal Inflation Models

Applying the anholonomic frame deformation method, we construct various classes of cosmological solutions for effective Einstein -- Yang-Mills -- Higgs, and two measure theories. The types of models considered are Freedman-Lemaître-Robertson-Walker, Bianchi, Kasner and models with attractor configurations. The various regimes pertaining to plateau--type inflation, quadratic inflation, Starobinsky type and Higgs type inflation are presented.

physics.gen-ph↗

arXiv2016

Nonholonomic Jet Deformations and Exact Solutions for Modified Ricci Soliton and Einstein Equations

Let $\mathbf{g}$ be a pseudo--Riemanian metric of arbitrary signature on a manifold $\mathbf{V}$ with conventional $n+n$ dimensional splitting, $\ n\geq 2,$ determined by a nonholonomic (non--integrable) distribution $\mathcal{N}$ defining a generalized (nonlinear) connection and associated nonholonomic frame structures. We shall work with a correspondingly adapted linear metric compatible connection $\hat{\mathbf{D}}$ and its nonzero torsion $\hat{\mathcal{T}}$, both completely determined by $\mathbf{g}$. Our first goal is to prove that there are certain generalized frame and/or jet transforms and prolongations with $(\mathbf{g},\mathbf{V})\rightarrow (\hat{\mathbf{g}},\hat{\mathbf{V}})$ into explicit classes of solutions of some generalized Einstein equations $\hat{\mathbf{R}}\mathit{ic}=Λ\hat{\mathbf{g}}$, $Λ=const$, encoding various types of nonholonomic) Ricci soliton configurations and/or jet variables and symmetries. The second goal is to solve additional constraint equations for zero torsion, $\hat{\mathcal{T}}=0$, on generalized solutions constructed in explicit forms with jet variables and extract Levi-Civita configurations. This allows us to find generic off-diagonal exact solutions depending on all space time coordinates on $\mathbf{V}$ via generating and integration functions and various classes of constant jet parameters and associated symmetries. We shall study (third goal) how such generalized metrics and connections can be related by so-called "half-conformal" and/ or jet deformations of certain sub-classes of solutions with one, or two, Killing symmetries. Finally, there are considered some examples when exact solutions are constructed as nonholonomic jet prolongations of the Kerr metrics, with possible Ricci soliton deformations, and characterized by nonholonomic jet structures and generalized connections.

math-ph↗

arXiv2016

Multi-TeV flaring from blazars: Markarian 421 a case study

The TeV blazar Markarian 421 underwent multi-TeV flaring during April 2004 and simultaneously observed in x-ray and TeV energies. It was observed that the TeV outbursts had no counterparts in the lower energies, which implies that this might be an orphan flare. In the context of hadronic model, we have shown that this multi-TeV flaring can be produced due to the interaction of Fermi-accelerated protons of energy $\lesssim 168$ TeV with the background photons in the low energy tail of the synchrotron self-Compton spectrum of the blazar jet. We fit very well the flaring spectrum with this model. Based on this study, we speculate that Mrk 501 and PG 1553+113 are possible candidates for orphan flaring in the future.

astro-ph.HE↗