arXiv subjects

K. K. Singh

Publications and source records attributed to K. K. Singh.

At least 19 recordsLinked to original sources

arXiv2026

Peculiar Behavior of Optical Polarization in Blazar 1ES 1959+650: Role of Jet Magnetic Field and Geometry

Measurements of high degree of optical polarization from blazars provide a strong evidence for the optically thin synchrotron radiation from the relativistic leptons in the jet. This in turn characterizes the presence of a partially ordered magnetic field in the emission region. However, the topology of magnetic field and acceleration of electrons to ultrarelativistic energies within the blazar jet are poorly understood. In this work, we investigate the decade long optical light curve of the well-known blazar 1ES 1959+650 to probe the role of jet magnetic field in the peculiar behavior of the highly varying degree of linear polarization in the wavelength band 500- 700 nm. The optical light curves in V and R-bands exhibit a strong flare without any significant change in the contemporaneous degree of linear polarization. However, the degree of linear polarization shows strong positive and negative/anti-correlations with the V and R-band fluxes measured during the epochs before and after the flare, respectively. This can be satisfactorily explained using the helical magnetic field and transverse shock under the framework of a relativistic jet with modestly varying orientations. During the flare, the fractional linear polarization is explained by the turbulent emission multi-zone model.

astro-ph.HE↗

arXiv2026

A synchro-curvature treatment of gamma-ray luminosity trends in pulsars

In recent years, the Large Area Telescope (LAT) onboard the \emph{Fermi} satellite has detected more than 300 pulsars in the high energy range. The population studies of high energy pulsars show that the gamma ray luminosity of a pulsar ($L_γ$) can be expressed in terms of the spin down luminosity ($\dot{E}$) as $L_γ\propto \dot {E}^s$ having exponent $s\sim 0.68$. This high energy emission, assumed to originate far from the stellar surface and near the light cylinder, is usually studied in either purely curvature or purely synchrotron regime. In this work, we adopt a synchro-curvature radiation framework to understand the origin of gamma ray emission from the pulsar and its implications at the population-level. By comparing the observed cutoff energies of the differential gamma-ray spectra with the theoretical synchro-curvature predictions and enforcing radiation reaction approximation, we determine the equilibrium Lorentz factor and pitch angle of the emitting charged particles. This approach allows to quantify the relative roles of curvature and synchrotron radiation to the radiative losses, thereby providing a physically grounded interpretation of the luminosity trend across the pulsar population.

astro-ph.HE↗

arXiv2025

Identification of Gamma Ray Pulsar Candidates in the \emph{Fermi}-LAT 4FGL-DR4 Unassociated Sources Using Supervised Machine Learning

The Large Area Telescope (LAT) on board the \emph{Fermi} Gamma-ray Space Telescope has been continuously providing good quality survey data of the entire sky in the high energy range from 30 MeV to 500 GeV and above since August 2008. A succession of gamma-ray source catalogs is published after a comprehensive analysis of the \emph{Fermi}--LAT data. The most recent release of data in the fourth \emph{Fermi}--LAT catalog of gamma-ray sources (4FGL-DR4), based on the first 14 years of observations in the energy band 50 MeV-1 TeV, contains 7195 sources. A large fraction ($\sim$ 33\%) of this population has no known counterparts in the lower wave bands. Such high energy gamma-ray sources are referred to as unassociated or unidentified. An appropriate classification of these objects into known type of gamma-ray sources such as the active galactic nuclei or pulsars is essential for population studies and pointed multi-wavelength observations to probe the radiative processes. In this work, we perform a detailed classification of the unassociated sources reported in the 4FGL-DR4 catalog using two supervised machine learning techniques-Random Forest and Extreme Gradient Boosting. We mainly focus on the identification of new gamma-ray pulsar candidates by making use of different observational features derived from the long-term observations with the \emph{Fermi}--LAT and reported in the incremental 4FGL-DR4 catalog. We also explore the effects of data balancing approach on the classification of the \emph{Fermi}--LAT unassociated sources.

astro-ph.HE↗

arXiv2024

Very High-energy Gamma-Ray Episodic Activity of Radio Galaxy NGC 1275 in 2022-2023 Measured with MACE

The radio galaxy NGC 1275, located at the central region of Perseus cluster, is a well-known very high-energy (VHE) gamma-ray emitter. The Major Atmospheric Cherenkov Experiment Telescope has detected two distinct episodes of VHE (E > 80 GeV) gamma-ray emission from NGC 1275 during 2022 December and 2023 January. The second outburst, observed on 2023 January 10, was the more intense of the two, with flux reaching 58$\%$ of the Crab Nebula flux above 80 GeV. The differential energy spectrum measured between 80 GeV and 1.5 TeV can be described by a power law with a spectral index of $Γ= - 2.90 \pm 0.16_{stat}$ for both flaring events. The broadband spectral energy distribution derived from these flares, along with quasisimultaneous low-energy counterparts, suggests that the observed gamma-ray emission can be explained using a homogeneous single-zone synchrotron self-Compton model. The physical parameters derived from this model for both flaring states are similar. The intermediate state observed between two flaring episodes is explained by a lower Doppler factor or magnetic field, which subsequently returned to its previous value during the high-activity state observed on 2023 January 10.

astro-ph.HE↗

arXiv2024

Central Engine and Spectral Energy Distribution Properties of High Redshift Gamma Ray Blazars

We report on the properties of central engines in the $γ$-ray blazars located at high redshifts beyond z~>~0.4, where the extra-galactic background light (EBL) starts affecting their $γ$-ray spectra. The physical engine that provides power to the blazars of very high bolometric luminosity is assumed to be a highly collimated jet of matter moving relativistically away from the supermassive black hole (SMBH), located in the central region of the host galaxy, in a direction aligned toward the Earth. Due to their peculiar geometry and special physical conditions, blazars at redshifts beyond z~>~0.4 are bright enough to be detected in the $γ$-ray energy band. In this work, we investigate the physical properties of high-$z$ $γ$-ray blazars detected by the Large Area Telescope (LAT) on board the \emph{Fermi} satellite. We also study the properties of their emission regions and the central engines and discuss cosmological and astrophysical implications.

astro-ph.HE↗

arXiv2023

On the Maximum Energy Release from Formation of Static Compact Objects

Type II Supernova 1987A (SN 1987A), observed in 1987, released an energy of \mbox{$Q \approx 3 \times 10^{53}$ erg}}. This huge energy is essentially the magnitude of gravitational potential or self-gravitational energy (PE) of a new born cold neutron star having a gravitational compactness or redshift $z_b \approx 0.15$. One may wonder what could be the upper limit on the amount of energy that might be released with the formation of a cold Ultra Compact Object (UCO) with an arbitrary high $z_b$. Accordingly, here, for the first time, we obtain an analytical expression for the PE of a homogeneous general relativistic UCO assuming it to be cold and static. It is found that the PE of a homogeneous UCO of mass $M$ may exceed Mc$^2$ and be as large as 1.34 Mc$^2$. This result, though surprising, follows from an \textit{exact and correct} analytical calculation based on the standard General Theory of Relativity (GTR). Further, UCOs supported by tangential stresses may be inhomogeneous and much more massive than neutron stars with PE $\sim$ 2.1 Mc$^2$ Thus, in principle, formation of an UCO of a few solar masses ($M_\odot$) might release an energy $Q\sim10^{55}$ erg.

gr-qc↗

arXiv2023

Prospects of detecting gamma-ray signal of dark matter interaction with the MACE telescope

The MACE (Major Atmospheric Cherenkov Experiment) telescope has started its regular gamma-ray observations at Hanle in India. Located at an altitude of $\sim$ 4.3 km above sea level and equipped with a 21 m diameter large quasi-parabolic reflector, it has the capability to explore the gamma-ray sky in the energy range above 20 GeV with very high sensitivity. In this work, we present the results from the feasibility studies for searching high-energy gamma-ray signals from dark matter interaction in potential astrophysical environments. We study the impact of MACE response function and other instrumental characteristics to probe the velocity average interaction cross-section ($<σv>$) of the weakly interacting massive particles (WIMPs), expected from the thermal dark matter freeze-out during the decoupling era. We consider the presence of dark matter in the form of pure WIMPs in the mass range 200 GeV - 10 TeV to produce distinctive gamma-ray spectra through its self-annihilation into standard model particles using the Pythia simulation package. The convolution of gamma-ray spectra corresponding to different standard model channels with the MACE response function is used to estimate the upper limit on $<σv>$ for 100 hours of expected MACE observation of Segue1 (a dwarf spheroidal galaxy) which is a potential site of dark matter.

astro-ph.HE↗

arXiv2023

Classification of the \emph{Fermi}-LAT Blazar Candidates of Uncertain type using eXtreme Gradient Boosting

Machine learning based approaches are emerging as very powerful tools for many applications including source classification in astrophysics research due to the availability of huge high quality data from different surveys in observational astronomy. The Large Area Telescope on board \emph{Fermi} satellite (\emph{Fermi}-LAT) has discovered more than 6500 high energy gamma-ray sources in the sky from its survey over a decade. A significant fraction of sources observed by the \emph{Fermi}-LAT either remains unassociated or has been identified as \emph{Blazar Candidates of Uncertain type} (BCUs). We explore the potential of eXtreme Gradient Boosting (XGBoost)- a supervised machine learning algorithm to identify the blazar subclasses among a sample of 112 BCUs of the 4FGL catalog whose X-ray counterparts are available within 95$\%$ uncertainty regions of the \emph{Fermi}-LAT observations. We have used information from the multi-wavelength observations in IR, optical, UV, X-ray and $γ$-ray wavebands along with the redshift measurements reported in the literature for classification. Among the 112 uncertain type blazars, 62 are classified as BL Lacertae objects (BL Lacs) and 6 have been classified as Flat Spectrum Radio Quasars (FSRQs). This indicates a significant improvement with respect to the multi-perceptron neural network based classification reported in the literature. Our study suggests that the gamma-ray spectral index, and IR color indices are the most important features for identifying the blazar subclasses using the \emph{XGBoost} classifier. We also explore the importance of redshift in the classification BCU candidates.

astro-ph.HE↗

arXiv2021

Probing the evolution of the EBL photon density out to $z~\sim 1$ via $γ$-ray propagation measurements with Fermi

The redshift ($z$) evolution of the Extragalactic Background Light (EBL) photon density is very important to understand the history of cosmological structure formation of galaxies and stars since the epoch of recombination. The EBL photons with the characteristic spectral energy distribution ranging from ultraviolet/optical to far-infrared provide a major source of opacity of the Universe to the GeV-TeV $γ$-rays travelling over cosmological distances. The effect of the EBL is very significant through $γγ\rightarrow e^- e^+$ absorption process on the propagation of the $γ$-ray photons with energy $E >$ 50 GeV emitted from the sources at $z \sim 1$. This effect is characterized by the optical depth ($τ$) which strongly depends on $E$, $z$ and density of the EBL photons. The proper density of the EBL photons increases with $z$ due to expansion of the Universe whereas evolution of radiation sources contributing to the EBL leads to a decrease in the density with increasing $z$. Therefore, the resultant volumetric evolution of the EBL photon density is approximated by a modified redshift dependence. In this work, we probe evolution of the EBL photon density predicted by two prominent models using cosmic gamma-ray horizon ($τ(E,z)=$ 1) determined by the measurements from the \emph{Fermi}-Large Area Telescope (LAT) observations. The modified redshift dependence of the EBL photon density is optimized for a given EBL model by estimating the same gamma-ray horizon as predicted by the \emph{Fermi}-LAT observations. We further compare the optical depth estimates in the energy range $E =$ 4 GeV-1 TeV and redshift range $z =0.01-1$ from the \emph{Fermi}-LAT observations with the values derived from the two EBL models to further constrain the evolution of the EBL photon density in the $z~\sim 1$ Universe.

astro-ph.CO↗

arXiv2021

Properties of white dwarf in the binary system AR Scorpii and its observed features

The binary system AR Scorpii hosts an M-type main sequence cool star orbiting around a magnetic white dwarf in the Milky Way Galaxy. The broadband non-thermal emission over radio, optical and X-ray wavebands observed from AR Scorpii indicates strong modulations on the spin frequency of the white dwarf as well as the spin-orbit beat frequency of the system. Therefore, AR Scorpii is also referred to as a white dwarf pulsar wherein a fast spinning white dwarf star plays very crucial role in the broadband non-thermal emission. Several interpretations for the observed features of AR Scorpii appear in the literature without firm conclusions. In this work, we investigate connection between some of the important physical properties like spin-down power, surface magnetic field, equation of state, temperature and gravity associated with the white dwarf in the binary system AR Scorpii and its observational characteristics. We explore the plausible effects of white dwarf surface magentic field on the absence of substantial accretion in this binary system and also discuss the gravitational wave emission due to magnetic deformation mechanism.

astro-ph.HE↗

arXiv2020

Modelling the broadband emission from the white dwarf binary system AR Scorpii

In this work, we have analyzed the $γ$-ray data in the energy range 100 MeV to 500 GeV from the \emph{Fermi}-Large Area Telescope (LAT) observations for the period August 4, 2008 to March 31, 2019. The $γ$-ray emission from AR Scorpii over the last decade is not statistically significant and therefore 2$σ$ upper limit on the integral flux above 100 MeV has been estimated. We reproduce the non-thermal broadband spectral energy distribution of AR Scorpii using an emission model having two synchrotron components due to the relativistic electrons in very high magnetic fields. The first component (Synchrotron-1) broadly describes the emissions at radio to high energy X-rays through the synchrotron radiation originating from a spherical region of radius $\sim$ 1.8$\times$10$^{10}$ cm and a magnetic field strength of $\sim$ 10$^3$ Gauss. The second component (Synchrotron-2) which reproduces the X-ray emission at lower energies and predicts the $γ$-ray emission, originates from another spherical region with radius $\sim$ 1.4$\times$10$^{10}$ cm and a magnetic field strength of $\sim$ 10$^6$ Gauss. The relativistic electron populations in both the emission regions are described by a smooth broken power law energy distribution. The $γ$-ray emission predicted by the Synchrotron-2 model is below the broadband sensitivity of the \emph{Fermi}-LAT and is also consistent with the 95$\%$ confidence level upper limit on the integral flux above 100 MeV derived from more than 10 years of observations. According to our model, the binary system AR Scorpii could be a $γ$-ray source, although its emission level must be below the current detection limit of the \emph{Fermi}-LAT.

astro-ph.HE↗

arXiv2020

Characterization of Variability in Blazar Light curves

Blazars represent dominant population of the extragalactic $γ$-ray sources in the Universe. These sources exhibit some characteristic properties like strong and non-thermal continuum emission over the entire electromagnetic spectrum from radio to TeV $γ$-rays with rapid variability on all timescales. The emission at radio and optical wavelengths is highly polarized with significant variation. The fastest variability in the blazar emission is observed during the flaring activity which is an important observational property of blazars. In this paper, we describe various methods to characterize the temporal variability in the multi-wavelength light curves of blazars. We also provide a detailed description of the set of statistical parameters which are used to quantify the level of variability present in the time-series. Implications of the informations derived from the variability study to probe the physics of blazars using multi-wavelength observations are also discussed.

astro-ph.HE↗

arXiv2020

Extragalactic background light models and GeV-TeV observation of blazars

In this work, we use two different methods to determine the opacity of the TeV gamma-rays caused by the extragalactic background light (EBL) via e-e+ production due to photon-photon interaction. The first method, Model-Dependent Approach, uses various EBL models for estimating the opacity as a function of the redshift and energy of the TeV photons. The second method, Model-Independent Approach, relies on using the simultaneous observations of blazars in the MeV-GeV energy range from the Fermi-LAT and in the TeV band from the ground-based gamma-ray telescopes. We make the underline assumption that the extrapolation of the LAT spectrum of blazars to TeV energies is either a good estimate or an upper limit for the intrinsic TeV spectrum of a source. We apply this method on the simultaneous observations of a few blazars at different redshifts to demonstrate a comparative study of six prominent EBL models. Opacities of the TeV photons predicted by the model-independent approach are systematically larger than the ones estimated from the model-dependent method. Therefore, the gamma-ray observations of blazars can be used to set a strict upper limit on the opacity of the Universe to the TeV photons at a given redshift.

astro-ph.HE↗

arXiv2020

Understanding the giant gamma-ray outburst on June 16, 2015 from the blazar 3C 279

A very bright and fast varying gamma-ray flare has been detected from the blazar 3C 279 on June 16, 2015. We have studied the broadband spectral energy distribution of the source during the flaring episode and in the low activity state using a simple one zone leptonic model. We find that an electron energy distribution described by a broken power law can be used to reproduce the broadband emissions during the high and low activity states. The flux measurements at radio, infrared and optical frequencies are reproduced by the synchrotron emission resulting from the relativistic electrons in a jet magnetic field strength of 0.37 G. The gamma-ray emission from the blazar 3C 279 is attributed to the Comptonization of the IR seed photons from the dusty torus with a temperature of 870 K. The outburst from the source observed on June 16, 2015 can be ascribed to an efficient acceleration process associated with a sudden enhancement in the electron energy density in the emitting region with respect to the low activity state. The fast gamma-ray variability at a minute timescale implies that the emission during the flare originates from a more compact region and the size of the emission zone in the low activity state is found to be four times larger than that during the flare. We have also used the model parameters derived from the broadband spectral energy distribution modelling to investigate a few physical properties of the jet during the outburst.

astro-ph.HE↗

arXiv2019

Long term multi-wavelength view of the blazar 1ES 1218+304

In this work, we present a multi-wavelength study of the blazar 1ES 1218+304 using near simultaneous observations over 10 years during the period September 1, 2008 to August 31, 2018 (MJD 54710-58361). We have analyzed data from \emph{Swift}-UVOT, \emph{Swift}-XRT and \emph{Fermi}-LAT to study the long term behaviour of 1ES 1218+304 in different energy bands over the last decade. We have also used the archival data from OVRO, MAXI and \emph{Swift}-BAT available during the above period. The near simultaneous data on 1ES 1218+304 suggest that the long term multi-wavelength emission from the source is steady and does not show any significant change in the source activity. The optical/UV fluxes are found to be dominated by the host galaxy emission and can be modelled using the $PEGASE$ code. However, the time averaged X-ray and $γ$-ray emisions from the source are reproduced using a single zone leptonic model with log-parabolic distribution for the radiating particles. The intrinsic very high energy $γ$-ray emission during a low activity state of the source is broadly consistent with the predictions of the leptonic model for blazars. We have investigated the physical properties of the jet and the mass of the super massive black hole at the center of the host galaxy using long term X-ray observations from the \emph{Swift}-XRT which is in agreement with the value derived using blackbody approximation of the host galaxy. We also discuss the extreme nature of the source on the basis of X-ray and $γ$-ray observations.

astro-ph.HE↗

arXiv2019

Results from the long-term Fermi-LAT observations of the white dwarf binary pulsar AR Scorpii

The discovery of the white dwarf binary system AR Scorpii (AR Sco) with its fascinating non-thermal dominated multi-frequency emission has sparked renewed interest in potential high energy gamma-ray emission from white dwarf pulsars. The Spectral Energy Distribution (SED) below and above optical shows evidence of non-thermal synchrotron emission, with pulsed emission in optical and X-ray bands at the white dwarf spin period ($P_{*} = 117 \, \mbox{s}$) as well as a beat period ($P_{\rm b} = 118.1 \, \mbox{s}$) with the binary period. From an energy perspective, the highly magnetic rotating white dwarf can accelerate particles to TeV energies. In this study, a search for high energy gamma-ray emission was conducted between 100 MeV - 500 GeV by analysing the newly available Fermi-LAT Pass 8 data with the new Fermi 1.0.1 Science Tools. Binned likelihood analysis was done using power law, broken power law and log parabola models. From the selected Region of Interest (ROI) centred on AR Sco's position, we calculated a significance of $\sqrt{TS}\leq$ 3.87$σ$ for the integrated gamma-ray activity between 100 MeV - 500 GeV at a photon flux level of 0.486 $\pm$ 0.261 x 10$^{-8}$ photons cm$^{-2}$ s$^{-1}$ using the broken power law model. This resulted in a 3$σ$ upper-limit detection from the position of AR Sco. The location of AR Sco inside the Rho Ophiuchi (Rho Oph) molecular cloud complex combined with the poor spatial resolution of Fermi-LAT, complicates any positive identification of low-level gamma-ray activity at the location that coincides with the position of AR Sco.

astro-ph.HE↗

arXiv2019

Intrinsic GeV-TeV gamma-ray emission from EHSP blazars

Extremely High Synchrotron Peak (EHSP) blazars are observed to form a small population of sources with high energy hump peaking at TeV energies in their broad-band spectral energy distributions. The observed gamma-ray emission from these sources at GeV-TeV energies is described by unusual hard spectral indices. The observed spectral and temporal characteristics of these sources challenge the standard leptonic models for the broad-band emissions from blazars. Therefore, such sources provide astrophysical sites to investigate directly the particle acceleration, cooling of relativistic particles and indirectly probe the cosmological quantities like extragalactic background light (EBL) and intergalactic magnetic field (IMF) in the Universe. In this study, we investigate the spectral properties of the gamma-ray emission from EHSP blazars using observations from the Fermi-LAT catalogues (3FGL and 3FHL) along with the TeV observations using ground based telescopes. The observed TeV gamma-ray spectra are corrected for the EBL absorption using the most recent and updated EBL models to determine the intrinsic spectrum at the source. The intrinsic TeV spectra are combined with the MeV-GeV observations from the Fermi-LAT to study the gamma-ray emission from EHSP blazars in the broad energy band. The intrinsic gamma-ray spectra are then used to estimate the position of high energy peak in the spectral energy distribution. We also present the qualitative description for the observed spectral properties of EHSP blazars using different physical scenarios and discuss their importance for the upcoming CTA observatory.

astro-ph.HE↗

arXiv2014

Search for TeV $γ$--ray emission from blazar 1ES1218+304 with TACTIC telescope during March-April 2013

In this paper, we present results of TeV $γ$--ray observations of the high synchrotron peaked BL Lac object 1ES 1218+304 (z=0.182) with the $TACTIC$ (TeV Atmospheric Cherenkov Telescope with Imaging Camera). The observations are primarily motivated by the unusually hard GeV-TeV spectrum of the source despite its relatively large redshift. The source is observed in the TeV energy range with the $TACTIC$ from March 1, 2013 to April 15, 2013 (MJD 56352--56397) for a total observation time of 39.62 h and no evidence of TeV $γ$--ray activity is found from the source. The corresponding 99$\%$ confidence level upper limit on the integral flux above a threshold energy of 1.1 TeV is estimated to be 3.41 $\times10^{-12}$ photons cm$^{-2}$ s$^{-1}$ (i.e $<23\%$ Crab Nebula flux) assuming a power law differential energy spectrum with photon index 3.0, as previously observed by the $MAGIC$ and $VERITAS$ telescopes. For the study of multi-wavelength emission from the source, we use nearly simultaneous optical, UV and and X--ray data collected by the UVOT and XRT instruments on board the \emph{Swift} satellite and high energy $γ$--ray data collected by the Large Area Telescope on board the \emph{Fermi} satellite. We also use radio data at 15 GHz from OVRO 40 m telescope in the same period. No significant increase of activity is detected from radio to TeV $γ$--rays from 1ES1218+304 during the period from March 1, 2013 to April 15, 2013.

astro-ph.HE↗