arXiv subjects

A. Tolamatti

Publications and source records attributed to A. Tolamatti.

6 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↗

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

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↗

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↗