arXiv subjects

V. S. Paliya

Publications and source records attributed to V. S. Paliya.

18 recordsLinked to original sources

arXiv2026

The Emerging Population of High-energy Emitting Radio Galaxies

High-energy emission from radio galaxies provides a unique laboratory to study the connection between accretion, jet formation, and particle acceleration in active galactic nuclei (AGN). The recent detection of $γ$-ray emission from misaligned radio galaxies - including Compact Symmetric Objects (CSOs), FR0, FRI/II, and even Giant Radio Galaxies (GRGs) - has shown that efficient particle acceleration is not limited to blazars, but occurs throughout the full radio-loud AGN population. This finding supports a unifying framework where leptonic synchrotron, synchrotron self-Compton (SSC), and external inverse-Compton (EIC) processes coexist across multiple spatial scales, from the inner jet and corona to the extended lobes, possibly with a hadronic contribution in dense environments. The Square Kilometre Array (SKA) will be pivotal in advancing this field. SKA1-Low will detect and characterize diffuse, low-surface-brightness emission tracing aged plasma and jet duty cycles. SKA1-Mid will enable high-resolution spectral and polarimetric studies of compact jets and nuclear regions, while SKA-VLBI will connect parsec- to kiloparsec-scale structures, identifying the exact sites of high-energy dissipation. In synergy with forthcoming high-energy missions such as NewAthena and CTAO, SKA will provide the first spatially resolved, multi-scale view of particle acceleration and energy release in misaligned AGN, unveiling the physical link between the central engine and its large-scale feedback on the host galaxy evolution.

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arXiv2025

Exploring the Most Extreme Gamma-Ray Blazars Using Broadband Spectral Energy Distributions

Extreme high-synchrotron peaked blazars (EHSPs) are rare high-energy sources characterised by synchrotron peaks beyond 10$^{17}$ Hz in their spectral energy distributions (SEDs). Their extreme properties challenge conventional blazar emission models and provide a unique opportunity to test the limits of particle acceleration and emission mechanisms in relativistic jets. However, the number of identified EHSPs is still small, limiting comprehensive studies of their population and characteristics. This study aims to identify new EHSP candidates and characterise their emission properties. A sample of 124 $γ$-ray blazars was analysed, selected for their high synchrotron peak frequencies and $γ$-ray emission properties, with a focus on sources showing low variability and good broadband data coverage. Their SEDs were constructed using archival multi-wavelength data from the SSDC SED Builder service, supplemented with recent Swift-UVOT, Swift-XRT, and Fermi-LAT observations. The SEDs were modelled with a one-zone synchrotron/synchrotron-self-Compton framework, classifying sources by synchrotron peak frequency. We identify 66 new EHSP candidates, significantly expanding the known population. A clear correlation between synchrotron peak frequency and the magnetic-to-kinetic energy density ratio is found, with the most extreme EHSPs nearing equipartition. Host galaxy emission is detected in many sources, but no significant differences are observed between elliptical and lenticular hosts. Our analysis suggests that nine high-synchrotron peaked/EHSPs could be observed by CTAO at $>5σ$ (20 at $>3σ$) in 20-hour exposures, indicating that while the overall detection rate remains modest, a subset of these sources is within reach of next-generation very-high-energy gamma-ray instruments.

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arXiv2025

Continuation of an Optical Spectroscopic Campaign of Fermi Blazar Candidates with TNG: Discovery of a New Changing-Look Blazar

Context. Blazars are a distinct subclass of active galactic nuclei (AGN), known for their fast variability, high polarization, and intense emission across the electromagnetic spectrum, from radio waves to gamma rays. Gamma-ray blazar candidates of uncertain type (BCU) are an ongoing challenge in gamma-ray astronomy due to difficulties in classification and redshift determination. Aims. This study continues an optical spectroscopic campaign aimed at identifying the characteristics of BCUs to improve classification and redshift estimates, particularly focusing on low-synchrotron-peak sources. Methods. We conducted a detailed analysis of optical spectroscopic data for a sample of 21 low-synchrotron-peak BCUs plus one bl lac with contradictory results in the literature, using the 3.58-m Telescopio Nazionale Galileo (TNG, La Palma, Spain). Results. Our analysis identifies 14 out of the 21 BCUs as flat-spectrum radio quasars (FSRQs), demonstrating the effectiveness of our selection criteria. Notably, four FSRQs have redshifts exceeding 1, including 4FGL J2000.0+4214 at z = 2.04. Six sources are classified as bl lacs, with one of them, 4FGL J0746.5-0719, showing a featureless spectrum in this work despite previously exhibiting strong lines, suggesting it may be a changing-look blazar. One source remains classified as a BCU due to a noisy spectrum. Additionally, we observed a bl lac object, 4FGL J1054.5+2211, due to inconsistent redshift estimates in the literature, but we could not confirm any redshift due to its featureless spectrum. Our findings provide insights into the classification and redshift estimation of blazar candidates, emphasizing the need for continued spectroscopic monitoring.

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arXiv2023

Constraints on redshifts of blazars from extragalactic background light attenuation using Fermi-LAT data

The extragalactic high-energy $γ$-ray sky is dominated by blazars, which are active galactic nuclei with their jets pointing towards us. Distance measurements are of fundamental importance yet for some of these sources are challenging because any spectral signature from the host galaxy may be outshone by the non-thermal emission from the jet. In this paper, we present a method to constrain redshifts for these sources that relies only on data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. This method takes advantage of the signatures that the pair-production interaction between photons with energies larger than approximately 10 GeV and the extragalactic background light leaves on $γ$-ray spectra. We find upper limits for the distances of 303 $γ$-ray blazars, classified as 157 BL Lacertae objects, 145 of uncertain class, and 1 flat-spectrum-radio quasar, whose redshifts are otherwise unknown. These derivations can be useful for planning observations with imaging atmospheric Cherenkov telescopes and also for testing theories of supermassive black hole evolution. Our results are applied to estimate the detectability of these blazars with the future Cherenkov Telescope Array, finding that at least 21 of them could be studied in a reasonable exposure of 20 h.

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arXiv2023

A new derivation of the Hubble constant from $γ$-ray attenuation using improved optical depths for the Fermi and CTA era

We present $γ$-ray optical-depth calculations from a recently published extragalactic background light (EBL) model built from multiwavelength galaxy data from the Hubble Space Telescope Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (HST/CANDELS). CANDELS gathers one of the deepest and most complete observations of stellar and dust emissions in galaxies. This model resulted in a robust derivation of the evolving EBL spectral energy distribution up to $z\sim 6$, including the far-infrared peak. Therefore, the optical depths derived from this model will be useful for determining the attenuation of $γ$-ray photons coming from high-redshift sources, such as those detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope, and for multi-TeV photons that will be detected from nearby sources by the future Cherenkov Telescope Array. From these newly calculated optical depths, we derive the cosmic $γ$-ray horizon and also measure the expansion rate and matter content of the Universe including an assessment of the impact of the EBL uncertainties. We find $H_{0}=61.9$ $^{+2.9}_{-2.4}$ km s$^{-1}$ Mpc$^{-1}$ when fixing $Ω_{m}=0.32$, and $H_{0}=65.6$ $^{+5.6}_{-5.0}$ km s$^{-1}$ Mpc$^{-1}$ and $Ω_{m}=0.19\pm 0.07$, when exploring these two parameters simultaneously.

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arXiv2022

BASS XXXIII: Swift-BAT blazars and their jets through cosmic time

We derive the most up-to-date Swift-Burst Alert Telescope (BAT) blazar luminosity function in the 14-195 keV range, making use of a clean sample of 118 blazars detected in the BAT 105-month survey catalog, with newly obtained redshifts from the BAT AGN Spectroscopic Survey (BASS). We determine the best-fit X-ray luminosity function for the whole blazar population, as well as for Flat Spectrum Radio Quasars (FSRQs) alone. The main results are: (1) at any redshift, BAT detects the most luminous blazars, above any possible break in their luminosity distribution, which means we cannot differentiate between density and luminosity evolution; (2) the whole blazar population, dominated by FSRQs, evolves positively up to redshift z~4.3, confirming earlier results and implying lower number densities of blazars at higher redshifts than previously estimated. The contribution of this source class to the Cosmic X-ray Background at 14-195 keV can range from 5-18%, while possibly accounting for 100% of the MeV background. We also derived the average 14 keV-10 GeV SED for BAT blazars, which allows us to predict the number counts of sources in the MeV range, as well as the expected number of high-energy (>100 TeV) neutrinos. A mission like COSI, will detect 40 MeV blazars and 2 coincident neutrinos. Finally, taking into account beaming selection effects, the distribution and properties of the parent population of these extragalactic jets are derived. We find that the distribution of viewing angles is quite narrow, with most sources aligned within < 5° of the line of sight. Moreover, the average Lorentz factor, <$Γ$>= 8-12, is lower than previously suggested for these powerful sources.

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arXiv2022

Hunting extreme BL Lacertae blazars with Fermi-LAT

The emission of very-high-energy photons (VHE, E>100 GeV$) in blazars is closely connected to the production of ultra-relativistic particles and the role of these gamma-ray sources as cosmic particle accelerators. This work focuses on a selection of 22 gamma-ray objects from the 2BIGB catalog of high-synchrotron-peaked sources, which are classified as blazar candidates of uncertain type in the 4FGL-DR2 catalog. We study these sources by means of a re-analysis of the first 10 years of gamma-ray data taken with the Fermi Large Area Telescope, including the attenuation by the extragalactic background light. Their broadband spectral energy distributions are also evaluated, using multi-wavelength archival data in the radio, optical, and X-ray bands, in terms of one-zone synchrotron-self-Compton models, adding an external Compton component when needed. Out of this analysis, we identify 17 new extreme high-synchrotron-peaked (EHSP) candidates and compare their physical parameters with those of prototypical EHSP blazars. Finally, the resulting models are used to assess their detectability by the present and future generation of ground-based imaging atmospheric Cherenkov telescopes. We find two VHE candidates within the reach of the current and next generation of Cherenkov telescopes: J0847.0-2336 and J1714.0-2029.

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arXiv2021

Fermi Large Area Telescope Performance After 10 Years Of Operation

The Large Area Telescope (LAT), the primary instrument for the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy gamma-ray telescope, covering the energy range from 30 MeV to more than 300 GeV. We describe the performance of the instrument at the 10-year milestone. LAT performance remains well within the specifications defined during the planning phase, validating the design choices and supporting the compelling case to extend the duration of the Fermi mission. The details provided here will be useful when designing the next generation of high-energy gamma-ray observatories.

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arXiv2020

The $γ$-ray Emission of Star-Forming Galaxies

A majority of the $γ$-ray emission from star-forming galaxies is generated by the interaction of high-energy cosmic rays with the interstellar gas and radiation fields. Star-forming galaxies are expected to contribute to both the extragalactic $γ$-ray background and the IceCube astrophysical neutrino flux. Using roughly 10\,years of $γ$-ray data taken by the {\it Fermi} Large Area Telescope, in this study we constrain the $γ$-ray properties of star-forming galaxies. We report the detection of 11 bona-fide $γ$-ray emitting galaxies and 2 candidates. Moreover, we show that the cumulative $γ$-ray emission of below-threshold galaxies is also significantly detected at $\sim$5\,$σ$ confidence. The $γ$-ray luminosity of resolved and unresolved galaxies is found to correlate with the total (8-1000\,$μ$m) infrared luminosity as previously determined. Above 1\,GeV, the spectral energy distribution of resolved and unresolved galaxies is found to be compatible with a power law with a photon index of $\approx2.2-2.3$. Finally, we find that star-forming galaxies account for roughly 5\,\% and 3\,\% of the extragalactic $γ$-ray background and the IceCube neutrino flux, respectively.

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arXiv2020

Fermi and Swift Observations of GRB 190114C: Tracing the Evolution of High-Energy Emission from Prompt to Afterglow

We report on the observations of gamma-ray burst (GRB) 190114C by the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory. The early-time observations reveal multiple emission components that evolve independently, with a delayed power-law component that exhibits significant spectral attenuation above 40 MeV in the first few seconds of the burst. This power-law component transitions to a harder spectrum that is consistent with the afterglow emission observed at later times. This afterglow component is clearly identifiable in the GBM and BAT light curves as a slowly fading emission component on which the rest of the prompt emission is superimposed. As a result, we are able to constrain the transition from internal shock to external shock dominated emission. We find that the temporal and spectral evolution of the broadband afterglow emission can be well modeled as synchrotron emission from a forward shock propagating into a wind-like circumstellar environment and find that high-energy photons observed by Fermi LAT are in tension with the theoretical maximum energy that can be achieved through synchrotron emission from a shock. These violations of the maximum synchrotron energy are further compounded by the detection of very high energy (VHE) emission above 300 GeV by MAGIC concurrent with our observations. We conclude that the observations of VHE photons from GRB 190114C necessitates either an additional emission mechanism at very high energies that is hidden in the synchrotron component in the LAT energy range, an acceleration mechanism that imparts energy to the particles at a rate that is faster than the electron synchrotron energy loss rate, or revisions of the fundamental assumptions used in estimating the maximum photon energy attainable through the synchrotron process.

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arXiv2019

Unresolved Gamma-Ray Sky through its Angular Power Spectrum

The gamma-ray sky has been observed with unprecedented accuracy in the last decade by the Fermi large area telescope (LAT), allowing us to resolve and understand the high-energy Universe. The nature of the remaining unresolved emission (unresolved gamma-ray background, UGRB) below the LAT source detection threshold can be uncovered by characterizing the amplitude and angular scale of the UGRB fluctuation field. This work presents a measurement of the UGRB autocorrelation angular power spectrum based on eight years of Fermi LAT Pass 8 data products. The analysis is designed to be robust against contamination from resolved sources and noise systematics. The sensitivity to subthreshold sources is greatly enhanced with respect to previous measurements. We find evidence (with $\sim$3.7$σ$ significance) that the scenario in which two classes of sources contribute to the UGRB signal is favored over a single class. A double power law with exponential cutoff can explain the anisotropy energy spectrum well, with photon indices of the two populations being 2.55 $\pm$ 0.23 and 1.86 $\pm$ 0.15.

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arXiv2018

Signatures of the disk-jet coupling in the Broad-line Radio Quasar 4C+74.26

Here we explore the disk-jet connection in the broad-line radio quasar 4C+74.26, utilizing the results of the multiwavelength monitoring of the source. The target is unique in that its radiative output at radio wavelengths is dominated by a moderately-beamed nuclear jet, at optical frequencies by the accretion disk, and in the hard X-ray range by the disk corona. Our analysis reveals a correlation (local and global significance of 96\% and 98\%, respectively) between the optical and radio bands, with the disk lagging behind the jet by $250 \pm 42$ days. We discuss the possible explanation for this, speculating that the observed disk and the jet flux changes are generated by magnetic fluctuations originating within the innermost parts of a truncated disk, and that the lag is related to a delayed radiative response of the disk when compared with the propagation timescale of magnetic perturbations along relativistic outflow. This scenario is supported by the re-analysis of the NuSTAR data, modelled in terms of a relativistic reflection from the disk illuminated by the coronal emission, which returns the inner disk radius $R_{\rm in}/R_{\rm ISCO} =35^{+40}_{-16}$. We discuss the global energetics in the system, arguing that while the accretion proceeds at the Eddington rate, with the accretion-related bolometric luminosity $L_{\rm bol} \sim 9 \times 10^{46}$ erg s$^{-1}$ $\sim 0.2 L_{\rm Edd}$, the jet total kinetic energy $L_\textrm{j} \sim 4 \times 10^{44}$ erg s$^{-1}$, inferred from the dynamical modelling of the giant radio lobes in the source, constitutes only a small fraction of the available accretion power.

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arXiv2018

Search for extended sources in the Galactic Plane using 6 years of Fermi-Large Area Telescope Pass 8 data above 10 GeV

The spatial extension of a gamma-ray source is an essential ingredient to determine its spectral properties as well as its potential multi-wavelength counterpart. The capability to spatially resolve gamma-ray sources is greatly improved by the newly delivered Fermi-Large Area Telescope (LAT) Pass 8 event-level analysis which provides a greater acceptance and an improved point spread function, two crucial factors for the detection of extended sources. Here, we present a complete search for extended sources located within 7 degrees from the Galactic plane, using 6 years of LAT data above 10 GeV. We find 46 extended sources and provide their morphological and spectral characteristics. This constitutes the first catalog of hard LAT extended sources, named the Fermi Galactic Extended Source Catalog, which allows a thorough study of the properties of the Galactic plane in the sub-TeV domain.

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arXiv2017

The second catalog of flaring gamma-ray sources from the Fermi All-sky Variability Analysis

We present the second catalog of flaring gamma-ray sources (2FAV) detected with the Fermi All-sky Variability Analysis (FAVA), a tool that blindly searches for transients over the entire sky observed by the Large Area Telescope (LAT) on board the \textit{Fermi} Gamma-ray Space Telescope. With respect to the first FAVA catalog, this catalog benefits from a larger data set, the latest LAT data release (Pass 8), as well as from an improved analysis that includes likelihood techniques for a more precise localization of the transients. Applying this analysis on the first 7.4 years of \textit{Fermi} observations, and in two separate energy bands 0.1$-$0.8 GeV and 0.8$-$300 GeV, a total of 4547 flares has been detected with a significance greater than $6σ$ (before trials), on the time scale of one week. Through spatial clustering of these flares, 518 variable gamma-ray sources are identified. Likely counterparts, based on positional coincidence, have been found for 441 sources, mostly among the blazar class of active galactic nuclei. For 77 2FAV sources, no likely gamma-ray counterpart has been found. For each source in the catalog, we provide the time, location, and spectrum of each flaring episode. Studying the spectra of the flares, we observe a harder-when-brighter behavior for flares associated with blazars, with the exception of BL Lac flares detected in the low-energy band. The photon indexes of the flares are never significantly smaller than 1.5. For a leptonic model, and under the assumption of isotropy, this limit suggests that the spectrum of the freshly accelerated electrons is never harder than $p\sim$2.

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arXiv2017

High-redshift blazars through nustar eyes

The most powerful sources among the blazar family are MeV blazars. Often detected at $z>2$, they usually display high X- and \gm-ray luminosities, larger-than-average jet powers and black hole masses $\gtrsim 10^9 M_{\odot}$. In the present work we perform a multiwavelength study of three high redshift blazars: 3FGL J0325.5+2223 ($z=2.06$), 3FGL J0449.0+1121 ($z= 2.15$), and 3FGL J0453.2$-$2808 ($z=2.56$), analysing quasi simultaneous data from GROND, \swift-UVOT and XRT, \nustar, and \fermi-LAT. Our main focus is on the hard X-ray band recently unveiled by \nustar~(3$-$79 keV) where these objects show a hard spectrum which enables us to constrain the inverse Compton peak and the jet power. We found that all three targets resemble the most powerful blazars, with the synchrotron peak located in the sub-millimeter range and the inverse Compton peak in the MeV range, and therefore belong to the MeV blazar class. Using a simple one zone leptonic emission model to reproduce the spectral energy distributions, we conclude that a simple combination of synchrotron and accretion disk emission reproduces the infrared-optical spectra while the X-ray to \gm-ray part is well reproduced by the inverse Compton scattering of low energy photons supplied by the broad line region. The black hole masses for each of the three sources are calculated to be $\gtrsim 4 \times 10^{8} M_{\odot}$. The three studied sources have jet power at the level of, or beyond, the accretion luminosity.

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arXiv2016

New High-z Fermi BL Lacs with the Photometric Dropout Technique

Determining redshifts for BL Lacertae (BL Lac) objects using the traditional spectroscopic method is challenging due to the absence of strong emission lines in their optical spectra. We employ the photometric dropout technique to determine redshifts for this class of blazars using the combined 13 broad-band filters from Swift-UVOT and the multi-channel imager GROND at the MPG 2.2 m telescope at ESO's La Silla Observatory. The wavelength range covered by these 13 filters extends from far ultraviolet to the near-Infrared. We report results on 40 new Fermi detected BL Lacs with the photometric redshifts determinations for 5 sources, with 3FGL J1918.2-4110 being the most distant in our sample at z=2.16. Reliable upper limits are provided for 20 sources in this sample. Using the highest energy photons for these Fermi-LAT sources, we evaluate the consistency with the Gamma-ray horizon due to the extragalactic background light.

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arXiv2016

Detection of Possible Quasi-periodic Oscillations in the Long-term Optical Light Curve of the BL Lac Object OJ 287

Detection of periodicity in the broad-band non-thermal emission of blazars has so far been proven to be elusive. However, there are a number of scenarios which could lead to quasi-periodic variations in blazar light curves. For example, orbital or thermal/viscous period of accreting matter around central supermassive black holes could, in principle, be imprinted in the multi-wavelength emission of small-scale blazar jets, carrying as such crucial information about plasma conditions within the jet launching regions. In this paper, we present the results of our time series analysis of $\sim 9.2$ year-long, and exceptionally well-sampled optical light curve of the BL Lac OJ 287. The study primarily uses the data from our own observations performed at the Mt. Suhora and Kraków Observatories in Poland, and at the Athens Observatory in Greece. Additionally, SMARTS observations were used to fill in some of the gaps in the data. The Lomb-Scargle Periodogram and the Weighted Wavelet Z-transform methods were employed to search for the possible QPOs in the resulting optical light curve of the source. Both the methods consistently yielded possible quasi-periodic signal around the periods of $\sim 400$ and $\sim 800$ days, the former one with a significance (over the underlying colored noise) of $\geq 99\%$. A number of likely explanations for such are discussed, with a preference given to a modulation of the jet production efficiency by highly magnetized accretion disks. This supports the previous findings and the interpretation reported recently in the literature for OJ 287 and other blazar sources.

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arXiv2016

NuSTAR, Swift, and GROND observations of the flaring MeV blazar: PMN J0641$-$0320

MeV blazars are a sub--population of the blazar family, exhibiting larger--than--average jet powers, accretion luminosities and black hole masses. Because of their extremely hard X--ray continua, these objects are best studied in the X-ray domain. Here, we report on the discovery by the $Fermi$ Large Area Telescope and subsequent follow-up observations with $NuSTAR$, $Swift$ and GROND of a new member of the MeV blazar family: PMN J0641$-$0320. Our optical spectroscopy provides confirmation that this is a flat--spectrum radio quasar located at a redshift of $z=1.196$. Its very hard $NuSTAR$ spectrum (power--law photon index of $\sim$1 up to $\sim$80 keV) indicates that the emission is produced via inverse Compton scattering off photons coming from outside the jet.The overall spectral energy distribution of PMN J0641$-$0320 is typical of powerful blazars and by reproducing it with a simple one-zone leptonic emission model we find the emission region to be located either inside the broad line region or within the dusty torus.

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