arXiv subjects

G. Harutyunyan

Publications and source records attributed to G. Harutyunyan.

16 recordsLinked to original sources

arXiv2026

Revisiting EBL Constraints from Gamma-Ray Observations: A Critical Assessment and Methodological Improvements

The extragalactic background light (EBL), ranging from the infrared to the ultraviolet bands, is the second most intense photon field in the universe, surpassed only by the cosmic microwave background (CMB). It is primarily generated by starlight in galaxies, either directly or through absorption by dust and re-emission at longer wavelengths. Very high energy (VHE, $E$ > $100\,$GeV) photons can be absorbed via $γγ$ interactions with the EBL during their propagation across cosmological distances, providing an indirect method to probe the EBL by studying its impact on the energy spectra of distant VHE sources. This paper examines the robustness of EBL constraints derived from gamma-ray data, critically assessing the assumptions made in previous studies about the intrinsic source spectra, the uncertainties of the observations and the validity of the analysis tools. We find that earlier studies likely underestimated the uncertainties in the EBL intensity constraints, underscoring the need to account for systematic uncertainties comprehensively. By employing a Monte Carlo (MC) simulation and a plausible model for unknown systematic errors, we compute more realistic uncertainties. Additionally, we discuss possible alternatives to set EBL constraints, relaxing the assumptions on the intrinsic spectra of gamma-ray sources, with the goal of achieving more robust constraints.

astro-ph.HE↗

arXiv2026

Long-term study of the gamma-ray emission of Cygnus X-3 with MAGIC and Fermi-LAT

Cygnus X-3 is a microquasar composed of a compact object of unknown nature closely orbiting around a Wolf-Rayet star. The particularities of this source make it a unique case among microquasars. This fact, together with its recent establishment as a PeV particle accelerator, makes Cygnus X-3 a very interesting target for the investigation of the physical processes leading to gamma-ray production. In this work, the TeV and GeV gamma-ray emission of Cygnus X-3 is studied in order to determine its origin and constrain the properties of the system. For that purpose, a point-like analysis of 130 h of data taken with the MAGIC telescopes between 2013 and 2024 was performed, which represents the largest available sample for Cygnus X-3 at $\sim$TeV energies. Additionally, contemporary data from Fermi-LAT were also analysed to better contextualize the MAGIC observations. For a more detailed investigation of the source physics, the data were divided into three subsets according to the flaring state of the source and orbital phase. No significant detection of Cygnus X-3 is found between 0.1 and 7 TeV for any of the datasets, and differential and integral flux upper limits are reported over the long-term monitoring of the source. The Fermi-LAT fluxes can be considered compatible with previous results, taking into account the different data samples used across studies. The MAGIC upper limits presented in this work represent the most constraining ones up to date at $\sim$TeV energies. An eventual detection of Cygnus X-3 at these energies would significantly constrain the source properties, and is not unreasonable to expect given that the source has already been detected in both the GeV and PeV regimes during flaring states. Further observations of Cygnus X-3 at energies above tens of GeV would be valuable for this purpose.

astro-ph.HE↗

arXiv2026

The January 2010 flare of Mrk421: Insights from a stochastic acceleration model

Mrk421 displayed its highest flux state ever observed in February of 2010 with very high TeV fluxes and interesting cross-band correlations and a spectral energy distribution (SED) evolution not entirely consistent with the standard single zone leptonic synchrotron self-Compton model. The source was already in a high state in January 2010 and displayed strong variability in the days preceding the highest state. We study the temporal evolution of the spectra in January to extract information about the particle dynamics and the physical properties of the emission region. We build up on the temporal variability and correlations studied in the previous work (MAGIC collaboration - Abe et al. 2025) and attempt to improve the SED model fits with a physics oriented approach. The multi-wavelength data was processed and the SEDs were fit using JetSeT. The SED evolution and cross band correlations were modelled using leptonic log-parabola with a low energy power-law branch (LPPL) and pile-up distributions that are predicted in a stochastic acceleration scenario. A simplified temporal evolution model was developed and fit to the SEDs and the resulting trends and phenomenology were characterised in context of theoretical literature. An expanding emission region model was also tested. We find the spectral variability to be well in agreement with stochastic acceleration. Our analysis suggests that the standard LPPL distribution develops a Maxwellian pile-up component at the transition from acceleration to cooling dominated phase on 3 nights in the dataset, as also hinted by the very-high energy and X-ray light curves. The resulting phenomenology of our sequential snapshot evolution SED model agrees well with theoretical and numerical simulation studies on temporal evolution using the diffusion equation approach.

astro-ph.HE↗

arXiv2026

MAGIC observations of NGC 4278. The first low-luminosity radio galaxy with compact jets detected at TeV energies

The Large High Altitude Air Shower Observatory (LHAASO) Collaboration has recently reported the first detection at TeV energies of a low-luminosity radio galaxy, NGC 4278. The aim of this work is to investigate the high-energy properties of NGC 4278 during the flaring and subsequent quasi-quiescent states with the Florian Goebel Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes. NGC 4278 is located in the field of view of two blazars, 1ES 1215+303 and 1ES 1218+304, previously observed by the MAGIC telescopes. Therefore, we re-analyzed MAGIC observations made between 2010 and 2024 on these sources. We also modeled the broadband spectral energy distribution of the source during and after the flaring state at TeV energies. We did not detect any statistically significant $γ$-ray emission from NGC 4278 with MAGIC. The corresponding upper limits obtained using the entire MAGIC dataset ($F_{{\rm UL, }\, >150\, \mathrm{GeV}}=1.5 \times 10^{-12}\, \mathrm{ph \, s^{-1}\, cm^{-2}}$) are consistent with the LHAASO results. The best-fit models obtained for both emission states suggest that the emitting region is strongly particle-dominated, and an efficient acceleration mechanism has to be in action in order to reach TeV energies. The transition between the flaring and quasi-quiescent state cannot be explained by a simple radiative cooling of the emitting particles. The inferred jet power, of the order of $L_{\rm jet}\sim 10^{42}\, \mathrm{erg\,s^{-1}}$, is dominated by the kinetic component in both states and it is in a good agreement with previous, time-averaged observational estimates, supporting the idea that such high-energy flares might be recurrent. The jet, however, remains too weak to break the host-galaxy confinement.

astro-ph.HE↗

arXiv2025

A comprehensive view of PKS 2155-304 from 2008 to 2023 through a multi-epoch modeling of its spectral energy distributions

We present a detailed investigation of the temporal and spectral evolution of the emission from the blazar PKS 2155-304, a high-synchrotron-peaked (HSP) blazar. Using $γ$-ray, X-ray, optical/UV, and infrared data assembled from the Markarian Multiwavelength Data Center, we constructed multi-band light curves and temporally resolved spectral energy distributions (SEDs) of PKS 2155-304 to probe the origin of its emission. The light curves show significant variability, with fractional variability peaking at 0.75 in X-rays, 0.4 in the optical/UV, and 0.65 in $γ$-ray band-consistent with expectations for HSPs. Segmenting the $γ$-ray light curve with Bayesian blocks, we defined 253 time-resolved epochs with adequate multi-band coverage and categorized them into quiescent states (QS), multiwavelength flares (MWF), $γ$-ray flares ($γ$F), X-ray flares (XF), and optical/UV flares (OUF). Each SED is modeled within a synchrotron self-Compton (SSC) framework that self-consistently evolves particle injection and cooling; a neural-network surrogate is used to accelerate parameter inference. Kolmogorov-Smirnov tests reveal state-dependent parameter variations relative to QS: (i) during MWF, the magnetic field B, electron luminosity $L_{e}$, maximum electron Lorentz factor $γ_{max}$, and Doppler factor $δ$ differ significantly; (ii) during $γ$F, a harder electron index p is estimated; (iii) XF shows higher B and $γ_{max}$ with a more compact emitting region; and (IV) during OUF, changes in B, $L_{e}$, $γ_{max}$, $δ$, and p are found while the emitting-zone size remains approximately constant. The jet power is electron-dominated (magnetic-to-electron power ratio $η_{B}\simeq0.09-0.17$), with $η_{B}$ rising during XF. These results suggest that variations in acceleration efficiency and magnetization drive band-dependent flaring in PKS 2155-304.

astro-ph.HE↗

arXiv2025

Modeling of the time-resolved spectral energy distribution of blazar OJ 287 from 2008 to 2023: a comprehensive multi-epoch study

We present a comprehensive analysis of the time-resolved spectral energy distributions (SEDs) of the blazar OJ 287 over a 15-year period (2008-2023), using multi-wavelength data. In the $γ$-ray band, multiple flaring episodes were observed, with the strongest flare reaching a peak flux of $(5.60\pm1.11)\times10^{-7}\:{\rm photons\:cm^{-2}\:s^{-1}}$ on MJD 55869.03 (04 November 2011). In the optical/UV band, the source was in an active state between MJD 57360 (04 December 2015) and 57960 (26 July 2017), during which the highest flux of $(1.07\pm0.02)\times10^{-10}\:{\rm erg\:cm^{-2}\:s^{-1}}$ was observed on MJD 57681.23 (20 October 2016). In the X-ray band, both the flux and spectral index exhibit variability. To investigate the origin of the broadband emission from OJ 287, we systematically modeled 739 quasi-simultaneous SEDs using a leptonic model that self-consistently accounts for particle injection and cooling. This analysis is possible thanks to the recent development of a surrogate neural-network-based model, trained on kinetic simulations. This innovative, time-resolved, neural network-based approach overcomes the limitations of traditional single-epoch SED modeling, enabling to explore the temporal evolution of key model parameters, such as the magnetic field strength, Doppler factor, and electron injection distribution, across different states of the source. We identified distinct emission states characterized by unique combinations of magnetic field $ B $, electron index $ p $, and Doppler boost $ δ$, associated to different underlying mechanisms such as varying acceleration processes (e.g., shocks, turbulence) and magnetic confinement. The analysis provides insights into the jet physics processes, including particle acceleration mechanisms and dynamic changes in the jet structure.

astro-ph.HE↗

arXiv2024

Markarian Multiwavelength Data Center (MMDC): A Tool for Retrieving and Modeling Multi-temporal, Multi-wavelength and Multi-messenger Data from Blazar Observations

The Markarian Multiwavelength Data Center (MMDC) is a web-based tool designed for accessing and retrieving multiwavelength and multimessenger data from blazar observations. MMDC facilitates the construction and interactive visualization of time-resolved multi-band spectral energy distributions (SEDs) of blazars by integrating: \textit{(i)} archival data from over 80 catalogs and databases, \textit{(ii)} optical data from all-sky survey facilities such as ASAS-SN, ZTF, and Pan-STARRS, and \textit{(iii)} newly analyzed datasets in the optical/UV band from \textit{Swift}-UVOT, in the X-ray band from \textit{Swift}-XRT and NuSTAR observations, and the high-energy $γ$-ray band from \textit{Fermi}-LAT observations. MMDC distinguishes itself from other online platforms by the large quantity of available data. For instance, it includes data from all blazar observations by \textit{Swift} and NuSTAR, as well as the results of detailed spectral analysis in the $γ$-ray band during different emission states, covering the period from 2008 to 2023. Another important distinguishing feature of MMDC is its ability to enable precise, self-consistent theoretical modeling of the observed data using machine learning algorithms trained on leptonic and lepto-hadronic models, which consider the injection of particles and all relevant cooling processes. MMDC is an innovative tool which significantly enhances blazar research by providing a comprehensive framework for data accessibility, analysis, and theoretical interpretation, thereby advancing our understanding of blazar emissions and the underlying astrophysical processes.

astro-ph.HE↗

arXiv2024

Broadband Study of Gamma-Ray Blazars at Redshifts $z=2.0-2.5$

High redshift blazars are among the most powerful non-explosive sources in the Universe and play a crucial role in understanding the evolution of relativistic jets. To understand these bright objects, we performed a detailed investigation of the multiwavelength properties of 79 $γ$-ray blazars with redshifts ranging from z = 2.0 to 2.5, using data from Fermi LAT, Swift XRT/UVOT, and NuSTAR observations. In the $γ$-ray band, the spectral analysis revealed a wide range of flux and photon indices, from $5.32 \times 10^{-10}$ to $3.40 \times 10^{-7}$ photons cm$^{-2}$ s$^{-1}$ and from 1.66 to 3.15, respectively, highlighting the diverse nature of these sources. The detailed temporal analysis showed that flaring activities were observed in 31 sources. Sources such as 4C+71.07, PKS 1329-049, and 4C+01.02, demonstrated significant increase in the $γ$-ray luminosity and flux variations, reaching peak luminosity exceeding $10^{50}$ erg s$^{-1}$. The temporal analysis extended to X-ray and optical/UV bands, showed clear flux changes in some sources in different observations. The time-averaged properties of high redshift blazars were derived through modeling the spectral energy distributions with a one-zone leptonic scenario, assuming the emission region is within the broad-line region (BLR) and the X-ray and $γ$-ray emissions are due to inverse Compton scattering of synchrotron and BLR-reflected photons. This modeling allowed us to constrain the emitting particle distribution, estimate the magnetic field inside the jet, and evaluate the jet luminosity, which is discussed in comparison with the disk luminosity derived from fitting the excess in the UV band.

astro-ph.HE↗

arXiv2023

Origin of multiwavelength emission from flaring high redshift blazar PKS 0537-286

The high redhsift blazars powered by supermassive black holes with masses exceeding $10^9\:M_\odot$ have the highest jet power and luminosity and are important probes to test the physics of relativistic jets at the early epochs of the Universe. We present a multi-frequency spectral and temporal study of high redshift blazar PKS 0537-286 by analyzing data from Fermi-LAT, NuSTAR Swift XRT and UVOT. Although the time averaged $γ$-ray spectrum of the source is relatively soft (indicating the high-energy emission peak is below the GeV range), several prominent flares were observed when the spectrum hardened and the luminosity increased above $10^{49}\:{\rm erg\:s^{-1}}$. The X-ray emission of the source varies in different observations and is characterised by a hard spectrum $\leq1.38$ with a luminosity of $>10^{47}\:{\rm erg\:s^{-1}}$. The broadband spectral energy distribution in the quiescent and flaring periods was modeled within a one-zone leptonic scenario assuming different locations of the emission region and considering both internal (synchrotron radiation) and external (from the disk, broad-line region and dusty torus) photon fields for the inverse Compton scattering. The modeling shows that the most optimistic scenario, from the energy requirement point of view, is when the jet energy dissipation occurs within the broad-line region. The comparison of the model parameters obtained for the quiescent and flaring periods suggests that the flaring activities are most likely caused by the hardening of the emitting electron spectral index and shifting of the cut-off energy to higher values.

astro-ph.HE↗

arXiv2022

Modeling the time variable spectral energy distribution of the blazar CTA 102 from 2008 to 2022

We present long-term multiwavelength observations of blazar CTA 102 ($z=1.037$). Detailed temporal and spectral analyses of $γ$-ray, X-ray and UV/optical data observed by {\it Fermi}-LAT, Swift XRT, NuSTAR and Swift-UVOT over a period of 14 years, between August 2008 and March 2022, was performed. We found strong variability of source emission in all the considered bands, especially in the $γ$-ray band it exhibited extreme outbursts when the flux crossed the level of $10^{-5}\:{\rm photon\:cm^{-2}\:s^{-1}}$. Using the Bayesian Blocks algorithm, we split the adaptively binned $γ$-ray light curve into 347 intervals of quiescent and flaring episodes and for each period built corresponding multiwavelength spectral energy distributions (SEDs), using the available data. Among the considered SEDs, 117 high-quality (quasi) contemporaneous SEDs which have sufficient multiwavelength data, were modeled using JetSeT framework within a one-zone leptonic synchrotron and inverse Compton emission scenario assuming the emitting region is within the broad-line-region and considering internal and external seed photons for the inverse Compton up-scattering. As a result of modeling, the characteristics of the relativistic electron distribution in the jet as well as jet properties are retrieved and their variation in time is investigated. The applied model can adequately explain the assembled SEDs and the modelling shows that the data in the bright flaring periods can be reproduced for high Doppler boosting and magnetic field. The obtained results are discussed in the context of particle cooling in the emitting region.

astro-ph.HE↗

arXiv2020

Multiwavelength Study of High-Redshift Blazars

High-redshift blazars are among the most powerful objects in the Universe. The spectral and temporal properties of thirty-three distant blazars ($z>2.5$) detected in the high energy $γ$-ray band are investigated by analyzing the {\it Fermi}-LAT and {\it Swift} UVOT/ XRT data. The considered sources have soft time averaged $γ$-ray spectra ($Γ_{\rm γ}\geq2.2$) whereas those that have been observed in the X-ray band have hard X-ray spectra ($Γ_{\rm X}=1.01-1.86$). The $γ$-ray flux of high-redshift blazars ranges from $4.84\times10^{-10}$ to $1.50\times10^{-7}\:{\rm photon\:cm^{-2}\:s^{-1}}$ and the luminosity is within $(0.10-5.54)\times10^{48}\:{\rm erg\:s^{-1}}$ which during the $γ$-ray flares increases up to $(0.1-1)\times10^{50}\:{\rm erg\:s^{-1}}$. In the X-ray band, only the emission of PKS 0438-43, B2 0743+25 and TXS 0222+185 is found to vary in different Swift XRT observations whereas in the $γ$-ray band, the emission is variable for fourteen sources: the flux of B3 1343+451 and PKS 0537-286 changes in sub-day scales, that of PKS 0347-211 and PKS 0451-28 in day scales, while the$γ$-ray variability of the others is in week or month scales. The properties of distant blazar jets are derived by modeling the multiwavelength spectral energy distributions within a one-zone leptonic scenario assuming that the X-ray and $γ$-ray emissions are produced from inverse Compton scattering of synchrotron and dusty torus photons. From the fitting, the emission region size is found to be $\leq0.05$ pc and the magnetic field and the Doppler factor are correspondingly within $0.10-1.74$ G and $10.0-27.4$. By modeling the optical-UV excess, we found that the central black hole masses and accretion disk luminosities are within $L_{\rm d}\simeq(1.09-10.94)\times10^{46}\:{\rm erg \: s^{-1}}$ and $(1.69-5.35)\times10^{9}\:M_\odot$, respectively.

astro-ph.HE↗

arXiv2018

3D non-LTE corrections for Li abundance and 6Li/7Li isotopic ratio in solar-type stars I. Application to HD 207129 and HD 95456

Convective motions in solar-type stellar atmospheres induce Doppler shifts that affect the strengths and shapes of spectral absorption lines and create slightly asymmetric line profiles. 1D LTE studies of elemental abundances are not able to reproduce this phenomenon, which becomes particularly important when modeling the impact of isotopic fine structure, like the subtle depression created by the 6Li isotope on the red wing of the Li I resonance doublet line. The purpose of this work is to provide corrections for the lithium abundance, A(Li), and the 6Li/7Li ratio that can easily be applied to correct 1D LTE lithium abundances in G and F dwarf stars of approximately solar mass and metallicity for 3D and non-LTE (NLTE) effects. The corrections for A(Li) and 6Li/7Li are computed using grids of 3D NLTE and 1D LTE synthetic lithium line profiles, generated from 3D hydro-dynamical CO5BOLD and 1D hydrostatic model atmospheres, respectively. The 3D NLTE corrections are then approximated by analytical expressions as a function of the stellar parameters. These are applied to adjust the 1D LTE isotopic lithium abundances in two solar-type stars, HD 207129 and HD 95456. The derived 3D NLTE corrections range between -0.01 and +0.11 dex for A(Li), and between -4.9 and -0.4% for the 6Li/7Li ratio, depending on the adopted stellar parameters. In the case of the 6Li/7Li ratio, our corrections are always negative, showing that 1D LTE analysis can significantly overestimate (up to 4.9 percentage points) the presence of 6Li in the atmospheres of solar-like dwarf stars. These results emphasize the importance of reliable 3D model atmospheres combined with NLTE line formation for deriving precise isotopic lithium abundances. Although 3D NLTE spectral synthesis implies an extensive computational effort, the results can be made accessible with parametric tools like the ones presented in this work.

astro-ph.SR↗

arXiv2016

Anti-solar differential rotation on the active sub-giant HU Virginis

Measuring surface differential rotation (DR) on different types of stars is important when characterizing the underlying stellar dynamo. It has been suggested that anti-solar DR laws can occur when strong meridional flows exist. We aim to investigate the differential surface rotation on the primary star of the RS CVn binary HU Vir by tracking its starspot distribution as a function of time. We also aim to recompute and update the values for several system parameters of the triple system HU Vir (close and wide orbits). Time-series high-resolution spectroscopy for four continuous months was obtained with the 1.2-m robotic STELLA telescope. Nine consecutive Doppler images were reconstructed from these data, using our line-profile inversion code iMap. An image cross-correlation method was applied to derive the surface differential-rotation law for HU Vir. New orbital elements for the close and the wide orbits were computed using our new STELLA radial velocities (RVs) combined with the RV data available in the literature. Photometric observations were performed with the Amadeus Automatic Photoelectric Telescope (APT), providing contemporaneous Johnson-Cousins $V$ and $I$ data for approximately 20 years. This data was used to determine the stellar rotation period and the active longitudes. We confirm anti-solar DR with a surface shear parameter $α$ of -0.029 $\pm$ 0.005 and -0.026 $\pm$ 0.009, using single-term and double-term differential rotation laws, respectively. The best fit is achieved assuming a solar-like double-term law with a lap time of $\approx$ 400 d. Our orbital solutions result in a period of 10.387678 $\pm$ 0.000003 days for the close orbit and 2726 $\pm$ 7 d ($\approx$ 7.5 yr) for the wide orbit. A Lomb-Scarge (L-S) periodogram of the pre-whitened $V$-band data reveals a strong single peak providing a rotation period of 10.391 $\pm$ 0.008 d.

astro-ph.SR↗

arXiv2016

Abundance trend with condensation temperature for stars with different Galactic birth places

During the past decade, several studies reported a correlation between chemical abundances of stars and condensation temperature (also known as Tc trend). However, the real astrophysical nature of this correlation is still debated. The main goal of this work is to explore the possible dependence of the Tc trend on stellar Galactocentric distances, Rmean. We used high-quality spectra of about 40 stars observed with the HARPS and UVES spectrographs to derive precise stellar parameters, chemical abundances, and stellar ages. A differential line-by-line analysis was applied to achieve the highest possible precision in the chemical abundances. We confirm previous results that [X/Fe] abundance ratios depend on stellar age and that for a given age, some elements also show a dependence on Rmean. When using the whole sample of stars, we observe a weak hint that the Tc trend depends on Rmean. The observed dependence is very complex and disappears when only stars with similar ages are considered. To conclude on the possible dependence of the Tc trend on the formation place of stars, a larger sample of stars with very similar atmospheric parameters and stellar ages observed at different Galactocentric distances is needed

astro-ph.SR↗

arXiv2016

$ζ^2$ Ret, its debris disk, and its lonely stellar companion $ζ^1$ Ret. Different $T_{\mathrm{c}}$ trends for different spectra

Several studies have reported a correlation between the chemical abundances of stars and condensation temperature (known as Tc trend). Very recently, a strong Tc trend was reported for the $ζ$ Reticuli binary system, which consists of two solar analogs. The observed trend in $ζ^2$ Ret relative to its companion was explained by the presence of a debris disk around $ζ^2$ Ret. Our goal is to re-evaluate the presence and variability of the Tc trend in the $ζ$ Reticuli system and to understand the impact of the presence of the debris disk on a star. We used very high-quality spectra of the two stars retrieved from the HARPS archive to derive very precise stellar parameters and chemical abundances. We derived the stellar parameters with the classical (nondifferential) method, while we applied a differential line-by-line analysis to achieve the highest possible precision in abundances, which are fundamental to explore for very tiny differences in the abundances between the stars. We confirm that the abundance difference between $ζ^2$ Ret and $ζ^1$ Ret shows a significant ($\sim$ 2 $σ$) correlation with Tc. However, we also find that the Tc trends depend on the individual spectrum used (even if always of very high quality). In particular, we find significant but varying differences in the abundances of the same star from different individual high-quality spectra. Our results for the $ζ$ Reticuli system show, for example, that nonphysical factors, such as the quality of spectra employed and errors that are not accounted for, can be at the root of the Tc trends for the case of individual spectra.

astro-ph.SR↗

arXiv2011

Asymptotic Parametrices of Elliptic Edge Operators

We study operators on a singular manifold, here of conical or edge type, and develop a new general approach of representing asymptotics of solutions to elliptic equations close to the singularities. The idea is to construct so-called asymptotic parametrices with flat left -over terms. Our structures are motivated by models of particle physics with singular potentials that contribute embedded singularities in $\R^N$ of higher order, according to the number of particles.

math.AP↗