arXiv subjects

D. Zargaryan

Publications and source records attributed to D. Zargaryan.

5 recordsLinked to original sources

arXiv2022

And then they were two: detection of non-thermal radio emission from the bow shocks of two runaway stars

Winds from massive stars have recently been deemed promising sites for investigating relativistic particle acceleration. Particularly, the resulting bow shock from the interaction of the winds of runaway stars with interstellar matter has been observed at multiple wavelengths. Here we investigate the O4If star, BD+433654, the bow shock of which is, so far, the only one proven to radiate both thermally and non-thermally at radio frequencies. We also consider NGC7635 as a bow shock candidate and examine its apex for indications of thermal and non-thermal radio emission. We observed both sources with the VLA at 4-8 GHz and 8-12 GHz, and with the Effelsberg telescope at 4-8 GHz. We analysed data from both telescopes individually and combined, obtained their spectral index maps and calculated their Spectral Energy Distributions. We present the first high-resolution maps of radio emission from NGC7635. We find that both emit non-thermal emission in the radio regime, with the clearest evidence for NGC7635. Our results are less conclusive for BD+433654, as the emission from its bow shock becomes weaker and fainter at higher radio frequencies. Our results extend the previous radio results for the BD+433654 bow shock to higher frequencies. Modelling of our data for both sources shows that accelerated electrons at the wind termination shock are a plausible source for the non-thermal radio emission, but energetics arguments suggest that any non-thermal X-ray and $γ$-ray emission could be significantly below existing upper limits. Enhanced synchrotron emission from compressed Galactic cosmic rays in the radiative bow shock could also explain the radio emission from the BD+433654 bow shock but not NGC7635. Non-detection of point-like radio emission from BD+433654 puts an upper limit on the mass-loss rate of the star that is lower than values quoted in the literature. [abridged]

astro-ph.HE↗

arXiv2018

On the multi-wavelength Emission from CTA 102

We report on broadband observations of CTA 102 ($z=1.037$) during the active states in 2016-2017. In the $γ$-ray band, Fermi LAT observed several prominent flares which followed a harder-when-brighter behavior: the hardest photon index $Γ=1.61\pm 0.10$ being unusual for FSRQs. The peak $γ$-ray flux above 100 MeV $(3.55\pm0.55)\times10^{-5}\:{\rm photon\:cm^{-2}\:s^{-1}}$ observed on MJD 57738.47 within 4.31 minutes, corresponds to an isotropic $γ$-ray luminosity of $L_γ=3.25\times10^{50}\:{\rm erg\:s^{-1}}$, comparable with the highest values observed from blazars so far. The analyses of the Swift UVOT/XRT data show an increase in the UV/optical and X-ray bands which is contemporaneous with the bright $γ$-ray periods. The X-ray spectrum observed by Swift XRT and NuSTAR during the $γ$-ray flaring period is characterized by a hard photon index of $\sim1.30$. The shortest e-folding time was $4.08\pm1.44$ hours, suggesting a very compact emission region $R\leqδ\times2.16\times10^{14}$ cm. We modeled the spectral energy distribution of CTA 102 in several periods (having different properties in UV/optical, X-ray and $γ$-ray bands) assuming a compact blob inside and outside the BLR. We found that the high-energy data are better described when the infrared thermal radiation of the dusty torus is considered. In the flaring periods when the correlation between the $γ$-ray and UV/optical/X-ray bands is lacking, the $γ$-ray emission can be produced from the interaction of fresh electrons in a different blob, which does not make a dominant contribution at lower energies.

astro-ph.HE↗

arXiv2018

Fermi-LAT Observation of Non-Blazar AGNs

We report on a detailed investigation of the $γ$-ray emission from 26 non-blazar AGNs based on the Fermi LAT data accumulated for 7 years. The photon index of non-blazar AGNs changes in the range of 1.84-2.86 and the flux varies from a few times $10^{-9} photon\: cm^{-2} s^{-1}$ to $10^{-7} photon\: cm^{-2}s^{-1}$. Over long time periods, power-law provides an adequate description of the $γ$-ray spectra of almost all sources. Significant curvature is observed in the $γ$-ray spectra of NGC 1275, NGC 6251, SBS 0846+513 and PMN J0948+0022 and their spectra are better described by log-parabola or power-law with exponential cut-off models. The $γ$-ray spectra of PKS 0625-25 and 3C 380 show a possible deviation from a simple power-law shape, indicating a spectral cutoff around the observed photon energy of $E_{cut}=131.2\pm88.04$ GeV and $E_{cut}=55.57\pm50.74$ GeV, respectively. Our analysis confirms the previous finding of an unusual spectral turnover in the $γ$-ray spectrum of Cen A: the photon index changes from $2.75\pm0.02$ to $2.31\pm0.1$ at $2.35\pm0.08$ GeV. In the $Γ-L_γ$ plane, the luminosity of non-blazar AGNs is spread in the range of $10^{41}-10^{47}\: erg\: s^{-1}$, where the lowest luminosity have FRI radio galaxies (but typically appear with a harder photon index) and the highest- SSRQs/NLSY1s (with softer photon indexes). We confirm the previously reported short-timescale flux variability of NGC 1275 and 3C 120. The $γ$-ray emission from NLSY1s, 1H 0323+342, SBS 0846+513 and PMN J0948+0022, is variable, showing flares in short scales sometimes accompanied by a moderate hardening of their spectra (e.g., on MJD 56146.8 the $γ$-ray photon index of SBS 0846+513 was $1.73\pm0.14$). 3C 111, Cen A core, 3C 207, 3C 275.1, 3C 380, 4C+39.23B, PKS 1502+036 and PKS 2004-447 show a long-timescale flux variability in the $γ$-ray band.

astro-ph.HE↗

arXiv2017

Comparing 3C 120 jet emission at small and large scales

Context. Important information on the evolution of the jet can be obtained by comparing the physical state of the plasma at its propagation through the broad-line region (where the jet is most likely formed) into the intergalactic medium, where it starts to significantly decelerate. Aims. We compare the constraints on the physical parameters in the innermost ($\leq$ pc) and outer ($\geq$ kpc) regions of the 3C 120 jet by means of a detailed multiwavelength analysis and theoretical modeling of their broadband spectra. Methods.The data collected by Fermi LAT, Swift and Chandra are analyzed together and the spectral energy distributions are modeled using a leptonic synchrotron and inverse Compton model, taking into account the seed photons originating inside and outside of the jet. The model parameters are estimated using the MCMC method. Results. The $γ$-ray flux from the inner jet of 3C 120 was characterized by rapid variation from MJD 56900 to MJD 57300. Two strong flares were observed on April 24, 2015 when, within 19.0 minutes and 3.15 hours the flux was as high as $(7.46\pm1.56)\times10^{-6}photon\:cm^{-2}\:s^{-1}$ and $(4.71\pm0.92)\times10^{-6}photon\:cm^{-2}\:s^{-1}$ respectively. The broadband emission in the quiet and flaring states can be described as SSC emission while IC scattering of dusty torus photons cannot be excluded for the flaring states. The X-ray emission from the knots can be well reproduced by IC scattering of CMB photons only if the jet is highly relativistic (since even when $δ=10$ still $U_{\rm e}/U_B\geq80$). These extreme requirements can be somewhat softened assuming the X-rays are from the synchrotron emission of a second population of very-high-energy electrons. Conclusions. We found that the jet power estimated at two scales is consistent, suggesting that the jet does not suffer severe dissipation, it simply becomes radiatively inefficient.

astro-ph.HE↗

arXiv2014

On the gamma-ray emission from 3C 120

We report the analysis of Fermi Large Area Telescope data from five years of observations of the broad line radio galaxy 3C 120. The accumulation of larger data set results in the detection of high-energy $γ$-rays up to 10 GeV, with a detection significance of about $8.7σ$. A power-law spectrum with a photon index of $2.72\pm0.1$ and integrated flux of $F_γ=(2.35\pm0.5)\times10^{-8}\:\mathrm{photon\:cm}^{-2}s^{-1}$ above 100 MeV well describe the data averaged over five year observations. The variability analysis of the light curve with 180-, and 365- day bins reveals flux increase (nearly twice from its average level) during the last year of observation. This variability on month timescales indicates the compactness of the emitting region. The $γ$-ray spectrum can be described as synchrotron self-Compton (SSC) emission from the electron population producing the radio-to-X-ray emission in the jet. The required electron energy density exceeds the one of magnetic field only by a factor of 2 meaning no significant deviation from equipartition.

astro-ph.HE↗