arXiv subjects

N. Sahakyan

Publications and source records attributed to N. Sahakyan.

50 records · Page 3Linked to original sources

arXiv2018

The Origin of the Multiwavelength Emission of PKS 0502+049

The origin of the multiwavelength emission from PKS 0502+049 neighboring the first cosmic neutrino source TXS 0506+056 is studied using the data observed by Fermi-LAT and Swift UVOT/XRT. This source was in a flaring state in the considered bands before and after the neutrino observations in 2014-2015, characterized by hard emission spectra in the X-ray and $γ$-ray bands, $1.5-1.8$ and $\leq2.0$, respectively. During the neutrino observations, the $γ$-ray spectrum shows a deviation from a simple power-law shape, indicating a spectral cutoff at $E_c =8.50\pm2.06$ GeV. The spectral energy distributions of PKS 0502+049 are modeled within a one-zone leptonic scenario assuming that high energy $γ$-ray emission is produced either by IC scattering of synchrotron or dusty torus photons by the electron population that produce the radio-to-optical emission. Alternatively, the observed $γ$-rays are modeled considering inelastic interaction of protons, when the jet interacts with a dense gaseous target. During the neutrino observations, the $γ$-ray data are best described when the proton energy distribution is $E_p^{-2.61}$ and if the protons are effectively accelerated up to 10 PeV, the expected neutrino rate is $\sim1.1$ events within 110 days. In principle, if the $γ$-ray emission with a hard photon index observed during the flaring periods extends up to TeV, the expected rate can be somewhat higher, but such conditions are hardly possible. Within the hadronic interpretation, the $γ$-ray data can be reproduced only when the accretion rate of PKS 0502+049 is in the supper-Eddington regime, as opposed to the leptonic scenario. From the point of view of the necessary energetics as well as considering that the required parameters are physically reasonable, when the neutrinos were observed, the broadband emission from PKS 0502+049 is most likely of a leptonic origin.

astro-ph.HE↗

arXiv2018

On a GRB afterglow model consistent with hypernovae observations

We describe the afterglows of the long gamma-ray-burst (GRB) 130427A within the context of a binary-driven hypernova (BdHN). The afterglows originate from the interaction between a newly born neutron star ($ν$NS), created by an Ic supernova (SN), and a mildly relativistic ejecta of a hypernova (HN). Such a HN in turn results from the impact of the GRB on the original SN Ic. The mildly relativistic expansion velocity of the afterglow ($Γ\sim 3$) is determined, using our model independent approach, from the thermal emission between $196$~s and $461$~s. The power-law in the optical and X-ray bands of the afterglow is shown to arise from the synchrotron emission of relativistic electrons in the expanding magnetized HN ejecta. Two components contribute to the injected energy: the kinetic energy of the mildly relativistic expanding HN and the rotational energy of the fast rotating highly magnetized $ν$NS. We reproduce the afterglow in all wavelengths from the optical ($10^{14}$~Hz) to the X-ray band ($10^{19}$~Hz) over times from $604$~s to $5.18\times 10^6$~s relative to the Fermi-GBM trigger. Initially, the emission is dominated by the loss of kinetic energy of the HN component. After $10^5$~s the emission is dominated by the loss of rotational energy of the $ν$NS, for which we adopt an initial rotation period of $2$~ms and a dipole plus quadrupole magnetic field of $\lesssim \! 7\times 10^{12}$~G or $\sim \! 10^{14}$~G. This scenario with a progenitor composed of a CO$_{\rm core}$ and a NS companion differs from the traditional ultra-relativistic-jetted treatments of the afterglows originating from a single black hole.

astro-ph.HE↗

arXiv2018

Lepto-hadronic $γ$-ray and neutrino emission from the jet of TXS 0506+056

The observation of IceCube-170922A event from the direction of TXS 0506+056 when it was in its enhanced $γ$-ray emission state offers a unique opportunity to investigate the lepto-hadronic processes in blazar jets. Here, the observed broadband emission of TXS 0506+056 is explained by boosted synchrotron/synchrotron self Compton emission from the jet whereas the $γ$-ray data observed during the neutrino emission- by inelastic interactions of the jet-accelerated protons in a dense gaseous target. The proton energy distribution is $\sim E_{p}^{-2.50}$, calculated straightforwardly from the data obtained by Fermi-LAT and MAGIC and if such distribution continues up to $E_{c,p}=10$ PeV, the expected neutrino rate is as high as $\sim0.46$ events during the long active phase of the source or $\sim0.15$ if the activity lasts 60 days. In this interpretation, the energy content of the protons above $>$ GeV in blazar jets can be estimated as well: the required proton injection luminosity is $\simeq2.0\times10^{48}\:{\rm erg\:s^{-1}}$ exceeding $10^3$ times that of electrons $\simeq10^{45}\:{\rm erg\:s^{-1}}$ which are in equipartition with the magnetic field. As the required parameters are physically realistic, this can be an acceptable model for explanation of the neutrino and $γ$-ray emission from TXS 0506+056.

astro-ph.HE↗

arXiv2018

Dissecting the region around IceCube-170922A: the blazar TXS 0506+056 as the first cosmic neutrino source

We present the dissection in space, time, and energy of the region around the IceCube-170922A neutrino alert. This study is motivated by: (1) the first association between a neutrino alert and a blazar in a flaring state, TXS 0506+056; (2) the evidence of a neutrino flaring activity during 2014 - 2015 from the same direction; (3) the lack of an accompanying simultaneous $γ$-ray enhancement from the same counterpart; (4) the contrasting flaring activity of a neighbouring bright $γ$-ray source, the blazar PKS 0502+049, during 2014 - 2015. Our study makes use of multi-wavelength archival data accessed through Open Universe tools and includes a new analysis of Fermi-LAT data. We find that PKS 0502+049 contaminates the $γ$-ray emission region at low energies but TXS 0506+056 dominates the sky above a few GeV. TXS 0506+056, which is a very strong (top percent) radio and $γ$-ray source, is in a high $γ$-ray state during the neutrino alert but in a low though hard $γ$-ray state in coincidence with the neutrino flare. Both states can be reconciled with the energy associated with the neutrino emission and, in particular during the low/hard state, there is evidence that TXS 0506+056 has undergone a hadronic flare with very important implications for blazar modelling. All multi-messenger diagnostics reported here support a single coherent picture in which TXS 0506+056, a very high energy $γ$-ray blazar, is the only counterpart of all the neutrino emissions in the region and therefore the most plausible first non-stellar neutrino and, hence, cosmic ray source.

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↗

arXiv2017

Rapid Gamma-ray variability of NGC 1275

We report on a detailed analysis of the $γ$-ray light curve of NGC 1275 using the Fermi large area telescope data accumulated in 2008-2017. Major $γ$-ray flares were observed in October 2015 and December 2016/January 2017 when the source reached a daily peak flux of $(2.21\pm0.26)\times10^{-6}\:{\rm photon\:cm^{-2}\:s^{-1}}$, achieving a flux of $(3.48\pm0.87)\times10^{-6}\:{\rm photon\:cm^{-2}\:s^{-1}}$ within 3 hours, which corresponds to an apparent isotropic $γ$-ray luminosity of $\simeq3.84\times10^{45}\:{\rm erg\:s^{-1}}$. The most rapid flare had e-folding time as short as $1.21\pm0.22$ hours which had never been previously observed for any radio galaxy in $γ$-ray band. Also $γ$-ray spectral changes were observed during these flares: in the flux versus photon index plane the spectral evolution follows correspondingly a counter clockwise and a clockwise loop inferred from the light curve generated by an adaptive binning method. On December 30, 2016 and January 01, 2017 the X-ray photon index softened ($Γ_{\rm X}\simeq 1.75-1.77$) and the flux increased nearly $\sim3$ times as compared with the quiet state. The observed hour-scale variability suggests a very compact emission region ($R_γ\leq5.22\times10^{14}\:(δ/4)\:{\rm cm}$) implying that the observed emission is most likely produced in the subparsec-scale jet if the entire jet width is responsible for the emission. During the active periods the $γ$-ray photon index hardened, shifting the peak of the high energy spectral component to $>{\rm GeV}$, making it difficult to explain the observed X-ray and $γ$-ray data in the standard one-zone synchrotron self-Compton model.

astro-ph.HE↗

arXiv2017

High Energy Gamma-Ray Emission From PKS 1441+25

We present the $γ$-ray observations of the flat-spectrum radio quasar PKS 1441+25 (z=0.939), using the {\it Fermi} large Area Telescope data accumulated during January - December 2015. A $γ$-ray flare was observed in January 24, when the flux increased up to $(2.22\pm0.38)\times10^{-6}\;{\rm photon\:cm^{-2}\:s^{-1}}$ with the flux-doubling time scale being as short as $\sim1.44$ days. The spectral analysis shows that from April 13 to April 28, 2015 the MeV-to-GeV photon index has hardened and changes in the range of $Γ=(1.73-1.79)$ for most of the time. The hardest photon index of $Γ=1.54\pm0.16$ has been observed on MJD 57131.46 with $11.8σ$ which is not common for flat-spectrum radio quasars. For the same period the \gray spectrum shows a possible deviation from a simple power-law shape, indicating a spectral cutoff at $E_{\rm cut}=17.7\pm8.9$ GeV. The spectral energy distributions during quiescent and flaring states are modeled using one-zone leptonic models that include the synchrotron, synchrotron self Compton and external inverse Compton processes; the model parameters are estimated using the Markov Chain Monte Carlo method. The emission in the flaring states can be modeled assuming that either the bulk Lorentz factor or the magnetic field has increased. The modeling shows that there is a hint of hardening of the low-energy index ($\sim1.98$) of the underlying non-thermal distribution of electrons responsible for the emission in April 2015. Such hardening agrees with the $γ$-ray data, which pointed out a significant $γ$-ray photon index hardening on April 13 to 28, 2015.

astro-ph.HE↗

arXiv2015

Galactic sources of high energy neutrinos: Expectation from gamma-ray data

The recent results from ground based $γ$-ray detectors (HESS, MAGIC, VERITAS) provide a population of TeV galactic $γ$-ray sources which are potential sources of High Energy (HE) neutrinos. Since the $γ$-rays and $ν$ -s are produced from decays of neutral and charged pions, the flux of TeV $γ$-rays can be used to estimate the upper limit of $ν$ flux and vice versa; the detectability of $ν$ flux implies a minimum flux of the accompanying $γ$-rays (assuming the internal and the external absorption of $γ$-rays is negligible). Using this minimum flux, it is possible to find the sources which can be detected with cubic-kilometer telescopes. I will discuss the possibility to detect HE neutrinos from powerful galactic accelerators, such as Supernova Remnants (SNRs) and Pulsar Wind Nebulae (PWNe) and show that likely only RX J1713.7-3946 , RX J0852.0-4622 and Vela X can be detected by current generation of instruments (IceCube and Km3Net). It will be shown also, that galactic binary systems could be promising sources of HE $ν$ -s. In particular, $ν$-s and $γ$-rays from Cygnus X-3 will be discussed during recent gamma-ray activity, showing that in the future such kind of activities could produce detectable flux of HE $ν$-s

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↗

arXiv2013

Hadronic Gamma-Ray and Neutrino Emission from Cygnus X-3

Cygnus X-3 (Cyg X-3) is a remarkable Galactic microquasar (X-ray binary) emitting from radio to $γ$-ray energies. In this paper, we consider hadronic model of emission of $γ$-rays above 100 MeV and their implications. We focus here on the joint $γ$-ray and neutrino production resulting from proton-proton interactions within the binary system. We find that the required proton injection kinetic power, necessary to explain the $γ$-ray flux observed by AGILE and Fermi-LAT, is $L_p \sim 10^{38}\:\rm{erg\:s^{-1}}$, a value in agreement with the average bolometric luminosity of the hypersoft state (when Cygnus X-3 was repeatedly observed to produce transient $γ$-ray activity). If we assume an increase of the wind density at the superior conjunction, the asymmetric production of $γ$-rays along the orbit can reproduce the observed modulation. According to observational constraints and our modelling, a maximal flux of high-energy neutrinos would be produced for an initial proton distribution with a power-law index $α=2.4$. The predicted neutrino flux is almost two orders of magnitude less than the 2-month IceCube sensitivity at $\sim$1 TeV. If the protons are accelerated up to PeV energies, the predicted neutrino flux for a prolonged "soft X-ray state" would be a factor of about 3 lower than the 1-year IceCube sensitivity at $\sim$10 TeV. This study shows that, for a prolonged soft state (as observed in 2006) possibly related with $γ$-ray activity and a hard distribution of injected protons, Cygnus X-3 might be close to being detectable by cubic-kilometer neutrino telescopes such as IceCube.

astro-ph.HE↗

arXiv2013

Evidence for a second component in the high-energy core emission from Centaurus A?

We report on an analysis of \fermi data from four year of observations of the nearby radio galaxy Centaurus A (Cen A). The increased photon statistics results in a detection of high-energy ($>\:100$ MeV) $γ$-rays up to 50 GeV from the core of Cen A, with a detection significance of about 44$σ$. The average gamma-ray spectrum of the core reveals evidence for a possible deviation from a simple power-law. A likelihood analysis with a broken power-law model shows that the photon index becomes harder above $E_b \simeq 4$ GeV, changing from $Γ_1=2.74\pm0.03$ below to $Γ_2=2.09\pm0.20$ above. This hardening could be caused by the contribution of an additional high-energy component beyond the common synchrotron-self Compton jet emission. A variability analysis of the light curve with 15-, 30-, and 60-day bins does not provide evidence for variability for any of the components. Indications for a possible variability of the observed flux are found on 45-day time scale, but the statistics do not allow us to make a definite conclusion in this regards. We compare our results with the spectrum reported by H.E.S.S. in the TeV energy range and discuss possible origins for the hardening observed.

astro-ph.HE↗

arXiv2010

Weak coupling regime of the Landau-Zener transition for association of an atomic Bose-Einstein condensate

In the framework of a basic semiclassical time-dependent nonlinear two-state problem, we study the weak coupling limit of the nonlinear Landau-Zener transition at coherent photo- and magneto-association of an atomic Bose-Einstein condensate. Using an exact third-order nonlinear differential equation for the molecular state probability, we develop a variational approach which enables us to construct an accurate analytic approximation describing time dynamics of the coupled atom-molecular system for the case of weak coupling. The approximation is written in terms of the solution to an auxiliary linear Landau-Zener problem with some effective Landau-Zener parameter. The dependence of this effective parameter on the input Landau-Zener parameter is found to be unexpected: as the generic Landau-Zener parameter increases, the effective Landau-Zener parameter first monotonically increases (starting from zero), reaches its maximal value and then monotonically decreases again reaching zero at some point. The constructed approximation quantitatively well describes many characteristics of the time dynamics of the system, in particular, it provides a highly accurate formula for the final transition probability to the molecular state. The present result for the final transition probability improves the accuracy of the previous approximation by Ishkhanyan et al. [Phys. Rev. A 69, 043612 (2004); J. Phys. A 38, 3505 (2005)] by order of magnitude.

cond-mat.quant-gas↗