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A. M. Atoyan

Publications and source records attributed to A. M. Atoyan.

14 recordsLinked to original sources
arXiv2008

Section on Extragalactic Science Topics of the White Paper on the Status and Future of Ground-Based TeV Gamma-Ray Astronomy

This is a report on the findings of the extragalactic science working group for the white paper on the status and future of TeV gamma-ray astronomy. The white paper was commissioned by the American Physical Society, and the full white paper can be found on astro-ph (arXiv:0810.0444). This detailed section discusses extragalactic science topics including active galactic nuclei, cosmic ray acceleration in galaxies, galaxy clusters and large scale structure formation shocks, and the study of the extragalactic infrared and optical background radiation. The scientific potential of ground based gamma-ray observations of Gamma-Ray Bursts and dark matter annihilation radiation is covered in other sections of the white paper.

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arXiv2001

Very high energy gamma-rays from Centaurus X-3: indications and implications

A timing analysis of VHE gamma-ray observations of Cen X-3 taken with the University of Durham Mark 6 telescope was performed, using both Rayleigh and Bayesian statistics. Data from only 1 observation produced evidence for periodicity, in a 'soft' cut data set, with an overall chance probability <0.01 and blueshifted by 6.6ms from the nominal second harmonic of the pulse period. We show that the disappearance of the Rayleigh peak in the fully cut data set is consistent with a gamma-ray origin hypothesis. The second part of the paper considers theoretical models that could explain the existing data of Cen X-3 in high and very high energy gamma-rays. On the basis of energetics, all reasonable options for gamma-ray production must be connected to jets emerging from the inner accretion disc around the neutron star. One principal option is a large scale source, with R_s~10^13-14cm, assuming effective acceleration of electrons by shocks produced by the interaction of the jets with the dense atmosphere of the binary. The bulk of gamma-ray features could be explained by this model except modulations at the pulsar period, which would require a source with R_s<10^11cm. We consider hadronic and leptonic models for the formation of such a compact object. Both models predict the episodes of pulsed emission may be rare, of typical duration ~few hours, and the frequency of pulsations should be significantly shifted from the nominal frequency of the X-ray pulsar. The opportunities to distinguish between different models using future gamma-ray observations of this X-ray binary are discussed. (v. abridged)

astro-ph↗
arXiv2000

Early Starbursts and Magnetic Field Generation in Galaxy Clusters

We propose a mechanism for the early generation of the mean Intracluster magnetic field in terms of magnetized galactic winds. These winds are the result of starburst phases of the cluster galaxies, assumed to produce the predominant population of early type galaxies in mergers of gas-rich progenitors. After further cluster contraction typical field strengths are $10^{-7}$ Gauss. This estimate may increase to the level of $10^{-6}$ Gauss if more extreme galactic parameters, and subsequent shear amplification of the field are considered. The topology of the field is one of almost unconnected Wind Bubbles with Parker-type spiral field configurations over scales of the distance between galaxies. Further cluster accretion, that continues chaotically in space and time up to the present, will perturb these "large-scale" mean fields on smaller or at best comparable spatial scales. The small scale fields in the resulting turbulent fluctuation spectrum should be able to confine relativistic particles over times longer than the age of the Universe. The nonthermal particle content of galaxy clusters should therefore also have a "cosmological" hadronic component generated during the early starburst phase of the member galaxies. Already by itself it implies a nonthermal energy fraction of about 10 percent for the Intracluster gas which should then be detectable by future gamma-ray telescopes.

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arXiv2000

On the gamma-ray fluxes expected from Cassiopeia A

Based on the results of our previous study of the broad band synchrotron emission of Cas A in the framework of a spatial inhomogeneous model, we calculate the fluxes of gamma-radiation that can be expected from this supernova remnant in different energy bands. We show that at energies below 10 GeV gamma-ray fluxes detectable by forthcoming space-borne detectors should be produced due to bremsstrahlung of radio emitting electrons. We predict that the power-law index of the photon flux in the GeV region should be hard, close to the index ~2.2 expected for the acceleration spectrum of the electrons. Photon fluxes accessible to future ground-based gamma-ray detectors could also be expected at very high energies. The fluxes to be expected due to bremsstrahlung and inverse Compton radiation of relativistic electrons at TeV energies should be very steep, and strongly dependent on the characteristic magnetic fields in Cas A. We could expect also significant fluxes of pi^0 -decay gamma-rays produced by relativistic protons. These fluxes may extend with a hard spectrum beyond TeV energies as far as the protons would be accelerated to energies beyond 100 TeV. The hardness of the gamma-ray spectrum at TeV energies could in principle allow one to distinguish between electronic and hadronic origins of those gamma-rays. We discuss also other implications, such as relativistic particle content, or physical parameters in the source, that could be derived from future gamma-ray flux measurements in different energy bands.

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arXiv2000

On energy-dependent propagation effects and acceleration sites of relativistic electrons in Cassiopeia A

We consider the effect of energy dependent propagation of relativistic electrons in a spatially inhomogeneous medium in order to interpret the broad-band nonthermal radiation of the young supernova remnant (SNR) Cassiopeia A. A two-zone model is proposed that distinguishes between compact, bright steep-spectrum radio knots and the bright fragmented radio ring on the one hand, and the remainder of the shell - the diffuse `plateau' - on the other hand. In the framework of this model it is possible to explain the basic features of the spectral and temporal evolution of the synchrotron radiation of Cas A if one assumes that these compact structures correspond to sites of efficient electron acceleration producing hard spectra of accelerated particles with power-law indices ~2.2. The resulting energy distribution of radio electrons in these compact structures becomes significantly steeper than the electron production spectrum on timescales of the energy dependent escape of these electrons into the surrounding diffuse plateau region. We argue that the steepness, rather than the hardness, of the radio spectra of compact bright structures in clumpy sources can in general be considered as a typical signature of sites where strong electron acceleration has built up high gradients in the spatial distribution of radio electrons. Subsequent diffusive escape then modifies their energy distribution, leading to spatial variations of spectral indices within the radio source. Qualitative and quantitative interpretations of a number of observational data of Cas A are given. Predictions following from the model are discussed.

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arXiv1999

Implications of a Nonthermal Origin of the Excess EUV Emission from the Coma Cluster of Galaxies

The inverse Compton (IC) interpretation of the excess EUV emission, that was recently reported from several clusters of galaxies, suggests that the amount of relativistic electrons in the intracluster medium is highly significant, W_e>10^{61} erg. Considering Coma as the prototype galaxy cluster of nonthermal radiation, we discuss implications of the inverse Compton origin of the excess EUV fluxes in the case of low intracluster magnetic fields of order 0.1 muG, as required for the IC interpretation of the observed excess hard X-ray flux, and in the case of high fields of order 1 muG as suggested by Faraday rotation measurements. Although for such high intracluster fields the excess hard X-rays will require an explanation other than by the IC effect, we show that the excess EUV flux can be explained by the IC emission of a `relic' population of electrons driven into the incipient intracluster medium at the epoch of starburst activity by galactic winds, and later on reenergized by adiabatic compression and/or large-scale shocks transmitted through the cluster as the consequence of more recent merger events. For high magnetic fields B > 1 muG the interpretation of the radio fluxes of Coma requires a second population of electrons injected recently. They can be explained as secondaries produced by a population of relativistic protons. We calculate the fluxes of gamma-rays to be expected in both the low and high magnetic field scenarios, and discuss possibilities to distinguish between these two principal options by future gamma-ray observations.

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arXiv1999

On the origin of TeV radiation of SN 1006

We discuss the link between the nonthermal X-radiation and TeV gamma-ray emission from SN 1006, and study the capabilities of both electronic and nucleonic models for explanation of the TeV flux observed from the northeast rim of SN 1006.

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arXiv1999

Radio spectrum of the Crab nebula as an evidence for fast initial spin of its pulsar

The origin of relativistic electrons in the Crab nebula which are producing the broad-band flat radio spectrum of this prototype plerion has proved difficult to understand. Here I show that these electrons can be naturally explained as a relic population of the pulsar wind electrons that have lost most of their energy in the expanding nebula because of intensive radiative and adiabatic cooling in the past. The observed radio spectrum suggests that the initial slowing-down time of the pulsar was about 30 yrs (or less), which implies that it has been born with a spin period P_0 ~ 3-5 ms, several times shorter than presently believed. Consistency of these results with the current data and the historical records is discussed.

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arXiv1998

Clusters of Galaxies: magnetic fields and nonthermal emission

The nonthermal particle content of galaxy clusters should in part have a cosmological component generated during the early starburst phase of the member galaxies. This is reviewed in the framework of a simple cluster formation model suggested previously. It implies a nonthermal energy fraction of about 10 percent for the Intracluster gas. We also propose a mechanism for the early generation of Intracluster magnetic fields in terms of Galactic Winds. It results in typical field strengths of about 0.1 microGauss. Such comparatively weak fields are consistent with an inverse Compton origin of the excess EUV and hard X-ray emission of the Coma cluster, given the radio synchrotron emission. The required relativistic electrons must have been accelerated rather recently, less than a few billion years ago, presumably in cluster accretion shocks. This is in contrast to the hadronic nonthermal component which accumulates on cosmological time scales, and whose pion-decay TeV gamma-ray emission is expected to be larger, or of the same order as the inverse Compton TeV emission. This gamma-radiation characterizes the energetic history of cluster formation and should be observable with future arrays of imaging atmospheric Cherenkov telescopes.

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arXiv1998

Nonthermal Radiation of the Crab Nebula

The radiation mechanisms contributing to formation of the nonthermal spectrum of the Crab Nebula, as well as the information that could be derived from future observations in different energy bands, are discussed.

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arXiv1997

Modelling of the nonthermal flares in the galactic microquasar GRS 1915+105

We study the temporal and spectral evolution of the radio flares detected from the relativistic ejecta in the microquasar GRS 1915+105, and develop a model which suggests that these flares are due to synchrotron radiation of relativistic electrons suffering adiabatic and radiative energy losses in fastly expanding plasmoids (radio clouds). Analytical solutions to the kinetic equation for the electrons in expanding magnetized clouds are found. Detailed comparison of the calculated radio fluxes with the ones detected from the prominent flare of GRS 1915+105 during March/April 1994 provides conclusive information on the basic parameters in the ejecta, such as the absolute values and temporal evolution of the magnetic field, speed of expansion, the rate of continuous injection of radio electrons into and their energy-dependent escape from the clouds, etc. The data of radio monitoring of the pair of resolved ejecta enable determination of parameters of the bulk motion of counter ejecta and the degree of asymmetry between them, and contain important information on the prime energy source for the accelerated electrons. Assuming that the electrons might be accelerated up to very high energies, we calculate the fluxes of synchrotron radiation up to hard X-rays/soft gamma-rays, and of the inverse Compton GeV/TeV gamma-rays expected during radio flares, and discuss the implications which could follow from either positive detection or flux upper limits of these energetic photons.

astro-ph↗
arXiv1997

GRS 1915+105: the flux ratio of twin radio clouds as a measure of asymmetry between counter jets

Resolution of both approaching and receding ejecta in the galactic microquasars makes possible to measure the flux ratio S_a/S_r of the twin ejecta, which contains an important information about the nature of the jets. We show that the flux ratio observed from GRS 1915+105 during the prominent March/April 1994 radio flare can be explained in terms of relativistic motion of discrete radio clouds, if one assumes that the twin ejecta are similar, but not completely identical, i.e. allowing for some asymmetry between the plasmoids in their speeds of propagation and/or luminosities. The recoil momentum due to asymmetrical ejection of the pair of plasmoids could be comparable with the momentum accumulated in the inner accretion disk. We suggest a possible explanation of the observed anticorrelation between the X-ray and radio flares as the result of drastic structural changes in the inner disk caused by production of powerful jets. The delay between the times of decline of X-rays and appearance of strong radio flares is explained as the time needed for expansion of radio clouds to become optically thin.

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arXiv1997

Study of the synchrotron and inverse Compton radiations of relativistic jets in GRS 1915+105

Comparison of the detailed calculations of the synchrotron radiation of expanding magnetized clouds with the temporal and spectral evolution of radio flares observed from the recently discovered galactic microquasar GRS 1915+105 imposes strong constraints on the model parameters of relativistic plasmoids. The data of radio monitoring of the pair of ejecta not only enable unambiguous determination of the aspect angle and the speeds of propagation of both components, but also contain an important information about the energy source for acceleration of the electrons, in particular, may distinguish between the scenarios of bow-shock powered and magnetized relativistic wind powered plasmoids. Within the framework of the synchrotron self Compton model we calculate the fluxes of synchrotron hard X-rays and GeV/TeV inverse Compton gamma-rays, and discuss constraints on the parameters of GRS 1915+105 which could be provided by positive detection or flux upper limits of these energetic photons.

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