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F. A. Aharonian

Publications and source records attributed to F. A. Aharonian.

At least 109 records · Page 6Linked to original sources
arXiv2001

Characteristics of air showers created by extremely high energy gamma-rays

The technique of adjoint cascade equations has been applied to calculate the properties of extremely high energy gamma-rays in the energy range 10^18--10^22 eV with taking into account the LPM effect and interactions of gamma-rays with the geomagnetic field. Such characteristics are analysed as the electron and muon contents at the observation level, the electron cascade curves, the lateral distribution functions of photoproduced muons.

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arXiv2001

A study of Tycho's SNR at TeV energies with the HEGRA CT-System

Tycho's supernova remnant (SNR) was observed during 1997 and 1998 with the HEGRA Cherenkov Telescope System in a search for gamma-ray emission at energies above ~1 TeV. An analysis of these data, ~65 hours in total, resulted in no evidence for TeV gamma-ray emission. The 3sigma upper limit to the gamma-ray flux (>1 TeV) from Tycho is estimated at 5.78x10^{-13} photons cm^{-2} s^{-1}, or 33 milli-Crab. We interpret our upper limit within the framework of the following scenarios: (1) that the observed hard X-ray tail is due to synchrotron emission. A lower limit on the magnetic field within Tycho may be estimated B>=22 microG, assuming that the RXTE-detected X-rays were due to synchrotron emission. However, using results from a detailed model of the ASCA emission, a more conservative lower limit B>=6 microG is derived. (2) the hadronic model of Drury, Aharonian & Voelk, and (3) the more recent time-dependent kinetic theory of Berezhko & Voelk. Our upper limit lies within the range of predicted values of both hadronic models, according to uncertainties in physical parameters of Tycho, and shock acceleration details. In the latter case, the model was scaled to suit the parameters of Tycho and re-normalised to account for a simplification of the original model. We find that we cannot rule out Tycho as a potential contributor at an average level to the Galactic cosmic-ray flux.

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arXiv2001

Proton synchrotron radiation of large-scale jets in active galactic nuclei

I propose a new mechanism for explanation of nonthernal X-ray emission of large-scale AGN jets. Namely, I assume that this radiation has synchrotron origin emitted by extremely high energy protons, and discuss implications of this model for several prominent hot spots and knots resolved by Chandra in Pictor A, 3C 120, PKS 0637-752, 3C 273.

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arXiv2000

Gamma Rays from Molecular Clouds

I discuss the spectral features of pi-0 decay gamma-radiation from clouds/targets located in proximity of relatively young proton accelerators, and speculate that such ``accelerator+target'' systems in our Galaxy can be responsible for a subset of unidentified EGRET sources. Also, I argue that the recent observations of high energy gamma-rays from the Orion complex contain evidence that the level of the ``sea'' of galactic cosmic rays may differ significantly from the flux and the spectrum of local (directly detected) particles.

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arXiv2000

TeV Gamma Rays from BL Lac Objects due to Synchrotron Radiation of Extremely High Energy Protons

One of remarkable features of the gamma ray blazar Markarian 501 is the reported shape of the TeV spectrum, which during strong flares of the source remains essentially stable despite dramatic variations of the absolute gamma-ray flux. I argue that this (to a large extent unexpected) behavior of the source could be explained assuming that the TeV gamma-ray emission is a result of synchrotron radiation of extremely high energy protons in highly magnetized compact regions of the jet.

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arXiv2000

Constraints on the Cosmic Infra-Red Background based on BeppoSAX and CAT spectra of Mkn 501

The TeV and X-ray data obtained by the imaging Cherenkov telescope CAT and X-ray satellite BeppoSAX during the remarkable flare of Mkn 501 in April 16, 1997 are used to constrain the flux of the Cosmic Infrared Background (CIB) using different CIB models. We show that a non-negligible absorption of gamma-rays due to the CIB could take place already in the low-energy (sub-TeV) domain of the spectrum of Mkn 501. This implies that the data of the low-energy threshold CAT telescope contain very important information about the CIB at short wavelengths, 0.4 mum <= lambda <= 3. mum. The analysis of almost simultaneous spectroscopic measurements of Mkn 501 in a high state by CAT and BeppoSAX in the framework of the standard homogeneous Synchrotron-Self-Compton (SSC) framework model leads to the conclusion that the density of the near-infrared background with typical ``starlight spectrum'' around 1 mum should be between 5 and 35 nW m^-2 sr^-1 (99 % CL), with most likely value around 20 nW m^-2 sr^-1. Also we argue that the CAT gamma-ray data alone allow rather robust upper limits on the CIB, lambda F_lambda <= 60 nW m^-2 sr^-1 at 1 mum, taking into account that for any reasonable scenario of gamma-ray production the differential intrinsic spectrum of gamma-ray hardly could be flatter than dN/dE == E^-1. This estimate agrees, within statistical and systematic uncertainties, with recent reports about tentative detections of the CIB at 2.2 and 3.5 mum by the Diffuse Infrared Background Experiment (DIRBE), as well as with the measurements of the background radiation at optical wavelengths from absolute photometry. We also discuss the impact of the intergalactic absorption effect in derivation of the SSC parameters for the jet in Mkn 501.

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arXiv2000

Very-high-energy gamma radiation associated with the unshocked wind of the Crab pulsar

We show that the relativistic wind in the Crab pulsar, which is commonly thought to be invisible in the region upstream of the termination shock at R < 0.1 pc, in fact could be directly observed through its inverse Compton gamm-ray emission. The search for such specific component of radiation in the gamma-ray spectrum of the Crab can provide unique information about the unshocked pulsar wind that is not accessible at other wavelengths.

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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

X-ray/TeV-gamma-ray observations of several strong flares of Mkn 501 during 1997 and implications

During more than 6 months in 1997, the BL Lac object Mkn 501 was in an exceptionally bright state, both in the X-ray band and in the Very High Energy (VHE) band. In this paper we present a multiwavelength study of Mkn 501 during this extraordinary outburst. We describe the analysis of a data base of X-ray observations acquired with the pointed X-ray telescopes on board the Rossi X-Ray Timing Explorer (RXTE) during April, May, and July, 1997. We combine this data set with detailed VHE spectral information obtained from simultaneous or nearly simultaneous observations with the stereoscopic Cherenkov telescope system of HEGRA. Several strong flares could clearly be resolved in both energy bands, making it possible to perform a detailed correlation analysis of the X-ray and VHE gamma-ray flux levels and spectra. We interpret the results in the framework of a Synchrotron Self Compton models and study the constraints on the model parameters. We show that the emission mechanism produces an Inverse Compton spectrum with a significant curvature in the TeV energy range and discuss the implications for estimates of the intergalactic extinction due to pair production processes of the VHE photons on the Diffuse Extragalactic Background Radiation.

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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

Observations of Mkn 421 during 1997 and 1998 in the energy range above 500 GeV with the HEGRA stereoscopic Cherenkov telescope system

Since its commissioning in fall 1996, the stereoscopic system of Imaging Atmospheric Cherenkov Telescope (IACTs) of HEGRA with an energy threshold of 500 GeV, an angular resolution of 0.1 degree and an energy resolution of 20% per individual photon, and an energy flux sensitivity nu F_nu at 1 TeV of 1E-11erg/cm**2 s (S/N = 5 sigma) for one hour of observation time has been used to monitor the BL Lac object Mkn 421 on a regular basis. In this letter, we report detailed temporal and spectral information about the TeV characteristics of Mkn 421 in 1997 and 1998. We study the light curve, the shortest time scales of flux variability, the differential spectra on diurnal basis for several days with good gamma-ray statistics and the time averaged energy spectrum. Special emphasis will be put on presenting the data taken during the world-wide April 1997 multiwavelength campaign. We compare the Mkn 421 results with the results obtained for the BL Lac object Mkn 501 and discuss possible implications for the emission mechanism and the Diffuse Intergalactic Background Radiation.

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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

Very High Energy Photons in Universe

I discuss the recent exciting results of discovery of TeV photons from several galactic and extragalactic sources, and highlight some objectives of VHE gamma ray astronomy that can be achieved in the near future by means of new generation ground-based detectors.

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arXiv1997

Stereo Imaging of VHE Gamma-Ray Sources

We describe the features of the stereoscopic approach in the imaging atmospheric Cherenkov technique, and discuss the performance of future low threshold telescope arrays.

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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.

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