arXiv subjects

F. A. Aharonian

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

113 records · Page 7Linked to original sources

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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arXiv1997

The HEGRA Experiment - Status and Recent Results

A report is given on the status and recent results from the HEGRA imaging atmospheric Cherenkov telescopes (IACT) concerning Mrk 501 and GRS 1915+105 and on recent results from the HEGRA arrays concerning galactic and extragalactic gamma-ray sources, counterparts of TeV Gamma-Ray-Bursts, cosmic ray anisotropies, energy spectrum and composition of primary cosmic rays. The report summarizes the presentations given at the 15th European Cosmic Ray Symposium 1996.

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arXiv1996

Constraints on the VHE Emissivity of the Universe from the Diffuse GeV Gamma-Ray Background

VHE (Very High Energy, E>100 GeV) radiation emitted at cosmological distances will pair produce on low-energy diffuse extragalactic background radiation before ever reaching us. This prevents us from directly seeing most of the VHE emission in the Universe. However, a VHE gamma-ray that pair produces initiates an electromagnetic pair cascade. At low energies, this secondary cascade radiation has a spectrum insensitive to the spectrum of the primary gamma-radiation and, unlike the original VHE radiation, IS observable. Motivated by new measurements of the extragalactic MeV-GeV diffuse gamma-ray background, we discuss the constraints placed on cosmological VHE source populations by requiring that the cascade background they produce not exceed the observed levels. We use a new, accurate cascading code and pay particular attention to the dependence of the constraints on the diffuse cosmic background at infrared/optical wavelengths. Despite considerable uncertainty in this background, we find that robust constraints may still be placed on the integrated emissivity of potential VHE sources in the Universe. The limits are tighter than those obtained by considering cascading on the microwave background alone and restrict significantly, for example, the parameter space available for the exotic particle physics scenarios recently proposed to explain the highest energy cosmic ray events. If direct emission from blazar AGN in fact accounts for most of the observed GeV background, these limits strengthen and rule out AGN emission scenarios which produce significant power above 300 GeV.

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arXiv1993

Very High Energy Gamma-Rays from AGN: Cascading on the Cosmic Background Radiation Fields and the Formation of Pair Halos

Recent high energy gamma-ray observations (E>100 MeV) of blazar AGN show emission spectra with no clear upper energy cutoff. AGN, considered to be possible sources for the highest energy cosmic rays, may have emission extending well into the VHE (very high energy, E>100 GeV) domain. Because VHE gamma-rays are absorbed by pair production on the intergalactic background radiation, much of this emission is not visible. Electromagnetic cascades initiated by absorbed VHE gamma-rays, however, may be observable. Since the velocities of e+e- pairs produced in a cascade are likely to be isotropized by an ambient random magnetic field, extended ``halos'' (R>1 Mpc) of pairs will be formed around AGN with VHE emission. Cascade radiation from these pair halos is emitted isotropically and is observable below a few TeV. The halo radiation can be distinguished by its characteristic variation in spectrum and intensity with angular distance from the central source. This variation depends weakly on the details of the central source model, e.g., the orientation and beaming/opening angle of an emitting jet. Limiting or determining the intensity of the pair halo can thus serve as a model-independent bound on or measure of the VHE power of AGN. Next-generation Cherenkov telescopes may be able to image a pair halo.

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