Searcharxiv⌕ Search

arXiv subjects

F. A. Aharonian

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

At least 91 records · Page 5Linked to original sources
arXiv2005

High-energy emission from off-axis relativistic jets

We analyze how the spectrum of synchrotron and inverse Compton radiation from a narrow relativistic jet changes with the observation angle. It is shown that diversity of acceleration mechanisms (in particular, taking the converter mechanism (Derishev et al. 2003) into account) allows for numerous modifications of the observed spectrum. In general, the off-axis emission in GeV-TeV energy range appears to be brighter, has a much harder spectrum and a much higher cut-off frequency compared to the values derived from Doppler boosting considerations alone. The magnitude of these effects depends on the details of particle acceleration mechanisms, what can be used to discriminate between different models. One of the implications is the possibility to explain high-latitude unidentified EGRET sources as off-axis but otherwise typical relativistic-jet sources, such as blazars. We also discuss the broadening of beam pattern in application to bright transient jet sources, such as Gamma-Ray Bursts.

astro-ph↗
arXiv2004

Electromagnetic radiation initiated by hadronic jets from microquasars in the ISM

Microquasars are potential candidates to produce a non-negligible fraction of the observed galactic cosmic rays. The protons accelerated at the jet termination shock interact with the interstellar medium and may produce detectable fluxes of extended emission at different energy bands: high-energy and very high-energy gamma-rays produced by neutral pion-decay, synchrotron and bremsstrahlung emission in a wide energy range generated by the secondary electrons produced by charged pion-decay. We discuss the association between this scenario and some of the unidentified EGRET sources in the galactic plane.

astro-ph↗
arXiv2004

The Crab nebula: Linking MeV synchrotron and 50 TeV inverse Compton photons

Pulsar wind driven synchrotron nebulae are offering a unique view on the connection of the pulsar wind and the surrounding medium. The Crab nebula is particularly well suited for detailed studies of the different emission regions. As inferred from the observed synchrotron emission extending beyond MeV energies, the Crab is a unique and extreme accelerator. In the framework of the synchrotron/inverse Compton emission model, the same electrons with energies exceeding $10^{15}$ eV that are responsible for the MeV synchrotron emission produce via inverse Compton scattering 10-50 TeV radiation which has recently been observed with the HEGRA system of ground based gamma-ray telescopes. Here we discuss the close relation of the two energy bands covered by INTEGRAL and ground based gamma-ray telescopes. Despite the lack of sufficient spatial resolution in both bands to resolve the emission region, correlation of the flux measurements in the two energy bands would allow to constrain the structure of the emission region. The emission region is expected to be a very compact region (limited by the life-time of the electrons) near the termination shock of the pulsar wind. We extend previous model calculations for the nebula's emission to include an additional compact non-thermal emission region recently detected at mm wavelengths.The overall good agreement of this model with data constrains additional emission processes (ions in the wind, inverse Compton from the unshocked wind) to be of little relevance.

astro-ph↗
arXiv2003

On the non-thermal high energy radiation of galaxy clusters

The origin of the nonthermal EUV and hard X-ray emission "excess" reported from some galaxy clusters has been intensively debated over last several years. The most favored models which refer this excess to relativistic electrons upscattering the 2.7 K CMBR generally requires very low magnetic field, significantly below the estimates derived from the Faraday Rotation Measurements, unless one invokes rather nonstandard assumptions concerning the energy and pitch angle distributions of nonthermal electrons. In this paper we suggest a new model assuming that the "nonthermal" excess is due to synchrotron radiation of ultrarelativistic (multi-TeV) electrons of "photonic" origin. These electrons are continuously introduced throughout the entire intracluster medium by very high energy (hypothetical) gamma-rays through interactions with the diffuse extragalactic radiation fields. We present numerical calculations for the Coma cluster, and briefly discuss implications of the model for other galaxy clusters both in the X- and gamma-ray energy domains.

astro-ph↗
arXiv2003

Particle Acceleration through Multiple Conversions from Charged into Neutral State and Back

We propose a new way of quick and very efficient acceleration of protons and/or electrons in relativistic bulk flows. The new mechanism takes advantage of conversion of particles from the charged state (protons or electrons/positrons) into neutral state (neutrons or photons) and back. In most cases, the conversion is photon-induced and requires presence of intense radiation fields, but the converter acceleration mechanism may also operate via inelastic nucleon-nucleon collisions. Like in the traditional model -- ``stochastic'' (or diffusive) acceleration, -- the acceleration cycle in our scenario consists of escape of particles from the relativistic flow followed by their return back after deflection from the ambient magnetic field. The difference is that the charge-changing reactions, which occur during the cycle, allow accelerated particles to increase their energies in each cycle by a factor roughly equal to the bulk Lorentz factor squared. The emerging spectra of accelerated particles can be very hard and their cut-off energy in some cases is larger than in the standard mechanism. This drastically reduces the required energy budget of the sources of the highest-energy particles observed in cosmic rays. Also, the proposed acceleration mechanism may serve as an efficient means of transferring the energy of bulk motion to gamma-radiation and, if the accelerated particles are nucleons, routinely produces high-energy neutrinos at $\sim 50 %$ relative efficiency.

astro-ph↗
arXiv2003

Fine-structure in the nonthermal X-ray emission of SNR RX J1713.7-3946 revealed by Chandra

We present morphological and spectroscopic studies of the northwest rim of the supernova remnant RX J1713.7-3946 based on observations by the Chandra X-ray observatory. We found a complex network of nonthermal (synchrotron) X-ray filaments, as well as a 'void' type structure -- a dim region of a circular shape -- in the northwest rim. It is remarkable that despite distinct brightness variations, the X-ray spectra everywhere in this region can be well fitted with a power-law model with photon index around 2.3. We briefly discuss some implications of these results and argue that the resolved X-ray features in the northwest rim may challenge the perceptions of standard (diffusive shock-acceleration) models concerning the production, propagation and radiation of relativistic particles in supernova remnants.

astro-ph↗
arXiv2002

An unidentified TeV source in the vicinity of Cygnus OB2

Deep observation (~113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air Cerenkov telescopes has serendipitously revealed a signal positionally inside the core of the OB association Cygnus OB2, at the edge of the 95% error circle of the EGRET source 3EG J2033+4118, and ~0.5deg north of Cyg X-3. The centre of gravity of the source is RA (J2000): 20hr 32m 07s +-9.2s (stat) +-2.2s (sys), Dec (J2000): +41deg 30' 30" +-2.0' (stat) +-0.4' (sys). The source is steady, has a post-trial significance of +4.6 sigma, indication for extension with radius 5.6' at the ~3sigma level, and has a differential power-law flux with hard photon index of -1.9 +-0.3 (stat) +-0.3 (sys). The integral flux above 1 TeV amounts ~3% that of the Crab. No counterpart for the TeV source at other wavelengths is presently identified, and its extension would disfavour an exclusive pulsar or AGN origin. If associated with Cygnus OB2, this dense concentration of young, massive stars provides an environment conducive to multi-TeV particle acceleration and likely subsequent interaction with a nearby gas cloud. Alternatively, one could envisage gamma-ray production via a jet-driven termination shock.

astro-ph↗
arXiv2002

Search for point sources of gamma radiation above 15 TeV with the HEGRA - AIROBICC array

A search for potential point sources of very high energy gamma rays has been carried out on the data taken simultaneously by the HEGRA AIROBICC and Scintillator arrays from August 1994 to March 2000. The list of sought sources includes supernova remnants, pulsars, AGNs and binary systems. The energy threshold is around 15 TeV. For the Crab Nebula, a modest excess of 2.5 standard deviations above the cosmic ray background has been observed. Flux upper limits (at 90% c.l.) of around 1.3 times the flux of the Crab Nebula are obtained, on average, for the candidate sources. A different search procedure has been used for an all-sky search which yields absolute flux upper limits between 4 and 9 crabs depending on declination, in the band from 0 to 60 degrees.

astro-ph↗
arXiv2002

A search for TeV gamma-ray emission from SNRs, pulsars and unidentified GeV sources in the Galactic plane in the longitude range between -2 deg and 85 deg

Using the HEGRA system of imaging atmospheric Cherenkov telescopes, one quarter of the Galactic plane (-2 deg < l < 85 deg) was surveyed for TeV gamma-ray emission from point sources and moderately extended sources (diameter <= 0.8 deg). The region covered includes 86 known pulsars (PSR), 63 known supernova remnants (SNR) and nine GeV sources, representing a significant fraction of the known populations. No evidence for emission of TeV gamma radiation was detected, and upper limits range from 0.15 Crab units up to several Crab units, depending on the observation time and zenith angles covered. The ensemble sums over selected SNR and pulsar subsamples and over the GeV-sources yield no indication for emission from these potential sources. The upper limit for the SNR population is at the level of 6.7% of the Crab flux and for the pulsar ensemble at the level of 3.6% of the Crab flux.

astro-ph↗
arXiv2002

Flat Spectrum X-ray Emission from the Direction of a Molecular Cloud Associated with SNR RX J1713.7-3946

We report on the discovery of a hard X-ray source with ASCA from a molecular cloud in the vicinity of the SNR RX J1713.7-3946. The energy spectrum (1--10 keV) shows a flat continuum which is described by a power-law with photon index 1.0 +-0.4. We argue that this unusually flat spectrum can be best interpreted in terms of characteristic bremsstrahlung emission from the ionization-loss-flattened distribution of either sub-relativistic protons or mildly-relativistic electrons. The strong shock of the SNR RX J1713.7-3946, which presumably interacts with the molecular cloud, as evidenced by observations of CO-lines, seems to be a natural site of acceleration of such sub- or mildly-relativistic nonthermal particles. However, the observed X-ray luminosity of 1.7 10^35 erg/s (for 6 kpc distance) requires that a huge kinetic energy of about 10^50 erg be released in the form of nonthermal particles to illuminate the cloud. The shock-acceleration at RX J1713.7-3946 can barely satisfy this energetic requirement, unless (i) the source is located much closer than 6 kpc and/or (ii) the mechanical energy of the explosion essentially exceeds 10^51 erg. Another possibility would be that an essential part of the "lost" energy is somehow converted to plasma waves, which return this energy to nonthermal particles through their turbulent reacceleration on plasma waves. Irrespective of mechanisms responsible for production of high-energy particles, the flat X-ray emission seems to be a signature of a new striking energetic phenomenon in molecular clouds.

astro-ph↗
arXiv2002

On the Potential of the Imaging Atmospheric Cherenkov Technique for Study of the Mass Composition of Primary Cosmic Radiation in the Energy Region above 30 TeV

We suggest a new approach to study the cosmis ray (CR) mass composition in the energy region from 30 TeV/nucleus up to the "knee" region, i.e. up to a few PeV/nucleus, using an array of imaging atmospheric Cherenkov telescopes (IACTs) of a special architecture. This array consists of telescopes with a relatively small mirror size (~10 square meters) separated from each other by large distances (~500 meters) and equipped by multichannel cameras with a modest pixel size (0.3-0.5 degree) and a sufficiently large viewing angle (6-7 degree). Compared to traditional IACT systems (like HEGRA, HESS or VERITAS) the IACT array considered here could provide a very large detection area (several square kilometers or more). At the same time, it allows an accurate measurement of the energy of CR induced air showers (the energy resolution ranges within 25-35%) and an effective separation of air showers created by different nuclei. Particularly, it is possible to enrich air showers belonging to the nucleus group assigned for selection up to ~90% purity at a detection efficiency of 15-20% of such showers.

astro-ph↗
arXiv2002

Constraints on the Extremely High-Energy Cosmic Ray Accelerators from Classical Electrodynamics

We find the general requirements, set by classical electrodynamics, to the sources of extremely high-energy cosmic rays (EHECRs). It is shown that the parameters of EHECR accelerators are strongly limited not only by the particle confinement in large-scale magnetic field or by the difference in electric potentials (generalized Hillas criterion), but also by the synchrotron radiation, the electro-bremsstrahlung, or the curvature radiation of accelerated particles. Optimization of these requirements in terms of accelerator's size and magnetic field strength results in the ultimate lower limit to the overall source energy, which scales as the fifth power of attainable particle energy. Hard gamma-rays accompanying generation of EHECRs can be used as a probe for potential acceleration sites. We apply the results to several populations of astrophysical objects -- potential EHECR sources -- and discuss their ability to accelerate protons to $10^{20}$ eV and beyond. A possibility to gain from ultrarelativistic bulk flows is emphasized, with Active Galactic Nuclei and Gamma-Ray Bursts being the examples.

astro-ph↗
arXiv2002

On the origin of highest energy gamma-rays from Mkn 501

The spectra of very high energy gamma-radiation from distant extragalactic objects suffer significant deformations during the passage of primary gamma-rays through the intergalactic medium. The recently reported fluxes of diffuse infrared background radiation indicate that we detect, most probably, heavily absorbed TeV radiation from Mkn 421 and Mkn 501. This implies that the absorption-corrected spectrum of Mkn 501 may contain a sharp pile-up which contradicts to the predictions of the conventional models of TeV blazars, and thus may leads to the so-called "IR background-TeV gamma-ray crisis". To overcome this difficulty, in this paper we propose two independent hypotheses assuming that (i) the TeV radiation from Mkn 501 has a secondary origin, i.e. it is formed during the development of electron-photon cascades in the intergalactic medium initiated by primary gamma-rays; (ii) the pile-up in the source spectrum is a result of comptonization (in deep Klein-Nishina regime) of ambient optical radiation by an ultrarelativistic conical cold outflow (jet) with bulk motion Lorentz factor Gamma_0 >= 3 10^7. Within the uncertainties caused by the limited energy resolution of spectral measurements, the observed TeV radiation of Mkn 501 formally can be explained by the intergalactic cascade gamma-rays, assuming however an extremely low intergalactic magnetic field in the direction to the source at the level of < 10^{-18} G. We also demonstrate that the "bulk motion comptonization" scenario can quite naturally reproduce the unusual spectral features in the absorption-corrected TeV spectrum of Mkn 501, and briefly discuss the astrophysical implications of this hypothesis.

astro-ph↗
arXiv2001

TeV Blazars and Cosmic Infrared Background Radiation

The recent developments in studies of TeV radiation from blazars are highlighted and the implications of these results for derivation of cosmologically important information about the cosmic infrared background radiation are discussed.

astro-ph↗
arXiv2001

External Compton emission from relativistic jets in Galactic black hole candidates and ultraluminous X-ray sources

Galactic binary systems that contain a black hole candidate emit hard X-rays in their low luminosity mode. We show that this emission can be understood as due to the Compton scattering of photons from the companion star and/or the accretion disk by relativistic electrons in a jet. The same electrons are also responsible for the radio emission. Two sources -- XTE J1118+480 and Cygnus X-1 -- are modelled as representatives of black holes with low and high luminosity companion stars respectively. We further show that the ultraluminous compact X-ray sources observed in nearby galaxies have the properties expected of stellar mass black holes with high luminosity companions in which the jet is oriented close to our line of sight.

astro-ph↗
arXiv2001

Limits on the TeV flux of diffuse gamma rays as measured with the HEGRA air shower array

Using data from the HEGRA air shower array, taken in the period from April 1998 to March 2000, upper limits on the ratio I_gamma /I_CR of the diffuse photon flux I_gamma to the hadronic cosmic ray flux I_CR are determined for the energy region 20 TeV to 100 TeV. The analysis uses a gamma-hadron discrimination which is based on differences in the development of photon- and hadron-induced air showers after the shower maximum. A method which is sensitive only to the non-isotropic component of the diffuse photon flux yields an upper limit of I_gamma /I_CR (at 54 TeV) < 2.0*10^-3 (at the 90% confidence level) for a sky region near the inner galaxy (20 degrees < galactic longitude <60 degrees and |galactic latitude|<5 degrees). A method which is sensitive to both the isotropic and the non-isotropic component yields global upper limits of I_gamma /I_CR (at 31 TeV) <1.2*10^-2 and I_gamma /I_CR (at 53 TeV) < 1.4*10^-2 (at the 90% confidence level).

astro-ph↗