arXiv subjects

D. Khangulyan

Publications and source records attributed to D. Khangulyan.

81 records · Page 5Linked to original sources

arXiv2009

The new surprising behaviour of the two "prototype" blazars PKS 2155-304 and 3C 279

Recent VHE observations have unveiled a surprising behaviour in two well-known blazars at opposite sides of the blazar sequence. PKS 2155-304 have shown for the first time in an HBL a large Compton dominance, high gamma-ray luminosities and a cubic relation between X-ray and VHE fluxes. 3C 279 is the first FSRQ detected at VHE. The high luminosity required to overcome the significant absorption caused by the BLR emission cannot be easily reconciled with the historical and quasi-simultaneous SED properties. Both cases shed a new light on the structure and ambient fields of blazars. Contrary to previous claims, it is also shown that 3C 279 --as any FSRQ-- cannot in general provide robust constraints on the EBL.

astro-ph.CO↗

arXiv2009

Study of the Spectral and Temporal Characteristics of X-Ray Emission of the Gamma-Ray Binary LS 5039 with Suzaku

We report on the results from Suzaku broadband X-ray observations of the galactic binary source LS5039. The Suzaku data, which have continuous coverage of more than one orbital period, show strong modulation of the X-ray emission at the orbital period of this TeV gamma-ray emitting system.The X-ray emission shows a minimum at orbital phase ~ 0.1, close to the so-called superior conjunction of the compact object, and a maximum at phase ~0.7, very close to the inferior conjunction of the compact object. The X-ray spectral data up to 70 keV are described by a hard power-law with a phase-dependent photon index which varies within Gamma ~1.45 - 1.61. The amplitude of the flux variation is a factor of 2.5, but is significantly less than that of the factor ~8 variation in the TeV flux. Otherwise the two light curves are similar, but not identical. Although periodic X-ray emission has been found from many galactic binary systems, the Suzaku result implies a phenomenon different from the "standard" origin of X-rays related to the emission of the hot accretion plasma formed around the compact companion object. The X-ray radiation of LS5039is likely to be linked to very-high-energy electrons which are also responsible for the TeV gamma-ray emission. While the gamma-rays are the result of inverse Compton scattering by electrons on optical stellar photons, X-rays are produced via synchrotron radiation. Yet, while the modulation of the TeV gamma-ray signal can be naturally explained by the photon-photon pair production and anisotropic inverse Compton scattering, the observed modulation of synchrotron X-rays requires an additional process, the most natural one being adiabatic expansion in the radiation production region.

astro-ph↗

arXiv2008

Clues to unveil the emitter in LS 5039: powerful jets vs colliding winds

LS 5039 is among the most interesting VHE sources in the Galaxy. Two scenarios have been put forward to explain the observed TeV radiation: jets vs pulsar winds. The source has been detected during the superior conjunction of the compact object, when very large gamma-ray opacities are expected. In addition, electromagnetic cascades, which may make the system more transparent to gamma-rays, are hardly efficient for realistic magnetic fields in massive star surroundings. All this makes unlikely the standard pulsar scenario for LS 5039, in which the emitter is the region located between the star and the compact object, where the opacities are the largest. Otherwise, a jet-like flow can transport energy to regions where the photon-photon absorption is much lower and the TeV radiation is not so severely absorbed.

astro-ph↗

arXiv2008

The magnetic field and the location of the TeV emitter in Cygnus X-1 and LS 5039

Cygnus X-1 and LS 5039 are two X-ray binaries observed at TeV energies. Both sources are compact systems, contain jet-like (radio) structures, and harbor very luminous O stars. A TeV signal has been found around the superior conjunction of the compact object in both objects, when the highest gamma-ray opacities are expected. We investigate the implications of finding TeV emission from Cygnus X-1 and LS 5039 around the superior conjunction, since this can give information on the system magnetic field and the location of the TeV emitter. Using the very high-energy spectra and fluxes observed around the superior conjunction in Cygnus X-1 and LS 5039, we compute the absorbed luminosity that is caused by pair creation in the stellar photon field for different emitter positions with respect to the star and the observer line of sight. The role of the magnetic field and electromagnetic cascading are discussed. For the case of inefficient electromagnetic cascading, the expected secondary synchrotron fluxes are compared with the observed ones at X-ray energies. We find that, in Cygnus X-1 and LS 5039, either the magnetic field in the star surroundings is much smaller than the one expected for O stars or the TeV emitter is located at a distance >10^12 cm from the compact object. Our results strongly suggest that the TeV emitters in Cygnus X-1 and LS 5039 are located at the borders of the binary system and well above the orbital plane. This would not agree with those models for which the emitter is well inside the system, like the innermost-jet region (Cygnus X-1 and LS 5039; microquasar scenario), or the region between the pulsar and the primary star (LS 5039; standard pulsar scenario).

astro-ph↗

arXiv2008

On the X-ray/TeV connection in Galactic jet sources

There are three Galactic jet sources, from which TeV emission has been detected: LS 5039, LS I +61 303 and Cygnus X-1. These three sources show power-law tails at X-rays and soft gamma-rays that could indicate a non-thermal origin for this radiation. In addition, all three sources apparently show correlated and complex behavior at X-ray and TeV energies. In some cases, this complex behavior is related to the orbital motion (e.g. LS 5039, LS I +61 303), and in some others it is related to some transient event occurring in the system (e.g. Cygnus X-1, and likely also LS I +61 303 and LS 5039). Based on modeling or energetic grounds, it seems difficult to explain the emission in the X-/soft gamma-ray and the TeV bands as coming from the same region (i.e. one-zone). We also point out the importance of the pair creation phenomena in these systems, which harbor a massive and hot star, for the radio and the X-ray emission, since a secondary pair radiation component may be significant in these energy ranges. Finally, we discuss that in fact the presence of the star can indeed have strong impact on, beside the non-thermal radiation production, the jet dynamics.

astro-ph↗

arXiv2008

Non-thermal emission from secondary pairs in close TeV binary systems

Massive hot stars produce dense ultraviolet (UV) photon fields in their surroundings. If a very high-energy (VHE) gamma-ray emitter is located close to the star, then gamma-rays are absorbed in the stellar photon field, creating secondary (electron-positron) pairs. We study the broadband emission of these secondary pairs in the stellar photon and magnetic fields. Under certain assumptions on the stellar wind and the magnetic field in the surroundings of a massive hot star, we calculate the steady state energy distribution of secondary pairs created in the system and its radiation from radio to gamma-rays. Under the ambient magnetic field, possibly high enough to suppress electromagnetic (EM) cascading, the energy of secondary pairs is radiated via synchrotron and single IC scattering producing radio-to-gamma-ray radiation. The synchrotron spectral energy distribution (SED) is hard, peaks around X-ray energies, and becomes softer. The IC SED is hard as well and peaks around 10 GeV, becoming also softer at higher energies due to synchrotron loss dominance. The radio emission from secondary pairs is moderate and detectable as a point-like and/or extended source. In X-rays, the secondary pair synchrotron component may be dominant. At energies <10 GeV, the secondary pair IC radiation may be the dominant primary gamma-ray emission and possibly detectable by the next generation of instruments.

astro-ph↗

arXiv2008

Formation of hard VHE gamma-ray spectra of blazars due to internal photon-photon absorption

The energy spectra of TeV gamma-rays from blazars, after being corrected for intergalatic absorption in the Extragalactic Background Light (EBL), appear unusually hard, a fact that poses challenges to the conventional models of particle acceleration in TeV blazars and/or to the EBL models. In this paper we show that the internal absorption of gamma-rays caused by interactions with dense narrow-band radiation fields in the vicinity of compact gamma-ray production regions can lead to the formation of gamma-ray spectra of an almost arbitrary hardness. This allows significant relaxation of the current tight constraints on particle acceleration and radiation models, although at the expense of enhanced requirements to the available nonthermal energy budget. The latter, however, is not a critical issue, as long as it can be largely compensated by the Doppler boosting, assuming very large ($\geq 30$) Doppler factors of the relativistically moving gamma-ray production regions. The suggested scenario of formation of hard gamma-ray spectra predicts detectable synchrotron radiation of secondary electron-positron pairs which might require a revision of the current ``standard paradigm'' of spectral energy distributions of gamma-ray blazars. If the primary gamma-rays are of hadronic origin related to $pp$ or $p γ$ interactions, the ``internal gamma-ray absorption'' model predicts neutrino fluxes close to the detection threshold of the next generation high energy neutrino detectors.

astro-ph↗

arXiv2007

Modeling interaction of relativistic and nonrelativistic winds in binary system PSR 1259-63/SS2883. I.Hydrodynamical limit

In this paper, we present a detailed hydrodynamical study of the properties of the flow produced by the collision of a pulsar wind with the surrounding in a binary system. This work is the first attempt to simulate interaction of the ultrarelativistic flow (pulsar wind) with the nonrelativistic stellar wind. Obtained results show that the wind collision could result in the formation of an "unclosed" (at spatial scales comparable to the binary system size) pulsar wind termination shock even when the stellar wind ram pressure exceeds significantly the pulsar wind kinetical pressure. Moreover, the post-shock flow propagates in a rather narrow region, with very high bulk Lorentz factor ($γ\sim100$). This flow acceleration is related to adiabatical losses, which are purely hydrodynamical effects. Interestingly, in this particular case, no magnetic field is required for formation of the ultrarelativistic bulk outflow. The obtained results provide a new interpretation for the orbital variability of radio, X-ray and gamma-ray signals detected from binary pulsar system PSR 1259-63/SS2883.

astro-ph↗

arXiv2005

On the formation of energy spectra of synchrotron X-rays and inverse Compton gamma-rays in binary systems with luminous optical stars

In this paper we study formation broad band energy spectra of electrons due to synchrotron radiation and inverse Compton (IC) scattering. The effect of transition of IC cooling from the Thomson regime to the Klein-Nishina regime can be very significant in the environments where radiation density dominates over the magnetic field density. We discuss impact of this effect on the formation of synchrotron and IC radiation components of TeV electrons in binary systems with very luminous optical stars. Such a scenario can be realized, for example, at the wind termination shock of the pulsar in PSR B1259-63/SS2883 or in the jets of microquasars. The calculations are based on the solution of Boltzmann equation using precise differential Compton cross-section.

astro-ph↗