ON THE JITTER RADIATION Kelner, S R; Aharonian, F A; Khangulyan, D
The Astrophysical journal,
09/2013, Letnik:
774, Številka:
1
Journal Article
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In a small-scale turbulent medium, when the nonrelativistic Larmor radius R sub(L) = mc super(2)/eB exceeds the correlation length lambda of the magnetic field, the magnetic Bremsstrahlung radiation ...of charged relativistic particles unavoidably proceeds to the so-called jitter radiation regime. The cooling timescale of parent particles is identical to the synchrotron cooling time, thus this radiation regime can be produced with very high efficiency in different astrophysical sources characterized by high turbulence. The jitter radiation has distinct spectral features shifted toward high energies, compared to synchrotron radiation. This effect makes the jitter mechanism an attractive broad-band gamma-ray production channel, which, in highly magnetized and turbulent environments, can compete or even dominate over other high-energy radiation mechanisms. In this paper, we present a novel study of the spectral properties of the jitter radiation performed within the framework of perturbation theory. The derived general expression for the spectral power of radiation is presented in a compact and convenient form for numerical calculations.
Context. High-mass microquasars may produce jets that will strongly interact with surrounding stellar winds on binary system spatial scales. Aims. We study the dynamics of the collision between a ...mildly relativistic hydrodynamical jet of supersonic nature and the wind of an OB star. Methods. We performed numerical 3D simulations of jets that cross the stellar wind with the code Ratpenat. Results. The jet head generates a strong shock in the wind, and strong recollimation shocks occur due to the initial overpressure of the jet with its environment. These shocks can accelerate particles up to TeV energies and produce gamma-rays. The recollimation shock also strengthens jet asymmetric Kelvin-Helmholtz instabilities produced in the wind/jet contact discontinuity. This can lead to jet disruption even for jet powers of several times 1036 erg s-1. Conclusions. High-mass microquasar jets likely suffer a strong recollimation shock that can be a site of particle acceleration up to very high energies, but also eventually lead to the disruption of the jet.
On the formation of TeV radiation in LS 5039 Khangulyan, D.; Aharonian, F.; Bosch-Ramon, V.
Monthly notices of the Royal Astronomical Society,
January 2008, Letnik:
383, Številka:
2
Journal Article
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The recent detections of TeV gamma-rays from compact binary systems show that relativistic outflows (jets or winds) are sites of effective acceleration of particles up to multi-TeV energies. In this ...paper, we discuss the conditions of acceleration and radiation of ultrarelativistic electrons in LS 5039, the gamma-ray emitting binary system for which the highest quality TeV data are available. Assuming that the gamma-ray emitter is a jet-like structure, we performed detailed numerical calculations of the energy spectrum and light curves accounting for the acceleration efficiency, the location of the accelerator, the speed of the emitting flow, the inclination angle of the system, as well as specific features related to anisotropic inverse Compton (IC) scattering and pair production. We conclude that the accelerator should not be deep inside the binary system unless we assume a very efficient acceleration rate. We show that within the IC scenario both the gamma-ray spectrum and flux are strongly orbital phase dependent. Formally, our model can reproduce, for specific sets of parameter values, the energy spectrum of gamma-rays reported by HESS for wide orbital phase intervals. However, the physical properties of the source can be constrained only by observations capable of providing detailed energy spectra for narrow orbital phase intervals (Δφ≪ 0.1).
Context. Stars and their winds can contribute to the non-thermal emission in extragalactic jets. Because of the complexity of jet-star interactions, the properties of the resulting emission are ...closely linked to those of the emitting flows. Aims. We simulate the interaction between a stellar wind and a relativistic extragalactic jet and use the hydrodynamic results to compute the non-thermal emission under different conditions. Methods. We performed relativistic axisymmetric hydrodynamical simulations of a relativistic jet interacting with a supersonic, non-relativistic stellar wind. We computed the corresponding streamlines out of the simulation results and calculated the injection, evolution, and emission of non-thermal particles accelerated in the jet shock, focusing on electrons or e±-pairs. Several cases were explored, considering different jet-star interaction locations, magnetic fields, and observer lines of sight. The jet luminosity and star properties were fixed, but the results are easily scalable when these parameters are changed. Results. Individual jet-star interactions produce synchrotron and inverse Compton emission that peaks from X-rays to MeV energies (depending on the magnetic field), and at ~100–1000 GeV (depending on the stellar type), respectively. The radiation spectrum is hard in the scenarios explored here as a result of non-radiative cooling dominance, as low-energy electrons are efficiently advected even under relatively high magnetic fields. Interactions of jets with cold stars lead to an even harder inverse Compton spectrum because of the Klein-Nishina effect in the cross section. Doppler boosting has a strong effect on the observer luminosity. Conclusions. The emission levels for individual interactions found here are in the line of previous, more approximate, estimates, strengthening the hypothesis that collective jet-star interactions could significantly contribute at high energies under efficient particle acceleration.
ABSTRACT
We present a numerical study of the properties of the flow produced by the collision of a magnetized anisotropic pulsar wind with the circumbinary environment. We focus on studying the ...impact of the high wind magnetization on the geometrical structure of the shocked flow. This work is an extension of our earlier studies that focused on a purely hydrodynamic interaction and weak wind magnetization. We consider the collision in the axisymmetric approximation, i.e. the pulsar rotation axis is assumed to be oriented along the line between the pulsar and the optical star. The increase of the magnetization results in the expansion of the opening cone in which the shocked pulsar wind propagates. This effect is explained in the frameworks of the conventional theory of collimation of magnetized winds. This finding has a direct implication for scenarios that involve Doppler boosting as the primary mechanism behind the GeV flares detected with the Fermi Large Area Telescope from PSR B1259−63/LS 2883. The maximum enhancement of the apparent emission is determined by the ratio of 4$\pi$ to the solid in which the shocked pulsar wind propagates. Our simulations suggest that this enhancement factor is decreased by the impact of the magnetic field.
ABSTRACT
Isolated black holes (IBHs) are not usually considered to be important astrophysical sources, since, even in the case of a high accretion rate, an accretion disc rarely can be formed due to ...the small angular momentum of the infalling matter. Thus, such systems are not expected to feature thermal disc emission which makes the dominant contribution to the radiative output of binary systems harbouring a BH. Moreover, due to their relatively modest accretion rates, these objects are not conventionally treated as feasible jet sources. However, the large number of IBHs in the Galaxy, estimated to be ∼108, implies a very high density of 10−4 pc−3 and an average distance between IBHs of ∼10 pc. Our study shows that the magnetic flux, accumulated on the horizon of an IBH because of accretion of interstellar matter, allows the Blandford–Znajeck mechanism to be activated. Thus, electron–positron jets can be launched. We have performed 2D numerical modelling which allowed the jet power to be estimated. Their inferred properties make such jets a feasible electron accelerator which, in molecular clouds (MCs), allows electron energy to be boosted up to ∼1 PeV. For the conditions expected in MCs, the radiative cooling time should be comparable to the escape time. Thus, these sources can contribute both to the population of unidentified point‐like sources and to the local cosmic‐ray (CR) electron spectrum. The impact of the generated electron CRs depends on the diffusion rate inside MCs. If the diffusion regime in a MC is similar to Galactic diffusion, the produced electrons should rapidly escape the cloud and contribute to the Galactic CR population at very high energies, >100 TeV. However, due to the modest jet luminosity (at the level of ∼1035 erg s−1) and low filling factor of MCs, these sources cannot make a significant contribution to the spectrum of CR electrons at lower energies. On the other hand, if the diffusion within MCs operates at a rate close to the Bohm limit, the CR electrons escaping from the source should be confined in the cloud, significantly contributing to the local density of CRs. The inverse Compton emission of these locally generated CRs may explain the variety of gamma‐ray spectra detected from nearby MCs.
Context. The evidence of line-like spectral features above 100 GeV, in particular at 130 GeV, which have been recently reported from some parts of the Galactic plane, poses serious challenges for any ...interpretation of this surprise discovery. It is generally believed that the unusually narrow profile of the spectral line cannot be explained by conventional processes in astrophysical objects, and, if real, is likely to be associated with dark matter. Aims. In this paper we argue that cold ultrarelativistic pulsar winds can be alternative sources of very narrow gamma-ray lines. Methods. We demonstrate that Comptonization of a cold ultrarelativistic electron-positron pulsar wind in the deep Klein-Nishina regime can readily provide very narrow (ΔE/E ≤ 0.2) distinct gamma-ray line features. To verify this prediction, we produced photon-count maps based on the Fermi LAT data in the energy interval 100 to 140 GeV. Results. We confirm earlier reports of the presence of marginal gamma-ray line-like signals from three regions of the Galactic plane. Although the maps show some structure inside these regions, unfortunately the limited photon statistics do not allow any firm conclusion in this regard. Conclusions. The confirmation of 130 GeV line emission by low-energy threshold atmospheric Cherenkov telescope systems, in particular by the new 28 m diameter dish of the H.E.S.S. array, would be crucial for resolving the spatial structure of the reported hotspots, and thus for distinguishing between the dark matter and pulsar origins of the “Fermi Lines”.
We propose a model to explain the ultra-bright GeV gamma-ray flares observed from the blazar 3C454.3. The model is based on the concept of a relativistic jet interacting with compact gas ...condensations produced when a star (a red giant) crosses the jet close to the central black hole. The study includes an analytical treatment of the evolution of the envelope lost by the star within the jet, and calculations of the related high-energy radiation. The model readily explains the day-long that varies on timescales of hours, GeV gamma-ray flare from 3C454.3, observed during 2010 November on top of a plateau lasting weeks. In the proposed scenario, the plateau state is caused by a strong wind generated by the heating of the stellar atmosphere due to nonthermal particles accelerated at the jet-star interaction region. The flare itself could be produced by a few clouds of matter lost by the red giant after the initial impact of the jet. In the framework of the proposed scenario, the observations constrain the key model parameters of the source, including the mass of the central black hole: M sub(BH) Asymptotically = to 10 super(9) M sub(middot in circle), the total jet power: L sub(j) Asymptotically = to 10 super(48) erg s super(-1), and the Doppler factor of the gamma-ray emitting clouds: delta Asymptotically = to20. Whereas we do not specify the particle acceleration mechanisms, the potential gamma-ray production processes are discussed and compared in the context of the proposed model. We argue that synchrotron radiation of protons has certain advantages compared to other radiation channels of directlyaccelerated electrons. An injected proton distribution is proportional to E super(-1) or harder below the relevant energies would be favored to alleviate the tight energetic constraints and to avoid the violation of the observational low-energy constraints.
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 non-relativistic 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 kinetic pressure. Moreover, the post-shock flow propagates in a rather narrow region, with very high bulk Lorentz factor (γ∼ 100). This flow acceleration is related to adiabatic 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 B1259−63/SS2883.
TeV light curve of PSR B1259–63/SS2883 Khangulyan, D.; Hnatic, S.; Aharonian, F. ...
Monthly notices of the Royal Astronomical Society,
September 2007, Letnik:
380, Številka:
1
Journal Article
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The inverse Compton (IC) scattering of ultrarelativistic electrons accelerated at the pulsar wind termination shock is generally believed to be responsible for TeV gamma-ray signal recently reported ...from the binary system PSR B1259−63/SS2883. While this process can explain the energy spectrum of the observed TeV emission, the gamma-ray fluxes detected by the Array of Imaging Atmospheric Cherenkov Telescopes (HESS) at different epochs do not agree with the published theoretical predictions of the TeV light curve. The main objective of this paper is to show that the HESS results can be explained, under certain reasonable assumptions concerning the cooling of relativistic electrons, by IC scenarios of gamma-ray production in PSR B1259−63. In this paper we study evolution of the energy spectra of relativistic electrons under different assumptions about the acceleration and energy-loss rates of electrons, and the impact of these processes on the light curve of IC gamma-rays. We demonstrate that the observed TeV light curve can be explained (i) by adiabatic losses which dominate over the entire trajectory of the pulsar with a significant increase towards the periastron or (ii) by the ‘early’ (sub-TeV) cut-offs in the energy spectra of electrons due to the enhanced rate of Compton losses close to the periastron. The first four data points obtained just after periastron comprise an exception – possibly due to interaction with the Be star disc, which introduces additional physics not included in the presented model. The calculated spectral and temporal characteristics of the TeV radiation provide conclusive tests to distinguish between these two working hypotheses. The Compton deceleration of the electron–positron pulsar wind contributes to the decrease of the non-thermal power released in the accelerated electrons after the wind termination, and thus to the reduction of the IC and synchrotron components of radiation close to the periastron. Although this effect alone cannot explain the observed TeV and X-ray light curves, the Comptonization of the cold ultrarelativistic wind leads to the formation of gamma-radiation with a specific line-type energy spectrum. While the HESS data already constrain the Lorentz factor of the wind, Γ≤ 106 (for the most likely orbit inclination angle i= 35°, and assuming an isotropic pulsar wind), future observations of this object with GLAST should allow a deep probe of the wind Lorentz factor in the range between 104 and 106.
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