Cотрудники института
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Cотрудники института |
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Деришев Евгений Владимирович, Образование: Область научных интересов: Профессиональная карьера: Награды, премии, гранты: Педагогическая деятельность: Количество публикаций: E.V. Derishev, "Synchrotron emission in the fast cooling regime: which spectra can be explained?", Astrophysics and Space Science, v.309, pp. 157-161 (2007) We consider the synchrotron emission from relativistic shocks assuming that the radiating electrons cool rapidly (either through synchrotron or any other radiation mechanism). It is shown that the theory of synchrotron emission in the fast cooling regime can account for a wide range of spectral shapes. In particular, the magnetic field, which decays behind the shock front, brings enough flexibility to the theory to explain the majority of gamma-ray burst spectra even in the parameter-free fast cooling regime. Also, we discuss whether location of the peak in observed spectral energy distributions of gamma-ray bursts and active galactic nuclei can be made consistent with predictions of diffusive shock acceleration theory, and find that the answer is negative. This result is a strong indication that a particle injection mechanism, other than the standard shock acceleration, works in relativistic shocks. E.V. Derishev, F.A. Aharonian, Vl.V. Kocharovsky, "Off-axis emission from relativistic plasma flows", The Astrophysical Journal, v.655, pp. 980-988 (2007) We show that there is no universal law describing how the spectra and luminosity of synchrotron and inverse Compton radiation from relativistic jets change with increasing observation angle. Instead, the physics of particle acceleration leaves pronounced imprints in the observed spectra and allows for a freedom in numerous modifications of them. The impact of these effects is the strongest for high-energy radiation and depends on the details of the particle acceleration mechanism(s). Thus, the observed impact is sensitive to the models' details, and one can use that sensitivity to discriminate between various (but maybe rather similar) models. Generally, the beam patterns of relativistic jets in the GeV-TeV spectral domain are much wider than the inverse Lorentz factor. The off-axis emission in this energy range appears to be brighter and has a much harder spectrum and a much higher cutoff frequency compared to the values derived from Doppler-boosting considerations alone. The implications include the possibility of explaining high-latitude unidentified EGRET sources as off-axis but otherwise typical relativistic jet sources, such as blazars, and the prediction of GeV-TeV afterglow from transient jet sources, such as gamma-ray bursts. We also discuss the phenomenon of beam pattern broadening as applied to neutrino emission. E.V. Derishev, F.A. Aharonian, V.V. Kocharovsky, and Vl.V. Kocharovsky, "Particle Acceleration through Multiple Conversions from Charged into Neutral State and Back", Physical Review D, v.68, 043003(1-10) (2003) We propose a new means for a quick and efficient acceleration of protons and/or electrons in relativistic bulk flows. The maximum attainable particle energies are limited either by radiative losses or by the condition of confinement in the magnetic field. The new mechanism takes advantage of a conversion of particles from the charged state (protons, electrons or positrons) into a neutral state (neutrons or photons) and back. In most cases, the conversion is photon induced and requires the presence of intense radiation fields, but under special circumstances the converter acceleration mechanism may operate via other charge-changing reactions, for example, inelastic nucleon-nucleon collisions. As in the traditional, “stochastic” (or diffusive) acceleration models, the acceleration cycle in the proposed scenario consists of the 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 much larger than 2 and usually roughly equal to the bulk Lorentz factor squared. The emerging spectra of accelerated particles can be very hard and their maximum energy in some cases is larger than in the standard mechanism. This significantly reduces the required energy budget of the sources of the highest-energy particles observed in cosmic rays. The proposed acceleration mechanism has a distinctive feature—it unavoidably creates neutral beams, consisting of photons, neutrinos, or neutrons, whose beam pattern may be much broader than the inverse Lorentz factor of the relativistic flow. Also, the new mechanism may serve as an efficient means of transferring the energy of bulk motion to gamma radiation and, if the accelerated particles are nucleons, inevitably produces high-energy neutrinos at a relative efficiency approaching ?50%. F.A. Aharonian, A.A. Belyanin, E.V. Derishev, V.V. Kocharovsky, and Vl.V. Kocharovsky, "Constraints on the extremely high-energy cosmic ray accelerators from classical electrodynamics", Physical Review D, v.66, 023005(1-7) (2002) We formulate the general requirements, set by classical electrodynamics, on 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 fields 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 an accelerator's size and magnetic field strength results in the ultimate lower limit to the overall source energy budget, which scales as the fifth power of attainable particle energy. Hard γ rays accompanying generation of EHECRs can be used to probe potential acceleration sites. We apply the results to several populations of astrophysical objects-potential EHECR sources-and discuss their ability to accelerate protons to 1020 eV and beyond. The possibility of gain from ultrarelativistic bulk flows is addressed, with active galactic nuclei and gamma-ray bursts being the examples. E.V. Derishev, V.V. Kocharovsky, Vl.V. Kocharovsky, "Physical parameters and emission mechanism in gamma-ray bursts", Astronomy and Astrophysics, v.372, pp. 1071-1077 (2001) Detailed information on the physical parameters in the sources of cosmological Gamma-Ray Bursts (GRBs) is obtained from few plausible assumptions consistent with observations. We consider monoenergetic injection of electrons and let them cool self-consistently, taking into account Klein-Nishina cut-off in electron-photon scattering. The general requirements posed by the assumptions on the emission mechanism in GRBs are formulated. It is found that the observed radiation in the sub-MeV energy range is generated by the synchrotron emission mechanism, though about ten per cent of the total GRB energy should be converted via the inverse Compton (IC) process into the ultra-hard spectral domain (above 100 GeV). We estimate the magnetic field strength in the emitting region, the Lorentz factor of accelerated electrons, and the typical energy of IC photons. We show that there is a synchrotron-self-Compton constraint which limits the parameter space available for GRBs that are radiatively efficient in the sub-MeV domain. This concept is analogous to the line-of-death relation existing for pulsars and allows us to derive the lower limits on both GRB duration and the timescale of GRB variability. The upper limit on the Lorentz factor of GRB fireballs is also found. We demonstrate that steady-state electron distribution consistent with the Compton losses may produce different spectral indices, e.g., 3/4 as opposed to the figure 1/2 widely discussed in the literature. It is suggested that the changes in the decline rate observed in the lightcurves of several GRB afterglows may be due to either a transition to efficient IC cooling or the time evolution of Klein-Nishina and/or Compton spectral breaks, which are the general features of self-consistent electron distribution. E.V. Derishev, V.V. Kocharovsky, Vl.V. Kocharovsky, "The Neutron Component in Fireballs of Gamma-Ray Bursts: Dynamics and Observable Imprints", The Astrophysical Journal, v.521, pp. 640-649 (1999) We analyze the dynamics of a neutron-proton relativistic wind, paying particular attention to fireballs of cosmological gamma-ray bursts (GRBs). Specific effects of the neutron component depend on whether the final Lorentz factor of a plasma wind exceeds some critical value or not. In the first case, velocity decoupling of the neutron and proton flows takes place, giving rise to an electromagnetic cascade induced by pion production in inelastic collisions of nucleons. Otherwise, all nucleons in the wind behave as a single fluid. In both cases neutrons can strongly influence a GRB by changing the dynamics of a shock initiated by protons in the surrounding medium. Conditions for the decoupling of the neutron flow as well as observational consequences of the resulting pion-induced cascade are discussed, including preburst of high-energy photons and neutrinos and annihilation afterglow of a huge number of ejected electron-positron pairs. The critical value of the Lorentz factor is estimated to lie in the range expected for cosmological GRBs, so there possibly exist two different populations of bursts. A number of tests for decoupling of the neutron flow is suggested. The results obtained for the radiation-driven wind allow straightforward generalization for winds driven by other mechanisms, e.g., for the MHD winds. E.V. Derishev, V.V. Kocharovsky, Vl.V. Kocharovsky, "Lightcurves of cosmological gamma-ray bursts", Astronomy and Astrophysics, v.345, L51-L54 (1999) It is shown that in a typical fireball of cosmological GRB a neutron component is initially present and has the energy of the order of the proton-electron component. Free neutrons change essentially the dynamics of a relativistic shock formed by the ejected material in the surrounding medium. The neutron flow may decouple from the proton one or may not. Also, neutrons may decay before the shock of proton origin decelerates significantly or after that. According to these possibilities there are four types of bursts possessing lightcurves with different global appearance. We point out a number of lightcurve features expected to be correlated. For example, the height, delay time and duration of the secondary pulse are related to each other. We show that two-peaked lightcurves may be as common as single-peaked ones.
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