The mystery of spectral breaks: Lyman continuum absorption by photon-photon pair production in the Fermi GeV spectra of bright blazars

We reanalyze Fermi/LAT gamma-ray spectra of bright blazars with a higher photon statistics than in previous works and with new Pass 7 data representation. In the spectra of the brightest blazar 3C 454.3 and possibly of 4C +21.35 we detect breaks at 5 GeV (in the rest frame) associated with the photon-photon pair production absorption by He II Lyman continuum (LyC). We also detect confident breaks at 20 GeV associated with hydrogen LyC both in the individual spectra and in the stacked redshift-corrected spectrum of several bright blazars. The detected breaks in the stacked spectra univocally prove that they are associated with atomic ultraviolet emission features of the quasar broad-line region (BLR). The dominance of the absorption by hydrogen Ly complex over He II, rather small detected optical depth, and the break energy consistent with the head-on collisions with LyC photons imply that the gamma-ray emission site is located within the BLR, but most of the BLR emission comes from a flat disk-like structure producing little opacity. Alternatively, the LyC emission region size might be larger than the BLR size measured from reverberation mapping, and/or the gamma-ray emitting region is extended. These solutions would resolve a long-standing issue how the multi-hundred GeV photons can escape from the emission zone without being absorbed by softer photons.Comment: 7 pages, 6 figures; accepted to Ap

Gamma-ray burst spectra from continuously accelerated electrons

We discuss here constraints on the particle acceleration models from the observed gamma-ray bursts spectra. The standard synchrotron shock model assumes that some fraction of available energy is given instantaneously to the electrons which are injected at high Lorentz factor. The emitted spectrum in that case corresponds to the spectrum of cooling electrons, F_\nu ~ \nu^{-1/2}, is much too soft to account for the majority of the observed spectral slopes. We show that continuous heating of electrons over the life-time of a source is needed to produce hard observed spectra. In this model, a prominent peak develops in the electron distribution at energy which is a strong function of Thomson optical depth \tau_T of heated electrons (pairs). At \tau_T>1, a typical electron Lorentz factor \gamma ~ 1-2 and quasi-thermal Comptonization operates. It produces spectrum peaking at a too high energy. Optical depths below 10^{-4} would be difficult to imagine in any physical scenario. At \tau_T =10^{-4}-10^{-2}, \gamma ~ 30-100 and synchrotron self-Compton radiation is the main emission mechanism. The synchrotron peak should be observed at 10--100 eV, while the self-absorbed low-energy tail with F_\nu ~ \nu^2 can produce the prompt optical emission (like in the case of GRB 990123). The first Compton scattering radiation by nearly monoenergetic electrons peaks in the BATSE energy band and can be as hard as F_\nu ~ \nu^1 reproducing the hardness of most of the observed GRB spectra. The second Compton peak should be observed in the high-energy gamma-ray band, possibly being responsible for the 10-100 MeV emission detected in GRB 941017. A significant electron-positron pair production reduces the available energy per particle, moving spectral peaks to lower energies as the burst progresses.Comment: 4 pages, 1 figure, Il nuovo cimento C, in press. Proceedings of the 4th Workshop Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 200

A photon breeding mechanism for the high-energy emission of relativistic jets

We propose a straightforward and efficient mechanism for the high-energy emission of relativistic astrophysical jets associated with an exchange of interacting high-energy photons between the jet and the external environment. Physical processes playing the main role in this mechanism are electron-positron pair production by photons and the inverse Compton scattering. This scenario has been studied analytically as well as with numerical simulations demonstrating that a relativistic jet (with the Lorentz factor larger than 3--4) moving through the sufficiently dense, soft radiation field inevitably undergoes transformation into a luminous state. The process has a supercritical character: the high-energy photons breed exponentially being fed directly by the bulk kinetic energy of the jet. Eventually particles feed back on the fluid dynamics and the jet partially decelerates. As a result, a significant fraction (at least 20 per cent) of the jet kinetic energy is converted into radiation mainly in the MeV -- GeV energy range. The mechanism maybe responsible for the bulk of the emission of relativistic jets in active galactic nuclei, microquasars and gamma-ray bursts.Comment: 10 pages, 9 figures; MNRAS, in pres