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

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

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

Laboratory and theoretical models of planetary-scale instabilities and waves

The continuous low-g environment of the orbiting space shuttle provided a setting for conducting geophysical fluid model experiments with a completely consistent representation of sphericity and the resultant radial gravity found on astrogeophysical objects. This is possible because in zero gravity an experiment can be constructed that has its own radial buoyancy forces. The dielectric forces in a liquid, which are linearly dependent on fluid temperature, give rise to an effectively radial buoyancy force when a radial electrostatic field is applied. The Geophysical Fluid Flow Cell (GFFC) experiment is an implementation of this idea in which fluid is contained between two rotating hemispheres that are differentially heated and stressed with a large ac voltage. The GFFC flew on Spacelab 3 in May 1985. Data in the form of global Schlieren images of convective patterns were obtained for a large variety of configurations. These included situations of rapid rotation (large Taylor numbers), low rotation, large and small thermal forcing, and situations with applied meridional temperature gradients. The analysis and interpretation of the GFFC-85 data are being conducted. Improvements were developed to the GFFC instrument that will allow for real-time (TV) display of convection data and for near-real-time interactive experiments. These experiments, on the transition to global turbulence, the breakdown of rapidly rotating convective planforms and other phenomena, are scheduled to be carried out on the International Microgravity Laboratory (IML-1) aboard the shuttle in June 1990

Blind search for the real sample: Application to the origin of ultra-high energy cosmic rays

We suggest a method for statistical tests which does not suffer from a posteriori manipulations with tested samples (e.g. cuts optimization) and does not require a somewhat obscure procedure of the penalty estimate. The idea of the method is to hide the real sample (before it has been studied) among a large number of artificial samples, drawn from a random distribution expressing the null hypothesis, and then to search for it as the one demonstrating the strongest hypothesized effect. The statistical significance of the effect in this approach is the inverse of the maximal number of random samples at which the search was successful. We have applied the method to revisit the problem of correlation between the arrival directions of ultra-high energy cosmic rays and BL Lac objects. No significant correlation was found.Comment: 4 pages, 1 figure, accepted to ApJ Letter

Models of neutron star atmospheres enriched with nuclear burning ashes

Low-mass X-ray binaries hosting neutron stars (NS) exhibit thermonuclear (type-I) X-ray bursts, which are powered by unstable nuclear burning of helium and/or hydrogen into heavier elements deep in the NS "ocean". In some cases the burning ashes may rise from the burning depths up to the NS photosphere by convection, leading to the appearance of the metal absorption edges in the spectra, which then force the emergent X-ray burst spectra to shift toward lower energies. These effects may have a substantial impact on the color correction factor $f_c$ and the dilution factor $w$, the parameters of the diluted blackbody model $F_E \approx w B_E(f_c T_{eff})$ that is commonly used to describe the emergent spectra from NSs. The aim of this paper is to quantify how much the metal enrichment can change these factors. We have developed a new NS atmosphere modeling code, which has a few important improvements compared to our previous code required by inclusion of the metals. The opacities and the internal partition functions (used in the ionization fraction calculations) are now taken into account for all atomic species. In addition, the code is now parallelized to counter the increased computational load. We compute a detailed grid of atmosphere models with different exotic chemical compositions that mimic the presence of the burning ashes. From the emerging model spectra we compute the color correction factors $f_c$ and the dilution factors $w$ that can then be compared to the observations. We find that the metals may change $f_c$ by up to about 40%, which is enough to explain the scatter seen in the blackbody radius measurements. The presented models open up the possibility for determining NS mass and radii more accurately, and may also act as a tool to probe the nuclear burning mechanisms of X-ray bursts.Comment: 14 pages, 7 figures, to be published in A&

An Off-line Scan of the BATSE Daily Records and a Large Uniform Sample of Gamma-Ray Bursts

During a scan of the archival BATSE daily records covering the entire 9.1 years (TJD 8369-11690) of the BATSE operation, 3906 gamma-ray bursts (GRBs) have been detected. 2068 of these GRBs are previously known BATSE triggers while 1838 of them are new non-triggered bursts. It is important that all events were detected in the same type of data and were processed with the same procedure. Therefore these 3906 GRBs constitute a uniform sample. We have created a publically available electronic data base containing this sample. We describe the procedures of the data reduction, the selection of the GRB candidates, and the statistical tests for possible non-GRB contaminations. We also describe a novel test burst method used to measure the scan efficiency and the information obtained using the test bursts. Our scan decreases the BATSE detection threshold to ~0.1 photons/sec/cm2. As a first result, we show that the differential log N - log P distribution corrected for the detection efficiency extends to low brightnesses without any indication of a turn-over. Any reasonable extrapolation of the new log N - log P to lower brightnesses imply a rate of several thousands of GRBs in the Universe per year.Comment: 14 pages; 23 figures; revised version accepted to ApJ; electronic version of Table 2 is available at ftp://ftp.astro.su.se/pub/head/grb/catalogs/etable2.txt The GRB archive is available at http://www.astro.su.se/groups/head/grb_archive.htm

Gamma-ray emission of relativistic jets as a supercritical process

Supercriticality of the same kind as that in a nuclear pile can take place in high-energy astrophysical objects producing a number of impressive effects. For example, it could cause an explosive release of the energy of a cloud of ultrarelativistic protons into radiation. More certainly, supercriticality should be responsible for energy dissipation of very energetic relativistic fluids such as ultrarelativistic shocks in gamma-ray bursts and jets in active galactic nuclei (AGNs). In this case, the photon breeding process operates. It is a kind of the converter mechanism with the high-energy photons and e^+ e^- pairs converting into each other via pair production and inverse Compton scattering. Under certain conditions, which should be satisfied in powerful AGNs, the photon breeding mechanism becomes supercritical: the high-energy photons breed exponentially until their feedback on the fluid changes its velocity pattern. Then the system comes to a self-adjusting near-critical steady state. Monte-Carlo simulations with the detailed treatment of particle propagation and interactions demonstrate that a jet with the Lorentz factor Gamma ~ 20 can radiate away up to a half of its total energy and for Gamma=40 the radiation efficiency can be up to 80 per cent. Outer layers of the jet decelerate down to a moderate Lorentz factor 2-4, while the spine of the jet has the final Lorentz factor in the range 10-20 independently on the initial Gamma. Such sharp deceleration under the impact of radiation must cause a number of interesting phenomena such as formation of internal shocks and an early generation of turbulence.Comment: 4 pages, 4 figures, to appear in the proceedings of the HEPRO conference, September 24-28, 2007, Dublin, Irelan

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

We re-analyze Fermi/LAT &gamma; -ray spectra of bright blazars using the new Pass 7 version of the detector response files and detect breaks at &sim;5 GeV in the rest-frame spectra of 3C 454.3 and possibly also 4C +21.35, associated with the photon&ndash;photon pair production absorption by the He ii Lyman continuum (LyC). We also detect significant breaks at &sim;20 GeV associated with hydrogen LyC in both the individual spectra and 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 the hydrogen Ly complex over He ii, a small detected optical depth, and break energy consistent with 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 the long-standing issue of how the multi-hundred GeV photons can escape from the emission zone without being absorbed by softer photons.&nbsp;</p