5 research outputs found
Constraining Emission Models of Luminous Blazar Sources
Many luminous blazars which are associated with quasar-type active galactic
nuclei display broad-band spectra characterized by a large luminosity ratio of
their high-energy (gamma-ray) and low-energy (synchrotron) spectral components.
This large ratio, reaching values up to 100, challenges the standard
synchrotron self-Compton models by means of substantial departures from the
minimum power condition. Luminous blazars have also typically very hard X-ray
spectra, and those in turn seem to challenge hadronic scenarios for the high
energy blazar emission. As shown in this paper, no such problems are faced by
the models which involve Comptonization of radiation provided by a broad
line-region, or dusty molecular torus. The lack or weakness of bulk Compton and
Klein-Nishina features indicated by the presently available data favors
production of gamma-rays via up-scattering of infrared photons from hot dust.
This implies that the blazar emission zone is located at parsec-scale distances
from the nucleus, and as such is possibly associated with the extended,
quasi-stationary reconfinement shocks formed in relativistic outflows. This
scenario predicts characteristic timescales for flux changes in luminous
blazars to be days/weeks, consistent with the variability patterns observed in
such systems at infrared, optical and gamma-ray frequencies. We also propose
that the parsec-scale blazar activity can be occasionally accompanied by
dissipative events taking place at sub-parsec distances and powered by internal
shocks and/or reconnection of magnetic fields. These could account for the
multiwavelength intra-day flares occasionally observed in powerful blazars
sources.Comment: 34 pages, accepted for publication in the Astrophysical Journa
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On the Energy Spectra of GeV/TeV Cosmic Ray Leptons
Recent observations of cosmic ray electrons from several instruments have revealed various degrees of deviation in the measured electron energy distribution from a simple power-law, in a form of an excess around 0.1 to 1 TeV energies. An even more prominent deviation and excess has been observed in the fraction of cosmic ray positrons around 10 and 100 GeV energies. These observations have received considerable attention and many theoretical models have been proposed to explain them. The models rely on either dark matter annihilation/decay or specific nearby astrophysical sources, and involve several additional assumptions regarding the dark matter distribution or particle acceleration. In this paper we show that the observed excesses in the electron spectrum may be easily reproduced without invoking any unusual sources other than the general diffuse Galactic components of cosmic rays. The model presented here assumes a power-law injection of electrons (and protons) by supernova remnants, and evaluates their expected energy spectrum based on a simple kinetic equation describing the propagation of charged particles in the interstellar medium. The primary physical effect involved is the Klein-Nishina suppression of the electron cooling rate around TeV energies. With a very reasonable choice of the model parameters characterizing the local interstellar medium, we can reproduce the most recent observations by Fermi and HESS experiments. Interestingly, in our model the injection spectral index of cosmic ray electrons becomes comparable to, or even equal to that of cosmic ray protons. The Klein-Nishina effect may also affect the propagation of the secondary e{sup {+-}} pairs, and therefore modify the cosmic ray positron-to-electron ratio. We have explored this possibility by considering two mechanisms for production of e{sup {+-}} pairs within the Galaxy. The first is due to the decay of {pi}{sup {+-}}'s produced by interaction of cosmic ray nuclei with ambient protons. The second source discussed here is due to the annihilation of the diffuse Galactic {gamma}-rays on the stellar photon field. We find that high positron fraction increasing with energy, as claimed by the PAMELA experiment, cannot be explained in our model with the conservative set of the model parameters. We are able, however, to reproduce the PAMELA (as well as Fermi and HESS) results assuming high values of the starlight and interstellar gas densities, which would be more appropriate for vicinities of supernova remnants. A possible solution to this problem may be that cosmic rays undergo most of their interactions near their sources due to the efficient trapping in the far upstream of supernova shocks by self-generated, cosmic ray-driven turbulence
On the Radio and Optical Luminosity Evolution of Quasars
We calculate simultaneously the radio and optical luminosity evolutions of
quasars, and the distribution in radio loudness R defined as the ratio of radio
and optical luminosities, using a flux limited data set containing 636 quasars
with radio and optical fluxes from White et al. We first note that when dealing
with multivariate data it is imperative to first determine the true
correlations among the variables, not those introduced by the observational
selection effects, before obtaining the individual distributions of the
variables. We use the methods developed by Efron and Petrosian which are
designed to obtain unbiased correlations, distributions, and evolution with
redshift from a data set truncated due to observational biases. It is found
that the population of quasars exhibits strong positive correlation between the
radio and optical luminosities. With this correlation, whether intrinsic or
observationally induced accounted for, we find that there is a strong
luminosity evolution with redshift in both wavebands, with significantly higher
radio than optical evolution. We also construct the local radio and optical
luminosity functions and the density evolution. Finally, we consider the
distribution of the radio loudness parameter R obtained from careful treatment
of the selection effects and luminosity evolutions with that obtained from the
raw data without such considerations. We find a significant difference between
the two distributions and no clear sign of bi-modality in the true distribution
for the range of R values considered. Our results indicate therefore, somewhat
surprisingly, that there is no critical switch in the efficiency of the
production of disk outflows/jets between very radio quiet and very radio loud
quasars, but rather a smooth transition. Also, this efficiency seems higher for
the high-redshift and more luminous sources in the considered sample.Comment: 15 pages, 15 figures, accepted to ApJ, updated to in press versio
Discovery of GeV Emission from the Circinus galaxy with the Fermi-LAT
We report the discovery of gamma-ray emission from the Circinus galaxy using
the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope.
Circinus is a nearby (~4 Mpc) starburst with a heavily obscured Seyfert-type
active nucleus, bipolar radio lobes perpendicular to the spiral disk, and
kpc-scale jet-like structures. Our analysis of 0.1-100 GeV events collected
during 4 years of LAT observations reveals a significant (~ 7.3 sigma) excess
above the background. We find no indications of variability or spatial
extension beyond the LAT point-spread function. A power-law model used to
describe the 0.1-100 GeV gamma-ray spectrum yields a flux of
(18.8+/-5.8)x10^{-9} ph cm^{-2} s^{-1} and photon index 2.19+/-0.12,
corresponding to an isotropic gamma-ray luminosity of 3 x 10^{40} erg s^{-1}.
This observed gamma-ray luminosity exceeds the luminosity expected from
cosmic-ray interactions in the interstellar medium and inverse Compton
radiation from the radio lobes. Thus the origin of the GeV excess requires
further investigation.Comment: 7 pages, 7 figures, accepted for publication in the Astrophysical
Journa
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Automatic Quenching of High Energy gamma-ray Sources by Synchrotron Photons
Here we investigate evolution of a magnetized system, in which continuously produced high energy emission undergoes annihilation on a soft photon field, such that the synchrotron radiation of the created electron-positron pairs increases number density of the soft photons. This situation is important in high energy astrophysics, because, for an extremely wide range of magnetic field strengths (nano to mega Gauss), it involves {gamma}-ray photons with energies between 0.3GeV and 30TeV. We derive and analyze the conditions for which the system is unstable to runaway production of soft photons and ultrarelativistic electrons, and for which it can reach a steady state with an optical depth to photon-photon annihilation larger than unity, as well those for which efficient pair loading of the emitting volume takes place. We also discuss the application of our analysis to a realistic situation involving astrophysical sources of a broad-band {gamma}-ray emission and briefly consider the particular case of sources close to active supermassive black holes