Mrk 421, Mrk 501, and 1ES 1426+428 at 100 GeV with the CELESTE Cherenkov Telescope

We have measured the gamma-ray fluxes of the blazars Mrk 421 and Mrk 501 in the energy range between 50 and 350 GeV (1.2 to 8.3 x 10^25 Hz). The detector, called CELESTE, used first 40, then 53 heliostats of the former solar facility "Themis" in the French Pyrenees to collect Cherenkov light generated in atmospheric particle cascades. The signal from Mrk 421 is often strong. We compare its flux with previously published multi-wavelength studies and infer that we are straddling the high energy peak of the spectral energy distribution. The signal from Mrk 501 in 2000 was weak (3.4 sigma). We obtain an upper limit on the flux from 1ES 1426+428 of less than half that of the Crab flux near 100 GeV. The data analysis and understanding of systematic biases have improved compared to previous work, increasing the detector's sensitivity.Comment: 15 pages, 14 figures, accepted to A&A (July 2006) August 19 -- corrected error in author lis

Gamma-Ray Studies of Blazars: Synchro-Compton Analysis of Flat Spectrum Radio Quasars

We extend a method for modeling synchrotron and synchrotron self-Compton radiations in blazar jets to include external Compton processes. The basic model assumption is that the blazar radio through soft X-ray flux is nonthermal synchrotron radiation emitted by isotropically-distributed electrons in the randomly directed magnetic field of outflowing relativistic blazar jet plasma. Thus the electron distribution is given by the synchrotron spectrum, depending only on the Doppler factor $\delta_{\rm D}$ and mean magnetic field $B$, given that the comoving emission region size scale R_b^\prime \lesssim c \dD t_v/(1+z), where $t_v$ is variability time and $z$ is source redshift. Generalizing the approach of Georganopoulos, Kirk, and Mastichiadis (2001) to arbitrary anisotropic target radiation fields, we use the electron spectrum implied by the synchrotron component to derive accurate Compton-scattered $\gamma$-ray spectra throughout the Thomson and Klein-Nishina regimes for external Compton scattering processes. We derive and calculate accurate $\gamma$-ray spectra produced by relativistic electrons that Compton-scatter (i) a point source of radiation located radially behind the jet, (ii) photons from a thermal Shakura-Sunyaev accretion disk and (iii) target photons from the central source scattered by a spherically-symmetric shell of broad line region (BLR) gas. Calculations of broadband spectral energy distributions from the radio through $\gamma$-ray regimes are presented, which include self-consistent $\gamma\gamma$ absorption on the same radiation fields that provide target photons for Compton scattering. Application of this baseline flat spectrum radio/$\gamma$-ray quasar model is considered in view of data from $\gamma$-ray telescopes and contemporaneous multi-wavelength campaigns.Comment: Accepted by ApJ. 22 pages, 12 figures, 2 tables. Minor revisions to figures and tex

Canonical high power blazars

The jets of powerful blazars propagate within regions relatively dense of radiation produced externally to the jet. This radiation is a key ingredient to understand the origin of the high energy emission of blazars, from the X-ray to the gamma-ray energy band. These external radiation fields control the amount of the inverse Compton radiation with respect to the synchrotron flux. Therefore the predicted spectral energy distribution (SED) will depend on where the jet dissipates part of its energy to produce the observed radiation. We investigate in detail how the SED changes as a function of the location of the jet dissipation region, by assuming rather "standard" (i.e. "canonical") prescriptions for the accretion disk and its X-ray corona, the profile of the jet magnetic field and the external radiation. The magnetic energy density of a "canonical" jet almost never dominates the radiative cooling of the emitting electrons, and consequently the inverse Compton flux almost always dominates the bolometric output. This is more so for large black hole masses. Dissipation taking place beyond the broad line region is particularly interesting, since it accounts in a simple way for the largest inverse Compton to synchrotron flux ratios accompanied by an extremely hard X-ray spectrum. Furthermore it makes the high power blazars at high redshift useful tools to study the optical to UV cosmic backgrounds.Comment: Revised version accepted for publication in MNRA

Numerical simulations of radiation from blazar jets

We present a description of our numerical code BLAZAR. This code calculates spectra and light curves of blazars during outbursts. The code is based on a model in which the non-thermal flares in blazars are produced in thin shells propagating down a conical jet with relativistic velocities. Such shells may represent layers of a shocked plasma, enclosed between the forward and reverse fronts of an internal shock. In the model adopted by us, the production of non-thermal radiation is assumed to be dominated by electrons and positrons which are accelerated directly, rather then injected by pair cascades. The code includes synchrotron emission and inverse-Compton process as the radiation mechanisms. Both synchrotron photons and external photons are included as the seed photons for Comptonization. At the present stage, the code is limited to treat the inverse Compton process only within the Thomson limit and is specialized to model radiation production in the flat spectrum radio quasars. As an example, we present the results of modeling an outburst in 3C 279 - the most extensively monitored gamma-ray - bright quasar.Comment: Replaced with revised version (significant changes); 12 pages, 5 figures, accepted for publication in A&A