Entity

Time filter

Source Type


Dermer C.D.,U.S. Navy | Yan D.,CAS Institute of High Energy Physics | Yan D.,Yunnan University | Zhang L.,Yunnan University | And 2 more authors.
Astrophysical Journal | Year: 2015

Fermi-LAT analyses show that the γ-ray photon spectral indices of a large sample of blazars correlate with the peak synchrotron frequency according to the relation . The same function, with different constants d and k, also describes the relationship between and peak Compton frequency . This behavior is derived analytically using an equipartition blazar model with a log-parabola description of the electron energy distribution (EED). In the Thomson regime, for external Compton (EC) processes and for synchrotron self-Compton (SSC) processes, where b is the log-parabola width parameter of the EED. The BL Lac object Mrk 501 is fit with a synchrotron/SSC model given by the log-parabola EED, and is best fit away from equipartition. Corrections are made to the spectral-index diagrams for a low-energy power-law EED and departures from equipartition, as constrained by absolute jet power. Analytic expressions are compared with numerical values derived from self-Compton and EC scattered γ-ray spectra from Ly broad-line region and IR target photons. The versus behavior in the model depends strongly on b, with progressively and predictably weaker dependences on γ-ray detection range, variability time, and isotropic γ-ray luminosity. Implications for blazar unification and blazars as ultra-high energy cosmic-ray sources are discussed. Arguments by Ghisellini et al. that the jet power exceeds the accretion luminosity depend on the doubtful assumption that we are viewing at the Doppler angle. © 2015. The American Astronomical Society. All rights reserved.. Source


Ortega R.,Bordeaux Gradignan Center of Nuclear Studies
Particle and fibre toxicology | Year: 2014

The mechanisms of toxicity of metal oxide particles towards lung cells are far from being understood. In particular, the relative contribution of intracellular particulate versus solubilized fractions is rarely considered as it is very challenging to assess, especially for low-solubility particles such as cobalt oxide (Co3O4). This study was possible owing to two highly sensitive, independent, analytical techniques, based on single-cell analysis, using ion beam microanalysis, and on bulk analysis of cell lysates, using mass spectrometry. Our study shows that cobalt oxide particles, of very low solubility in the culture medium, are readily incorporated by BEAS-2B human lung cells through endocytosis via the clathrin-dependent pathway. They are partially solubilized at low pH within lysosomes, leading to cobalt ions release. Solubilized cobalt was detected within the cytoplasm and the nucleus. As expected from these low-solubility particles, the intracellular solubilized cobalt content is small compared with the intracellular particulate cobalt content, in the parts-per-thousand range or below. However, we were able to demonstrate that this minute fraction of intracellular solubilized cobalt is responsible for the overall toxicity. Cobalt oxide particles are readily internalized by pulmonary cells via the endo-lysosomal pathway and can lead, through a Trojan-horse mechanism, to intracellular release of toxic metal ions over long periods of time, involving specific toxicity. Source


Dermer C.,U.S. Navy | Lott B.,Bordeaux Gradignan Center of Nuclear Studies
Journal of Physics: Conference Series | Year: 2012

Some questions raised by Fermi-LAT data about blazars are summarized, along with attempts at solutions within the context of leptonic models. These include both spectral and statistical questions, including the origin of the GeV breaks in low-synchrotron peaked blazars, the location of the gamma-ray emission sites, the correlations in the spectral energy distributions with luminosity, and the difficulty of synchrotron/SSC models to fit the spectra of some TeV blazars. © Published under licence by IOP Publishing Ltd. Source


Lott B.,Bordeaux Gradignan Center of Nuclear Studies
International Journal of Modern Physics D | Year: 2010

The first three months of sky-survey operation with the Large Area Telescope (LAT) on board the Fermi satellite revealed 132 bright sources at |b| > 10° with test statistic greater than 100 (corresponding to about 10σ). Two methods, based on the CGRaBS, CRATES and BZCat catalogs, indicated high-confidence associations of 106 of these sources with known AGNs. This sample is referred to as the LAT Bright AGN Sample (LBAS). It contains two radio galaxies, namely Centaurus A and NGC 1275, and 104 blazars consisting of 58 flat spectrum radio quasars (FSRQs), 42 BL Lac objects, and four blazars with unknown classification. Remarkably, the LBAS includes 10 high-energy-peaked BL Lacs. Only 33 of the sources, plus two at |b| < 10°, were previously detected with EGRET, probably due to variability. The analysis of the gamma-ray properties of the LBAS sources reveals that the average GeV spectra of BL Lac objects are significantly harder than the spectra of FSRQs. Other spectral and variability blazar properties are discussed. Some prominent Fermi-detected radiogalaxies are presented. © 2010 World Scientific Publishing Company. Source


Dermer C.D.,U.S. Navy | Cerruti M.,Harvard - Smithsonian Center for Astrophysics | Cerruti M.,University of Paris Descartes | Lott B.,Bordeaux Gradignan Center of Nuclear Studies | And 2 more authors.
Astrophysical Journal | Year: 2014

Blazar spectral models generally have numerous unconstrained parameters, leading to ambiguous values for physical properties like Doppler factor δD or fluid magnetic field B′. To help remedy this problem, a few modifications of the standard leptonic blazar jet scenario are considered. First, a log-parabola function for the electron distribution is used. Second, analytic expressions relating energy loss and kinematics to blazar luminosity and variability, written in terms of equipartition parameters, imply δD, B′, and the peak electron Lorentz factor . The external radiation field in a blazar is approximated by Lyα radiation from the broad-line region (BLR) and 0.1 eV infrared radiation from a dusty torus. When used to model 3C 279 spectral energy distributions from 2008 and 2009 reported by Hayashida et al., we derive δD20-30, B′few G, and total (IR + BLR) external radiation field energy densities u10-2-10-3 erg cm -3, implying an origin of the γ-ray emission site in 3C 279 at the outer edges of the BLR. This is consistent with the γ-ray emission site being located at a distance R ≲ Γ2 ct var0.1(Γ/30)2(t var/104 s) pc from the black hole powering 3C 279's jets, where t var is the variability timescale of the radiation in the source frame, and at farther distances for narrow-jet and magnetic-reconnection models. Excess ≳ 5 GeV γ-ray emission observed with Fermi LAT from 3C 279 challenges the model, opening the possibility of a second leptonic component or a hadronic origin of the emission. For low hadronic content, absolute jet powers of 10% of the Eddington luminosity are calculated. © 2014. The American Astronomical Society. All rights reserved. Source

Discover hidden collaborations