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# FERMI LARGE AREA TELESCOPE CONSTRAINTS ON THE GAMMA-RAY OPACITY OF THE UNIVERSE

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The Astrophysical Journal (Impact Factor: 6.73). 01/2010; 723:1082-1096. DOI: 10.1088/0004-637X/723/2/1082

ABSTRACT The extragalactic background light (EBL) includes photons with wavelengths from ultraviolet to infrared, which are effective at attenuating gamma rays with energy above ~10 GeV during propagation from sources at cosmological distances. This results in a redshift- and energy-dependent attenuation of the γ-ray flux of extragalactic sources such as blazars and gamma-ray bursts (GRBs). The Large Area Telescope on board Fermi detects a sample of γ-ray blazars with redshift up to z ~ 3, and GRBs with redshift up to z ~ 4.3. Using photons above 10 GeV collected by Fermi over more than one year of observations for these sources, we investigate the effect of γ-ray flux attenuation by the EBL. We place upper limits on the γ-ray opacity of the universe at various energies and redshifts and compare this with predictions from well-known EBL models. We find that an EBL intensity in the optical-ultraviolet wavelengths as great as predicted by the "baseline" model of Stecker et al. can be ruled out with high confidence.

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##### Article: On the detection of TeV gamma-rays from GRB with km3 neutrino telescopes - I. Muon event rate from single GRBs
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ABSTRACT: This is a preliminary study to examine the prospect of detecting Tetraelectronvolt 1012 eV (TeV) photons from gamma-ray bursts (GRB) using km-sized neutrino telescopes, specifically for the ANTARES neutrino telescope. Although optimized to detect upgoing neutrino-induced muons, neutrino telescopes nevertheless have a potential to detect high-energy photons by detecting downgoing muons from the electromagnetic cascade induced by the interaction of TeV photons with the Earth's atmosphere. The photon energy spectrum of a GRB is modelled by a simple power law and is normalized by simple energy considerations. Taking into account the absorption of TeV photons by cosmic infrared backgrounds, an optical depth table calculated from a model by Finke, Razzaque & Dermer is used and the arriving number of photons on top of the Earth atmosphere is determined. Muon production in the atmosphere is determined by considering two main channels of muon production: pion photoproduction and direct muon pair production. The muon energy loss during their traverse from the surface to the bottom of the sea is determined using the standard muon energy loss formula. Assuming different detector sizes, the number of detectable muons from single GRB events located at different redshifts and zenith distances is determined. The background is calculated assuming it consists primarily of cosmic ray induced downgoing muons. The detection significance is calculated and it can be concluded that to obtain at least 3sigma detection significance, a typical GRB has to be located at redshift z&lsim; 0.07 if the detector's muon effective area is Amueff˜ 10-2 km2, or redshift z&lsim; 0.15, if the muon effective area is Amueff˜ 1 km2.
Monthly Notices of the Royal Astronomical Society 01/2011; 418:1774-1786. · 5.52 Impact Factor
• ##### Article: Multi-frequency emission analysis and constraining redshift of 3C 66A
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ABSTRACT: The emission mechanism of TeV blazar 3C 66A is studied employing multi-wavelength observational data. The GeV spectrum observed by \textit{Fermi}-LAT cannot be explained by pure synchrotron self-Compton (SSC) radiation. A multi-component leptonic jet model including the absorption of extragalactic background light reproduces the quasi-simultaneous spectral energy distribution. The external Compton component of the broad line region (BLR) is important for emission above 10 GeV, while emission from 0.1 GeV to 10 GeV is dominated by the SSC component. Our model implies that the redshift of 3C 66A should be between 0.15 and 0.31, the most likely value being 0.21. Our results also indicate that the $\gamma$-ray emission region is beyond the inner zone of BLR. The effects of model parameters and assumptions on the estimating of the redshift are discussed.
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