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The EBL can be thought of as consisting of two spectral humps (see fig 1). The left hump, located at UV-OpaticalOptical-NearInfrared wavelengths, corresponds to the radiated output from stars. The second hump in the other hand corresponds to dust emission resulting from the absorption and re-emission of starlight by the interstellar medium within galaxies (other more "exotic" EBL contributors are also possible, see [1] for a review). The EBL is therefore a relic of the star formation and evolution processes, and its measurement provides a fundamental insight into the history of the universe [2].
Direct measurements of the EBL intensity at UV to Infrared wavelenghts wavelengths are very difficult. First, the EBL has no spectral signature to look for, since its spectrum depends in a nontrivial way on the characteristics of the sources, on their cosmic history, and on the process of dust formation around these sources. Second and more important, the EBL flux is excessively weak with respect to the foreground from other celestial sources (interplanetary dust, stars and interstellar medium in the galaxy, etc.).
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Chen, Reyes & Ritz [8] illustrated the potential of GLAST to probe the optical-UV EBL via the measurement and statistical analysis of the flux-ratio F (E > 10GeV)/F (E > 1GeV) for a large number (>5000) of blazars. An alternate method presented here considers the spectrum steepening of individual blazars by means of a functional form with adjustable parameters that are fitted with gtlikelihood. This analysis in particular provides a measurement of the energy cutoff observed in the source with respect to an assumed intrinsic spectrum (see fig. 2).
In the absence of information regarding the intrinsic spectrum of the source (from multi-wavelength observations and blazar emission models), a simple power law is used in order to keep the number of free parameters to a minimum. The bias introduced by individual sources given this particular oversimplification is expected to become less significant when many sources are considered together as a population.
A one-year-long simulation of the ~300 blazars expected to be the brightest in the gamma-ray sky as seen by GLAST
was performed. The simulation included galactic and extragalactic gamma-ray backgrounds and a detailed model for the variability and spectrum of such blazars. To simulate the EBL attenuation we use the "Best Fit" model from Kneiske et al [9].
Figure 3 2 presents a scatter plot of the energy cutoff vs redshift obtained from the analysis of the Monte Carlo simulation. This relation was first introduced by Fazio & Stecker in 1970 [10] as a way to relate the energy
cutoff with the redshift of the source. Kneiske et al 9 have proposed to use the Fazio-Stecker relation (FSR) to
compare EBL models with the FSR distribution obtained from observations. This idea is implemented here by considering the FSR obtained after determination of the cutoff energies of the brightest blazars expected to be observed with GLAST. In the absence of blazar intrinsic absorption and strong blazar spectrum curvature, the data points in the FSR plot will converge (amid statistical fluctuations) to the true curve due to EBL absorption. If it turns out, however, that this is not the case for a few or most blazars, their measured cut-off energies would spread below the EBL-induced value, but never above. This would enable at least and upper limit on EBL attenuation (least-attenuated flux in a particular redshift range).
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This is not the only type of method. EBL absorption can also be measured by using blazar emission models to predict the unattenuated spectrum of a few blazars through fitting of multi-wavelength data. Furthermore, blazars are not the only class of extragalactic gamma-ray sources, GRBs are also located at cosmological distances (observed up to z>~6) and will experience the same kind of EBL attenuation (Nukri and Komin, Fred Piron and V. Pelassa [12] were able to measure energy cutoffs in the spectra of a couple of blazars from the Service Challenge simulation). These two possibilities constitute independent types of analysis with respect to the one illustrated here, and when considered together, they will validate and complement each other.
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