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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-Opatical-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 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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The EBL is strongly connected to gamma-ray astrophysics because high energy gamma-rays (E > 10 GeV) emitted by extragalactic sources are subject to absorption due to pair-production with EBL photons. One exciting consequence of this effect is that the magnitude of this absorption can then be used to measure (or at least constrain) the column density of background photons between the source and the observer [3].
The cross section for pair-production depends strongly on the total center-of-momentum energy of the reaction. Gamma-rays detected by ground-based telescopes (with energy E >200 GeV) are subject to strong attenuation by the near- and mid-infrared part of the EBL, limiting O(>100 GeV) probes of the EBL to relatively low redshifts (z<~0.5). GLAST, on the other hand, is sensitive to the less drastic attenuation by the UV-optical part of the EBL, with no attenuation expected (at any redshift) for photons with energy below 10 GeV. Thus, EBL attenuation will not limit GLAST's ability to detect distant gamma-ray sources.
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GLAST will allow for a completely new approach to EBL studies, namely, study of the effects of EBL attenuation on a large number of blazars as a function of redshift. This is possible thanks to GLAST's sensitivity and wide bandpass, which will allow the number of known blazars to increase from about one hundred [4] to one thousand or more [5, 6, 7], with redshifts up to z > 3-5. Furthermore, because gamma-ray sources to be observed by GLAST are distributed over a wide range of redshifts, EBL studies with GLAST could potentially probe not only the total level of the background radiation (as observed in the present epoch, i.e. z~0), but its evolution as well.
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).
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Figure 3 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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An observation of a redshift-dependent effect does not guarantee actual absorption by EBL background. There would be a possibility that spectral evolution of gamma-ray blazars might coincidentally mimic EBL attenuation. For example, if blazars that formed in the early universe suffered more internal attenuation than younger blazars, a similar effect could be observed. Such possibility has been proposed by Anita Reimer [11] after modeling the intrinsic absorption of gamma-rays with photons from the accretion disk and broad-line region of blazars during periods of strong accretion. Given the blazar emission model considered in her study, and assuming a correlation between accretion history and black hole mass, Anita found that the intrinsic opacity of blazars is redshift-dependent (through black hole mass evolution), and thus, it mimics EBL attenuation.
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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 Fred Piron [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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In conclusion, the great window for high energy gamma-rays (10 GeV < E < 100 GeV) that GLAST will open will also provide unique insight into the optical-UV universe. Potentially, this will lay a path for a deeper understanding of the universe for many years to come.
[1] Kashlinsky review
[2] Hauser & Dwek
[3] Stecker, de Jager
[4] Dermer
[5] Chiang & Mukherjee
[6] Stecker & Salamon
[7] Narumoto & Totani
[8] Chen, Reyes & Ritz
[9] Kneiske et al
[10] Fazio & Stecker
[11] Anita's paper
[12] Nukri and Fred
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Figure 1. Schematic EBL spectrum as a function of wavelength. The EBL spectrum consists of two spectral humps: The blue hump at UV-Optical-NIR wavelengths is the radiated output from stars. The red hump at MIR (mid-infrared) and FIR (far-infrared) wavelengths results from the absorption and re-emission of starlight by the interstellar medium. The CMB spectrum (dashed black line) is presented here just for comparison purposes (since it is not considered part of the EBL). The location and size of the humps is just approximate, since the precise shape and intensity of the EBL is not completely constrained from observations.
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