EBL studies with GLAST
What is the EBL?
The Extragalactic Background Light (EBL) consists of the accumulated electromagnetic radiation in the universe since the decoupling of matter and radiation following the Big Bang. By definition, the EBL does not include foreground radiation from the Solar System, the Milky Way or other nearby galaxies, nor does it include the Cosmic Microwave Background radiation (CMB).
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Direct measurements of the EBL intensity at UV to Infrared 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.).
The EBL and GLAST
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.
Studying the EBL with GLAST
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.
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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).
Caveats
Blazars display strong variability in their intensity and spectra. For the study of EBL attenuation, blazar variability is both a nuisance and an opportunity. Variability is a nuisance because measuring the spectral steepening of a source is more difficult when such spectrum is changing constantly. In the case of the LAT, or any other space-based instrument, a precise measurement of the high energy spectrum of a source requires long integration times, and thus, the time-average steepening is what is actually measured. The impact of blazar variability has already been probed with the simulation and analysis described above, and as can be seen, it did not prevent a correct determination of the collective level of EBL attenuation experienced by blazars as a function of redshift. It should be noted however, that blazar variability is not well understood (this is something that GLAST will measure), and that the variability model used in the simulation might differ significantly from reality. Blazar variability could also represent an advantage, since the energy cut-off observed in a given blazar should be the same independently of the flaring state of the source, if due to EBL absorption. This would constitute a powerful check of the effectiveness of individual blazars as probe of the EBL.
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EBL studies like the one illustrated here will require redshift determinations for a large fraction of GLAST blazars. This is just another example of the importance of cross-wavelength studies: by using optical measurements of blazar redshifts, gamma-ray observations can effectively probe the optical-UV EBL.
Bottom line
Given enough observationally available gamma-ray sources at the relevant redshifts, GLAST observations could become a powerful cosmological probe of the high-redshift universe. Indeed, if enough of these sources are suitable for EBL studies (bright and free of intrinsic energy cutoffs at E<~100 GeV energies) GLAST will probe the UV-optical EBL density and its evolution over cosmic time.
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