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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 while the . 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 Kashlinsky 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 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 (by about four orders of magnitude) compared 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 (Stecker and de Jager) 3.
The cross section for pair-production depends strongly on the total center-of-momentum energy of the reaction. Gamma-rays detected by groundbased ground-based telescopes (with energy E > ~200 200 GeV) are subject to strong attenuation by the near- and mid-infrared part of the EBL, limiting O(~>100 >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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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 Dermer 4 to one thousand or more Chiang Mukherjee5, Stecker6, Narumoto and Totani7, 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 (2004) 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-wavelenght 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 2 3 presents the Energy Cutoff vs Redshift scatter plot 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 5 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 e-fold cut-off 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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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. The , 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.
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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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 crosswavelength cross-wavelength studies: by using optical measurements of blazar redshifts, gamma-ray observations can effectively probe the optical-UV EBL.
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This will not be a simple task. When considering blazars for example, the emission procesprocess(es) and intrinsic spectrum are not known. Conversely, blazars can not be completely understood if the effects of EBL absorption are not considered. GLAST represents a great opportunity to break this vicious circle by allowing the study of EBL attenuation with a large population of sources that are distributed over a wide range of redshifts. Analysis techniques like the one outlined here attempt to use this advantage by studying the collective behavior of blazars and its correlation with redshift.
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 multiwavelength 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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