EBL studies with GLAST
Luis C. Reyes
University of Maryland and NASA-GSFC
What is the EBL?
The Extragalactic Background Light (EBL) consists of is the accumulated electromagnetic radiation accumulated 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).
The EBL can be thought of as consisting spectrum consists of two spectral humps (see fig 1). The left hump, located at UV-Optical-NearInfrared wavelengths, corresponds to the radiated output from stars. The second hump in the other hand , meanwhile, 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 fosil 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 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 (such as 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 Due to the kinematics of pair-production depends strongly on the total center-of-momentum energy of the reaction. Gamma, 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) . Gamma-ray absorption in this energy regime is quite strong, and therefore, probes of the EBL by ground-based instruments is limited to relatively low redshifts (z<~0.52). 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 alone will not limit GLAST's ability to detect distant gamma-ray sources.
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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. 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 simplification 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] was used. Figure 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 GLASTsimulated blazars. The black squares in the plot indicate the energy cut-offs as determined from the likelihood fits (observations) and can be seen to reproduce very well the EBL model used for the simulation (Kneiske et al's "Best Fit"). Not all the sources considered in the simulation produced meaningful fits: for some blazars the error in the determination of the energy cutoff is greater than the value itself. This is due to the lack of photons at the highest energies for sources with soft intrinsic spectra (index > 2.5). Of the 165 blazars included in the simulation with redshift z > 0.5 (i.e. with EBL energy cut-offs that are in the energy range measured by GLAST) 97 of them yielded meaningful fits.
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Blazars display strong variability in their intensity and spectra. For the study of EBL attenuation, blazar variability is this constitutes 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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