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).
The EBL can be thought of as consisting of two spectral humps (see fig 1).
The left hump located at UV-Optical-NearInfrared wavelengths corresponds to the radiated output from stars
while the second hump 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
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.
Direct measurements of the EBL intensity at UV to InfraRed wavelenghts are very difficult. First, the EBL has no spectral
signature, 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 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 gamma-rays 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).
The cross section for pair-production depends strongly on the total center-of-momentum energy of the reaction.
Gamma-rays detected by groundbased 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.
GLAST, UV, more blazars.
A proposal to study 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 Dermer, Chiang, Stecker
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 to one thousand or more Chiang Mukherjee, Stecker, Narumoto and Totani, with redshifts
up to z > 3â?'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) 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-wavelenght 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.
Figure 3 presents the Energy Cutoff vs Redshift scatter plot obtained from the analysis of the Monte Carlo simulation. This relation was first introduced by Fazio & Stecker in 1970 as a way to relate the energy
cutoff with the redshift of the source. Kneiske et al 5 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 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).
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. 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 °-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 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.
The intrinsic energy cut-offs blazars are likely to vary significantly blazar-to-blazar, and thus, the energy cut-offs
for blazars in a given redshift bin would have larger scattering with respect to the mean than
in EBL-only absorption scenarios. This would allow at least an upper limit on EBL
attenuation by looking at the least-attenuated energy-cutoff in a particular
redshift bin. Furthermore, intrinsic opacity is likely to change within each blazar during
different emission states, allowing thus to constrain the nature of the observed energy cutoff.
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
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.
This will not be a simple task. When considering blazars for example, the emission proces(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
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 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.
Even after observation of a redshift-dependent effect, the possibility would remain that
the spectral evolution or observational selection of gamma-ray blazars mimic EBL absorption.
Detailed analyses will have to address the likelihood and impact of such scenarios. GLAST observations,
in any case, will provide an important constraint.
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.