The spectral shape of the blazar contribution to the gamma-ray background The spectral shape of the unresolved emission from different classes of gamma-ray emitters can be used to disentangle the contributions from these populations to the extragalactic gamma-ray background (EGRB). In the case of blazars which, in the GeV energy range (relevant to EGRET and GLAST) have spectra which can be described as power-laws, this spectral shape depends critically on the distribution of spectral indices of the unresolved sources. However, it is rather complicated to derive this distribution from observations. Even under the assumption that unresolved blazars have the same distribution of spectral indices as resolved blazars, one has to carefully account for the large and unequal uncertainties in measuring spectral indices for individual sources using the (few) photons detected from each. We have calculated the unabsorbed spectral shape of the unresolved blazar contribution to the EGRB starting from the spectral index distribution (SID) of resolved EGRET blazars derived through a maximum-likelihood analysis accounting for measurement errors. In addition, we have explicitly calculated the uncertainty in this theoretically predicted spectral shape.
Blazars are among the most spectacular high-energy particle accelerators and gamma-ray emitetrs in the universe. They have power-law spectra in GeV energies, with spectral indices measured by EGRET to vary from 1.5 to 3. The distribution of these indices encodes valuable information on particle acceleration and emission processes in blazars, as well as on the spectral shape of the emission from unresolved blazars, contribution to the isotropic diffuse background. We present a new method for deriving the distribution of blazar spectral indices, which indicates that blazar emission properties may change less from blazar to blazar than previously thought.
The large majority of EGRET point sources remain to this day without an identified low-energy counterpart. Whatever the nature of these sources, faint unresolved objects of the same class must have a
contribution to the diffuse gamma-ray background. We have tried to estimate whether it is likely to have an important contribution of unidentified sources to the extragalactic gamma-ray background - and we found that it is. GLAST will settle the issue, since the diffuse gamma-ray background it will measure will differ from the EGRET measurement, depending on the nature of the majority of unidentified sources.
The third EGRET catalog contains a large number of unidentified sources. This subset of objects is expected to include known gamma-ray emitters of Galactic origin such as pulsars and supernova remnants, in addition to an extragalactic population of blazars. We have taken a new approach to test whether most of these sources could be Galactic and where in the Galaxy they could be residing. If galaxies similar to the Milky host comparable populations of gamma-ray emitters, then whatever populations of objects live in our Galaxy should also exist in M31. The total luminosity of teh potentially local unidentified sources, placed at the distance of M31, should not then exceed the upper limit placed by EGRET on the M31 gamma-ray luminosity. Using this constraint, we found that it is highly improbable that a large number of the unidentified sources are members of a Galactic halo population, but that a distribution of the sources entirely in the disk and bulge is plausible.
Reference: Siegal-Gaskins, J.M., Pavlidou, V., Olinto, A. V., Brown, C., & Fields, B. D. 2006, to appear in APSS
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The gamma-ray background is a constant flux of gamma rays arriving uniformly from all directions. Persumably extragalactic in origin, the gamma-ray background encodes information about the highest-energy processes in the universe. Many possible sources have been proposed, some more exotic than others. We have carefully counted the gamma-ray photons expected from sources that are guaranteed to contribute to the gamma-ray background, in order to get a better estimate of the amount of yet unexplained flux, which can then be used to constrain previously undetected processes and exciting new physics.
Diffuse gamma-ray radiation in galaxies is produced by cosmic-ray interactions with the interstellar medium. With the completion of EGRET observations, the only galaxy other than the Milky Way which has been detected in gamma-rays was the Large Magellanic Cloud. Assuming the cosmic ray flux in each galaxy scales with its supernova rate, we calculated the expected diffuse gamma-ray flux from Local Group galaxies and determine their detectability by GLAST. We predict that M31 and the SMC will be easily detected, M33 will be a marginal case, while no other normal galaxy will make the cut.