62 research outputs found
Imprint of galaxy clustering in the cosmic gamma-ray background
Star-forming galaxies are predicted to contribute considerably to the cosmic gamma-ray background (CGB) as they are confirmed γ-ray emitters and are the most numerous population of γ-ray sources, although individually faint. Even though the Fermi Gamma-ray Space Telescope will be able to resolve few star-forming galaxies individually, their fractional contribution to the CGB should become far more significant than it was for past measurements of the CGB as many of the brighter, formerly unresolved sources will be resolved out. Thus, the clustering feature of galaxies imprinted on the CGB might be detectable by Fermi. In anticipation of such measurements, we calculate the predicted angular auto- and cross-power spectra of the CGB from normal galaxies. We find that the amplitude of the auto-power spectrum is smaller than that for other sources, such as blazars and dark matter annihilation; the shape is also characteristic. We also show that the cross-power spectrum with galaxy surveys features larger amplitude. Fermi should be able to detect the correlation signature in both the auto- and cross-power spectra at angular scales of ~1°–10° after 5 years of operation. Such a detection would be valuable in confirming the level of the star-forming galaxy contribution to the CGB and, more importantly, in serving as a tool in the effort to discriminate between possible origins of the CGB
The Spectral Index Distribution of EGRET Blazars: Prospects for GLAST
The intrinsic distribution of spectral indices in GeV energies of
gamma-ray--loud blazars is a critical input in determining the spectral shape
of the unresolved blazar contribution to the diffuse extragalactic gamma-ray
background, as well as an important test of blazar emission theories. We
present a maximum-likelihood method of determining the intrinsic spectral index
distribution (ISID) of a population of gamma-ray emitters which accounts for
error in measurement of individual spectral indices, and we apply it to EGRET
blazars. We find that the most likely Gaussian ISID for EGRET blazars has a
mean of 2.27 and a standard deviation of 0.20. We additionally find some
indication that FSRQs and BL Lacs may have different ISIDs (with BL Lacs being
harder). We also test for spectral index hardening associated with blazar
variability for which we find no evidence. Finally, we produce simulated GLAST
spectral index datasets and perform the same analyses. With improved statistics
due to the much larger number of resolvable blazars, GLAST data will help us
determine the ISIDs with much improved accuracy. Should any difference exist
between the ISIDs of BL Lacs and FSRQs or between the ISIDs of blazars in the
quiescent and flaring states, GLAST data will be adequate to separate these
ISIDs at a significance better than 3 sigma.Comment: 11 pages, 9 figures, emulateapj; accepted for publication in Ap
High Energy Polarization of Blazars : Detection Prospects
Emission from blazar jets in the ultraviolet, optical, and infrared is
polarized. If these low-energy photons were inverse-Compton scattered, the
upscattered high-energy photons retain a fraction of the polarization. Current
and future X-ray and gamma-ray polarimeters such as INTEGRAL-SPI, PoGOLITE,
X-Calibur, Gamma-Ray Burst Polarimeter, GEMS-like missions, ASTRO-H, and
POLARIX have the potential to discover polarized X-rays and gamma-rays from
blazar jets for the first time. Detection of such polarization will open a
qualitatively new window into high-energy blazar emission; actual measurements
of polarization degree and angle will quantitatively test theories of jet
emission mechanisms. We examine the detection prospects of blazars by these
polarimetry missions using examples of 3C 279, PKS 1510-089, and 3C 454.3,
bright sources with relatively high degrees of low-energy polarization. We
conclude that while balloon polarimeters will be challenged to detect blazars
within reasonable observational times (with X-Calibur offering the most
promising prospects), space-based missions should detect the brightest blazars
for polarization fractions down to a few percent. Typical flaring activity of
blazars could boost the overall number of polarimetric detections by nearly a
factor of five to six purely accounting for flux increase of the brightest of
the comprehensive, all-sky, Fermi-LAT blazar distribution. The instantaneous
increase in the number of detections is approximately a factor of two, assuming
a duty cycle of 20% for every source. The detectability of particular blazars
may be reduced if variations in the flux and polarization fraction are
anticorrelated. Simultaneous use of variability and polarization trends could
guide the selection of blazars for high-energy polarimetric observations.Comment: Matches published version in Ap
Robust identification of isotropic diffuse gamma rays from Galactic dark matter
Dark matter annihilation in Galactic substructure will produce diffuse
gamma-ray emission of remarkably constant intensity across the sky, making it
difficult to disentangle this Galactic dark matter signal from the
extragalactic gamma-ray background. We show that if Galactic dark matter
contributes a modest fraction of the measured emission in an energy range
accessible to the Fermi Gamma-ray Space Telescope, the energy dependence of the
angular power spectrum of the total measured emission could be used to
confidently identify gamma rays from Galactic dark matter substructure.Comment: 4 pages, 2 figures, added 1 reference, published in PR
Analytical Models for the Energetics of Cosmic Accretion Shocks, their Cosmological Evolution, and the Effect of Environment
We present an analytical description of the energetics of the population of
cosmic accretion shocks, for a concordance cosmology. We calculate how the
shock-processed accretion power and mass current are distributed among
different shock Mach numbers, and how they evolve with cosmic time. We
calculate the cumulative energy input of cosmic accretion shocks of any Mach
number to the intergalactic medium as a function of redshift, and we compare it
with the energy output of supernova explosions as well as with the energy input
required to reionize the universe. In addition, we investigate and quantify the
effect of environmental factors, such as local clustering properties and
filament preheating on the statistical properties of these shocks. We find that
the energy processed by accretion shocks is higher than the supernova energy
output for z<3 and that it becomes more than an order of magnitude higher in
the local universe. The energy processed by accretion shocks alone becomes
comparable to the energy required to reionize the universe by z~3.5. Finally,
we establish both qualitative and quantitatively that both local clustering as
well as filament compression and preheating are important factors in
determining the statistical properties of the cosmic accretion shock
population.Comment: 13 pages, 5 figures, emulateap
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