22,816 research outputs found
Optical depth for VHE gamma-rays from distant sources from a generic EBL density
Very-high-energy (VHE; E>100GeV) gamma-rays from distant sources suffer
attenuation through pair-production with low energy photons from the diffuse
extragalactic photon fields in the ultraviolet (UV) to far-infrared (FIR)
(commonly referred to as Extragalactic Background Light; EBL). When modeling
the intrinsic spectra of the VHE gamma-ray sources it is crucial to correctly
account for the attenuation. Unfortunately, direct measurements of the EBL are
difficult and the knowledge about the EBL over certain wavelength ranges is
poor. To calculate the EBL attenuation usually predictions from theoretical
models are used. Recently, the limits on the EBL from direct and indirect
methods have narrowed down the possible EBL range and many of the previous
models are in conflict with these limits. We propose a new generic EBL density
(not a complete model), which is in compliance with the new EBL limits. EBL
evolution with redshift is included in the calculation in a very simple but
effective ad-hoc way. Properties of this generic EBL are discussed.Comment: Proceedings of the workshop 'High Energy Phenomena in Relativistic
Outflows' (HEPRO), Dublin, 24-28 September 200
Probing the EBL evolution at high redshift using GRBs detected with the Fermi-LAT
The extragalactic background light (EBL), from ultraviolet to infrared
wavelengths, is predominantly due to emission from stars, accreting black holes
and reprocessed light due to Galactic dust. The EBL can be studied through the
imprint it leaves, via - absorption of high-energy photons, in
the spectra of distant -ray sources. The EBL has been probed through
the search for the attenuation it produces in the spectra of BL Lacertae (BL
Lac) objects and individual -ray bursts (GRBs). GRBs have significant
advantages over blazars for the study of the EBL especially at high redshifts.
Here we analyze a combined sample of twenty-two GRBs, detected by the Fermi
Large Area Telescope between 65 MeV and 500 GeV. We report a marginal detection
(at the ~2.8 level) of the EBL attenuation in the stacked spectra of
the source sample. This measurement represents a first constraint of the EBL at
an effective redshift of ~1.8. We combine our results with prior EBL
constraints and conclude that Fermi-LAT is instrumental to constrain the UV
component of the EBL. We discuss the implications on existing empirical models
of EBL evolution.Comment: on behalf of the Fermi-LAT collaboration, accepted for publication on
Ap
Measurement of the EBL through a combined likelihood analysis of gamma-ray observations of blazars with the MAGIC telescopes
The extragalactic background light (EBL) is the radiation accumulated through
the history of the Universe in the wavelength range from the ultraviolet to the
far infrared. Local foregrounds make the direct measurement of the diffuse EBL
notoriously difficult, while robust lower limits have been obtained by adding
up the contributions of all the discrete sources resolved in deep infrared and
optical galaxy observations. Gamma-ray astronomy has emerged in the past few
years as a powerful tool for the study of the EBL: very-high-energy (VHE)
photons traversing cosmological distances can interact with EBL photons to
produce ee pairs, resulting in an energy-dependent depletion of the
gamma-ray flux of distant sources that can be used to set constraints on the
EBL density. The study of the EBL is one of the key scientific programs
currently carried out by the MAGIC collaboration. We present here the results
of the analysis of 32 VHE spectra of 12 blazars in the redshift range 0.03 -
0.94, obtained with over 300 hours of observations with the MAGIC telescopes
between 2010 and 2016. A combined likelihood maximization approach is used to
evaluate the density and spectrum of the EBL most consistent with the MAGIC
observations. The results are compatible with state-of-the-art EBL models, and
constrain the EBL density to be roughly within of the nominal
value in such models. The study reveals no anomalies in gamma-ray propagation
in the large optical depth regime - contrary to some claims based on
meta-analyses of published VHE spectra.Comment: Proceedings of the 35th International Cosmic Ray Conference (ICRC
2017), Bexco, Busan, Korea (arXiv:1708.05153
Constraining Extragalactic Background Light From TeV Blazars
Our goal is to research the upper limits on the extragalactic background
light (EBL). The upper limits on the extragalactic background light (EBL),
using the Fermi and very high energy (VHE) spectra recently observed in TeV
blazars, are presented. We use an assumption that the VHE intrinsic photon
index cannot be harder than the Fermi index measured by the Fermi-LAT. Totally,
these upper limits on the EBL are compatible with ones given by most of EBL
models. However, the models of high EBL density are denied by TeV blazars.Comment: 4 pages, 2 figures, accepted by A&
The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000A (III): Cosmological Implications
(Abridged) We have used HST WFPC2 and ground-based spectroscopy to measure
the integrated extragalactic background light (EBL) at optical wavelengths. We
have also computed the integrated light from individual galaxy counts in the
images used to measure the EBL and in the Hubble Deep Field. We find that the
flux in galaxies as measured by standard galaxy photometry methods has
generally been underestimated by about 50%. Further, we find that the total
flux in individually detected galaxies is a factor of 2 to 3 less than the EBL
at 3000--8000A. We show that a significant fraction of the EBL may come from
normal galaxies at z<4, which are simply undetectable as a result of
K-corrections and cosmological surface brightness dimming. This is consistent
with recent redshift surveys at z<4. In the context of some simple models, we
discuss the constraints placed by the EBL on the evolution of the luminosity
density at z>1. Based on our optical EBL and published UV and IR EBL
measurements, we estimate that the total EBL from 0.1--1000 microns is 100+/-20
nW/m^2/sr. If the total EBL were produced entirely by stellar nucleosynthesis,
then we estimate that the total baryonic mass processed through stars is
Omega_* = 0.0062 (+/- 0.0022) h^{-2}, which corresponds to 0.33+/-0.12 Omega_B
for currently favored values of the baryon density. This estimate is smaller by
roughly 7% if 7 h_{0.7} nW/m^2/sr of the total EBL comes from accretion onto
central black holes. This estimate of Omega_* suggests that the universe has
been enriched to a total metal mass of 0.21(+/-0.13) Z_sun Omega_B. Our
estimate is consistent with other measurements of the cumulative metal mass
fraction of stars, stellar remnants, and the intracluster medium of galaxy
clusters in the local universe.Comment: Accepted for publication in ApJ, 20 pages using emulateapj.sty,
version with higher resolution figures available at
http://www.astro.lsa.umich.edu/~rab/publications.html or at
http://nedwww.ipac.caltech.edu/level5/Sept01/Bernstein3/frames.htm
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