64 research outputs found
Is the Universe More Transparent to Gamma Rays Than Previously Thought?
The MAGIC collaboration has recently reported the detection of the strong
gamma-ray blazar 3C279 during a 1-2 day flare. They have used their spectral
observations to draw conclusions regarding upper limits on the opacity of the
Universe to high energy gamma-rays and, by implication, upper limits on the
extragalactic mid-infrared background radiation. In this paper we examine the
effect of gamma-ray absorption by the extragalactic infrared radiation on
intrinsic spectra for this blazar and compare our results with the
observational data on 3C279. We find agreement with our previous results,
contrary to the recent assertion of the MAGIC group that the Universe is more
transparent to \gray s than our calculations indicate. Our analysis indicates
that in the energy range between ~80 and ~500 GeV, 3C279 has a best-fit
intrinsic spectrum with a spectral index ~1.78 using our fast evolution model
and ~2.19 using our baseline model. However, we also find that spectral indices
in the range of 0.0 to 3.0 are almost as equally acceptable as the best fit
spectral indices. Assuming the same intrinsic spectral index for this flare as
for the 1991 flare from 3C279 observed by EGRET, viz., 2.02, which lies between
our best fit indices, we estimate that the MAGIC flare was ~3 times brighter
than the EGRET flare observed 15 years earlier.Comment: version accepted for publication in ApJ Letter
An Empirical Determination of the Intergalactic Background Light Using NIR Deep Galaxy Survey Data out to 5 microns and the Gamma-ray Opacity of the Universe
We extend our previous model-independent determination of the intergalactic
background light (IBL), based purely on galaxy survey data, out to a wavelength
of 5 microns. Our approach enables us to constrain the range of photon
densities, based on the uncertainties from observationally determined
luminosity densities and colors. We further determine a 68% confidence upper
and lower limit on the opacity of the universe to gamma-rays up to energies of
1.6/(1+z) TeV. A comparison of our lower limit redshift-dependent opacity
curves to the opacity limits derived from the results of both ground-based air
Cherenkov telescope and Fermi-LAT observations of PKS 1424+240 allows us to
place a new upper limit on the redshift of this source, independent of IBL
modeling.Comment: version accepted for publication in the Astrophysical Journal, 23
pages, 4 figure
A Determination of the Intergalactic Redshift Dependent UV-Optical-NIR Photon Density Using Deep Galaxy Survey Data and the Gamma-ray Opacity of the Universe
We calculate the intensity and photon spectrum of the intergalactic
background light (IBL) as a function of redshift using an approach based on
observational data obtained in many different wavelength bands from local to
deep galaxy surveys. This allows us to obtain an empirical determination of the
IBL and to quantify its observationally based uncertainties. Using our results
on the IBL, we then place 68% confidence upper and lower limits on the opacity
of the universe to gamma-rays, free of the theoretical assumptions that were
needed for past calculations. We compare our results with measurements of the
extragalactic background light and upper limits obtained from observations made
by the Fermi Gamma-ray Space Telescope.Comment: 32 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
An Empirical Determination of the Intergalactic Background Light from UV to FIR Wavelengths Using FIR Deep Galaxy Surveys and the Gamma-ray Opacity of the Universe
We have previously calculated the intergalactic background light (IBL) as a
function of redshift in the far ultraviolet to near infrared range, based
purely on data from deep galaxy surveys. Here we utilize similar methods to
determine the mid- and far infrared IBL out to a wavelength of 850 microns. Our
approach enables us to constrain the range of photon densities, based on the
uncertainties from observationally determined luminosity densities and colors.
By also including the effect of the 2.7 K cosmic background photons, we
determine 68% confidence upper and lower limits on the opacity of the universe
to gamma-rays up to PeV energies. Our direct results on the IBL are consistent
with those from complimentary gamma-ray analyses using observations from the
Fermi -ray space telescope and the H.E.S.S. air Cherenkov telescope.
Thus, we find no evidence of previously suggested processes for the
modification of gamma-ray spectra other than that of absorption by pair
production alone.Comment: 33 pages, 11 figures, replacement matches article published in ApJ
827:6 (2016
Big Bang Nucleosynthesis: An Update
WThe current status of big bang nucleosynthesis is reviewed with an emphasis
on the comparison between the observational determination of the light element
abundances of \D, \he3, \he4 and \li7 and the predictions from theory. In
particular, we present new analyses for \he4 and \li7. Implications for physics
beyond the standard model are also discussed. Limits on the effective number of
neutrino flavors are also updated.Comment: 21 pages, latex, 9 uuencoded, tarred, compressed, ps figures minor
error in eq. 5 correcte
Searching for New Physics with Ultrahigh Energy Cosmic Rays
Ultrahigh energy cosmic rays that produce giant extensive showers of charged
particles and photons when they interact in the Earth's atmosphere provide a
unique tool to search for new physics. Of particular interest is the
possibility of detecting a very small violation of Lorentz invariance such as
may be related to the structure of space-time near the Planck scale of m. We discuss here the possible signature of Lorentz invariance
violation on the spectrum of ultrahigh energy cosmic rays as compared with
present observations of giant air showers. We also discuss the possibilities of
using more sensitive detection techniques to improve searches for Lorentz
invariance violation in the future. Using the latest data from the Pierre Auger
Observatory, we derive a best fit to the LIV parameter of , corresponding to an upper limit of at a
proton Lorentz factor of . This result has fundamental
implications for quantum gravity models.Comment: Minor text changes and some references corrected and updated. Version
published in New Journal of Physics 11 (2009) 08500
A Bayesian Estimate of the Primordial Helium Abundance
We introduce a new statistical method to estimate the primordial helium
abundance, Y_p from observed abundances in a sample of galaxies which have
experienced stellar helium enrichment. Rather than using linear regression on
metal abundance we construct a likelihood function using a Bayesian prior,
where the key assumption is that the true helium abundance must always exceed
the primordial value. Using a sample of measurements compiled from the
literature we find estimates of Y_p between 0.221 and 0.236, depending on the
specific subsample and prior adopted, consistent with previous estimates either
from a linear extrapolation of the helium abundance with respect to
metallicity, or from the helium abundance of the lowest metallicity HII region,
I Zw 18. We also find an upper limit which is insensitive to the specific
subsample or prior, and estimate a model-independent bound Y_p < 0.243 at 95%
confidence, favoring a low cosmic baryon density and a high primordial
deuterium abundance. The main uncertainty is not the model of stellar
enrichment but possible common systematic biases in the estimate of Y in each
individual HII region.Comment: 14 pages, latex, 3 ps figure
Gamma-Ray Absorption By The Cosmic Lyman Continuum From Star-forming Galaxies
Motivated by the discovery of the ultra-strong emission line starburst
galaxies (EELGs) known as "green pea galaxies", we consider here their
contribution to the intergalactic flux of ionizing UV at high redshifts. Most
galaxies that have been observed show a precipitous drop in their flux blueward
of the Lyman limit. However, recent observations of EELGs have discovered that
many more Lyman continuum photons escape from them into intergalactic space
than was previously suspected. We calculate their contribution to the
extragalactic background light (EBL). We also calculate the effect of these
photons on the absorption of high energy -rays. For the more distant
-ray sources, particularly at , the intergalactic opacity
above a few GeV is significantly higher than previous estimates which ignored
the Lyman continuum photons. We calculate the results of this increased opacity
on observed -ray spectra, which produces a high-energy turnover
starting at lower energies than previously thought, and a gradual spectral
steepening that may also be observable.Comment: 22 pages, 9 figures, accepted for publication in the Astrophysical
Journa
An Empirical Determination of the Intergalactic Background Light Using Near-Infrared Deep Galaxy Survey Data Out to 5 Micrometers and the Gamma-Ray Opacity of the Universe
We extend our previous model-independent determination of the intergalactic background light, based purely on galaxy survey data, out to a wavelength of 5 micrometers. Our approach enables us to constrain the range of photon densities, based on the uncertainties from observationally determined luminosity densities and colors. We further determine a 68% confidence upper and lower limit on the opacity of the universe to gamma-rays up to energies of 1.6/(1 + z) terraelectron volts. A comparison of our lower limit redshift-dependent opacity curves to the opacity limits derived from the results of both ground-based air Cerenkov telescope and Fermi-LAT observations of PKS 1424+240 allows us to place a new upper limit on the redshift of this source, independent of IBL modeling
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