4,785 research outputs found
Determination of intergalactic magnetic fields from gamma ray data
We report a measurement of intergalactic magnetic fields using combined data
from Atmospheric Cherenkov Telescopes and Fermi Gamma-Ray Space Telescope,
based on the spectral data alone. If blazars are assumed to produce both gamma
rays and cosmic rays, the observed spectra are not sensitive to the intrinsic
spectrum of the source, because, for a distant blazar, secondary photons
produced in line-of-sight cosmic-ray interactions dominate the signal. In this
case, we find 0.01 fG < B < 30 fG. If one excludes the cosmic-ray component,
the 0.01 fG lower limit remains, but the upper limit depends on the spectral
properties of the source. We present the allowed ranges for a variety of model
parameters.Comment: 13 pages, 3 figure
Time structure of gamma-ray signals generated in line-of-sight interactions of cosmic rays from distant blazars
Blazars are expected to produce both gamma rays and cosmic rays. Therefore,
observed high-energy gamma rays from distant blazars may contain a significant
contribution from secondary gamma rays produced along the line of sight by the
interactions of cosmic-ray protons with background photons. Unlike the standard
models of blazars that consider only the primary photons emitted at the source,
models which include the cosmic-ray contribution predict that even ~10 TeV
photons should be detectable from distant objects with redshifts as high as z>
0.1. Secondary photons contribute to signals of point sources only if the
intergalactic magnetic fields are very small, below ~10 femtogauss, and their
detection can be used to set upper bounds on magnetic fields along the line of
sight. Secondary gamma rays have distinct spectral and temporal features. We
explore the temporal properties of such signals using a semi-analytical
formalism and detailed numerical simulations, which account for all the
relevant processes, including magnetic deflections. In particular, we elucidate
the interplay of time delays coming from the proton deflections and from the
electromagnetic cascade, and we find that, at multi-TeV energies, secondary
gamma-rays can show variability on timescales of years for femtogauss magnetic
fields.Comment: 25 pages, 9 figure
TeV gamma rays from blazars beyond z=1?
At TeV energies, the gamma-ray horizon of the universe is limited to
redshifts z<<1, and, therefore, any observation of TeV radiation from a source
located beyond z=1 would call for a revision of the standard paradigm. While
robust observational evidence for TeV sources at redshifts z>1 is lacking at
present, the growing number of TeV blazars with redshifts as large as z~0.5
suggests the possibility that the standard blazar models may have to be
reconsidered. We show that TeV gamma rays can be observed even from a source at
z>1, if the observed gamma rays are secondary photons produced in interactions
of high-energy protons originating from the blazar jet and propagating over
cosmological distances almost rectilinearly. This mechanism was initially
proposed as a possible explanation for the TeV gamma rays observed from blazars
with redshifts z~0.2, for which some other explanations were possible. For TeV
gamma-ray radiation detected from a blazar with z>1, this model would provide
the only viable interpretation consistent with conventional physics. It would
also have far-reaching astronomical and cosmological ramifications. In
particular, this interpretation would imply that extragalactic magnetic fields
along the line of sight are very weak, in the range 0.01 < fG < 10 fG, assuming
random fields with a correlation length of 1 Mpc, and that acceleration of E>
0.1 EeV protons in the jets of active galactic nuclei can be very effective.Comment: 8 pages, 4 figure
Auschwitz has Happened: An Exploration of the Past, Present, and Future of Jewish Redemption
Ch. 1: Introduction: A Destruction without Adequate Precedent. Ch. 2: Rupture and the Holy Ideal: Redemption in the Hebrew Bible. Ch. 3: Giving the Sense: The Rise of Commentary. Ch. 4: Rabbi Eliezer’s Silence. Ch. 5: Gold and Glass: Ethical Rupture in Mystical Union? Ch. 6: Our Impossible Victory
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