46 research outputs found
Stochastic conversions of TeV photons into axion-like particles in extragalactic magnetic fields
Very-high energy photons emitted by distant cosmic sources are absorbed on
the extragalactic background light (EBL) during their propagation. This effect
can be characterized in terms of a photon transfer function at Earth. The
presence of extragalactic magnetic fields could also induce conversions between
very high-energy photons and hypothetical axion-like particles (ALPs). The
turbulent structure of the extragalactic magnetic fields would produce a
stochastic behaviour in these conversions, leading to a statistical
distribution of the photon transfer functions for the different realizations of
the random magnetic fields. To characterize this effect, we derive new
equations to calculate the mean and the variance of this distribution. We find
that, in presence of ALP conversions, the photon transfer functions on
different lines of sight could have relevant deviations with respect to the
mean value, producing both an enhancement or a suppression in the observable
photon flux with respect to the expectations with only absorption. As a
consequence, the most striking signature of the mixing with ALPs would be a
reconstructed EBL density from TeV photon observations which appears to vary
over different directions of the sky: consistent with standard expectations in
some regions, but inconsistent in others.Comment: v2: 22 pages, 5 eps figures. Minor changes. A reference added.
Matches the version published on JCA
Axion-like particle imprint in cosmological very-high-energy sources
Discoveries of very high energy (VHE) photons from distant blazars suggest
that, after correction by extragalactic background light (EBL) absorption,
there is a flatness or even a turn-up in their spectra at the highest energies
that cannot be easily explained by the standard framework. Here, it is shown
that a possible solution to this problem is achieved by assuming the existence
of axion-like particles (ALPs) with masses ~1 neV. The ALP scenario is tested
making use of observations of the highest redshift blazars known in the VHE
energy regime, namely 3C 279, 3C 66A, PKS 1222+216 and PG 1553+113. In all
cases, better fits to the observed spectra are found when including ALPs rather
than considering EBL only. Interestingly, quite similar critical energies for
photon/ALP conversions are also derived, independently of the source
considered.Comment: 12 pages, 2 figures, 2 tables; accepted by JCAP. Replaced to match
the accepted versio
The Cosmic Infrared Background Experiment (CIBER): The Wide-Field Imagers
We have developed and characterized an imaging instrument to measure the
spatial properties of the diffuse near-infrared extragalactic background light
in a search for fluctuations from z > 6 galaxies during the epoch of
reionization. The instrument is part of the Cosmic Infrared Background
Experiment (CIBER), designed to observe the extragalactic background light
above the Earth's atmosphere during a suborbital sounding rocket flight. The
imaging instrument incorporates a 2x2 degree field of view, to measure
fluctuations over the predicted peak of the spatial power spectrum at 10
arcminutes, and 7"x7" pixels, to remove lower redshift galaxies to a depth
sufficient to reduce the low-redshift galaxy clustering foreground below
instrumental sensitivity. The imaging instrument employs two cameras with
\Delta \lambda / \lambda ~0.5 bandpasses centered at 1.1 and 1.6 microns to
spectrally discriminate reionization extragalactic background fluctuations from
local foreground fluctuations. CIBER operates at wavelengths where the
electromagnetic spectrum of the reionization extragalactic background is
thought to peak, and complements fluctuations measurements by AKARI and Spitzer
at longer wavelengths. We have characterized the instrument in the laboratory,
including measurements of the sensitivity, flat-field response, stray light
performance, and noise properties. Several modifications were made to the
instrument following a first flight in 2009 February. The instrument performed
to specifications in subsequent flights in 2010 July and 2012 March, and the
scientific data are now being analyzed.Comment: 16 Pages, 16 figures, submitted to ApJS February 13 2012; accepted
June 20 2012 as part of CIBER Instrument Special Issu
The Second Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope
The second catalog of active galactic nuclei (AGNs) detected by the Fermi
Large Area Telescope (LAT) in two years of scientific operation is presented.
The Second LAT AGN Catalog (2LAC) includes 1017 gamma-ray sources located at
high Galactic latitudes (|b|>10{\deg}) that are detected with a test statistic
greater than 25 and associated statistically with AGNs. However some of these
are affected by analysis issues and some are associated with multiple AGNs.
Consequently we define a clean sample which includes 886 AGNs, comprising 395
BL Lacertae objects (BL Lacs), 310 flat-spectrum radio quasars (FSRQs), 157
candidate blazars of unknown type (i.e., with broad-band blazar characteristics
but with no optical spectral measurement yet), eight misaligned AGNs, four
narrow-line Seyfert 1 (NLS1s), 10 AGNs of other types and two starburst
galaxies. Where possible, the blazars have been further classified based on
their spectral energy distributions (SEDs) as archival radio, optical, and
X-ray data permit. While almost all FSRQs have a synchrotron-peak frequency
10^14 Hz, about half of the BL Lacs have a synchrotron-peak frequency >10^15
Hz. The 2LAC represents a significant improvement relative to the First LAT AGN
Catalog (1LAC), with 52% more associated sources. The full characterization of
the newly detected sources will require more broad-band data. Various
properties, such as gamma-ray fluxes and photon power law spectral indices,
redshifts, gamma-ray luminosities, variability, and archival radio
luminosities---and their correlations are presented and discussed for the
different blazar classes. The general trends observed in 1LAC are confirmed.Comment: Accepted for publication in The Astrophysical Journal. 103 pages, 45
figures, 9 tables. Full tables are available at
http://www.asdc.asi.it/fermi2lac