1,129 research outputs found
On the July 2007 flare of the blazar 3C 454.3
In July 2007, the blazar 3C 454.3 underwent a flare in the optical, reaching
R~13 on July 19. Then the optical flux decreased by one magnitude, being R~14
when the source was detected by the gamma-ray satellite AGILE, that observed
the source on July 24-30. At the same time, the Swift satellite performed a
series of snapshots. We can construct the simultaneous spectral energy
distribution using optical, UV, X-ray and gamma-ray data. These shows that an
increased gamma-ray flux is accompanied by a weaker optical/X-ray flux with
respect to the flare observed in the Spring 2005 by INTEGRAL and Swift. This
confirms earlier suggestions about the behaviour of the jet of 3C 454.3.Comment: 5 pages, 3 figures, MNRAS (Letters) in press (minor revision
Spine-sheath layer radiative interplay in subparsec-scale jets and the TeV emission from M87
Simple one-zone homogeneous synchrotron self-Compton models have severe
difficulties in explaining the TeV emission observed in the radiogalaxy M87.
Also the site of the TeV emission region is uncertain: it could be the
unresolved jet close to the nucleus, analogously to what proposed for blazars,
or an active knot, called HST-1, tens of parsec away. We explore the
possibility that the TeV emission of M87 is produced in the misaligned subpc
scale jet. We base our modelling on a structured jet, with a fast spine
surrounded by a slower layer. In this context the main site responsible for the
emission of the TeV radiation is the layer, while the (debeamed) spine accounts
for the emission from the radio to the GeV band: therefore we expect a more
complex correlation with the TeV component than that expected in one-zone
scenarios, in which both components are produced by the same region. Observed
from small angles, the spine would dominate the emission, with an overall
Spectral Energy Distribution close to those of BL Lac objects with a
synchrotron peak located at low energy (LBLs).Comment: 5 pages, 2 figures. Accepted for publication in MNRAS Letter
Constraining blazar distances with combined Fermi and TeV data: an empirical approach
We discuss a method to constrain the distance of blazars with unknown
redshift using combined observations in the GeV and TeV regimes. We assume that
the VHE spectrum corrected for the absorption through the interaction with the
Extragalactic Background Light can not be harder than the spectrum in the
Fermi/LAT band. Starting from the observed VHE spectral data we derive the
EBL-corrected spectra as a function of the redshift z and fit them with power
laws to be compared with power law fits to the LAT data. We apply the method to
all TeV blazars detected by LAT with known distance and derive an empirical law
describing the relation between the upper limits and the true redshifts that
can be used to estimate the distance of unknown redshift blazars. Using
different EBL models leads to systematic changes in the derived upper limits.
Finally, we use this relation to infer the distance of the unknown redshift
blazar PKS 1424+240.Comment: 5 pages, 4 figures, accepted for publication in MNRAS, minor revisio
Very high-energy constraints on the infrared extragalactic background light
Context. Measurements of the Extragalactic Background Light (EBL) are a
fundamental source of information on the collective emission of cosmic sources.
Aims. At infrared wavelengths, however, these measurements are precluded by
the overwhelming dominance from Interplanetary Dust emission and the Galactic
infrared foreground. Only at m, where the foregrounds are
minimal, has the Infrared EBL (IR EBL) been inferred from analysis of the COBE
maps. The present paper aims to assess the possibility of evaluating the IR EBL
from a few m up to the peak of the emission at >100 m using an
indirect method that avoids the foreground problem.
Methods. To this purpose we exploit the effect of pair-production from
gamma-gamma interaction by considering the highest energy photons emitted by
extragalactic sources and their interaction with the IR EBL photons. We
simulate observations of a variety of low redshift emitters with the
forthcoming Imaging Atmospheric Cherenkov Telescope (IACT) arrays (CTA in
particular) and water Cherenkov observatories (LHAASO, HAWC, SWGO) to assess
their suitability to constrain the EBL at such long wavelengths.
Results. We find that, even under the most extremely favorable conditions of
huge emission flares, extremely high-energy emitting blazars are not very
useful for our purpose because they are much too distant (>100 Mpc the nearest
ones, MKN 501 and MKN 421). Observations of more local Very High Energy (VHE)
emitting AGNs, like low-redshift radio galaxies (M87, IC 310, Centaurus A), are
better suited and will potentially allow us to constrain the EBL up to m
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