1,334 research outputs found
Gamma Rays From Blazars
Blazars are high-energy engines providing us natural laboratories to study
particle acceleration, relativistic plasma processes, magnetic field dynamics,
black hole physics. Key informations are provided by observations at
high-energy (in particular by Fermi/LAT) and very-high energy (by Cherenkov
telescopes). I give a short account of the current status of the field, with
particular emphasis on the theoretical challenges connected to the observed
ultra-fast variability events and to the emission of flat spectrum radio
quasars in the very high energy band.Comment: 12 pages, invited talk at "Gamma2016", Heidelberg July 11-15, 2016,
to be published in a special volume of the AIP Conference Proceeding
Recollimation shocks and radiative losses in extragalactic relativistic jets
We present the results of state-of-the-art simulations of recollimation
shocks induced by the interaction of a relativistic jet with an external
medium, including the effect of radiative losses of the shocked gas. Our
simulations confirm that -- as suggested by earlier semi-analytical models --
the post-shock pressure loss induced by radiative losses may lead to a
stationary equilibrium state characterized by a very strong focusing of the
flow, with the formation of quite narrow nozzles, with cross-sectional radii as
small as times the length scale of the jet. We also study the
time-dependent evolution of the jet structure induced of a density perturbation
injected at the flow base. The set-up and the results of the simulations are
particularly relevant for the interpretation of the observed rapid variability
of the -ray emission associated to flat spectrum radio quasars. In
particular, the combined effects of jet focusing and Doppler beaming of the
observed radiation make it possible to explain the sub-hour flaring events such
as that observed in the FSRQ PKS 1222+216 by MAGIC.Comment: 8 pages, 8 figures, Astronomy and Astrophysics accepte
Fermi/LAT broad emission line blazars
We study the broad emission line blazars detected in the gamma-ray band by
the Large Area Telescope onboard the Fermi satellite and with the optical
spectrum studied by Shaw et al. (2012, 2013). The observed broad line strength
provides a measure of the ionizing luminosity of the accretion disk, while the
gamma-luminosity is a proxy for the bolometric non-thermal beamed jet emission.
The resulting sample, composed by 217 blazars, is the best suited to study the
connection between accretion and jet properties. We compare the broad emission
line properties of these blazars with those of radio-quiet and radio-loud
quasars present in the Sloan Digital Sky Survey, to asses differences and
similarities of the disc luminosity and the virial black hole mass. For most
sources, we could derive the black hole mass by reproducing the IR-optical-UV
data with a standard accretion disc spectrum, and we compared the black hole
masses derived with the two methods. The distributions of the masses estimated
in the two ways agree satisfactorily. We then apply a simple, one-zone,
leptonic model to all the 217 objects of our sample. The knowledge of the black
hole mass and disc luminosity helps to constrain the jet parameters. On average
they are similar to what found by previous studies of smaller samples of
sources.Comment: Accepted for publication in MNRAS. 30 pages, 6 tables, 16 figures. It
contains full tables and all SEDs of the source
Extreme BL Lacs: probes for cosmology and UHECR candidates
High-energy observations of extreme BL Lac objects, such as 1ES0229+200 or
1ES 0347-121, recently focused interest both for blazar and jet physics and for
the implication on the extragalactic background light and intergalactic
magnetic field estimate. Moreover, their enigmatic properties have been
interpreted in a scenario in which their primary high- energy output is through
a beam of high-energy hadrons. However, despite their possible important role
in all these topics, the number of these extreme highly peaked BL Lac objects
(EHBL) is still rather small. Aiming at increase their number, we selected a
group of EHBL candidates considering those undetected (or only barely detected)
by the LAT onboard Fermi and characterized by a high X-ray versus radio flux
ratio. We assembled the multi-wavelength spectral energy distribution of the
resulting 9 sources, using available archival data of Swift, GALEX, and Fermi
satellites, confirming their nature. Through a simple one-zone synchrotron
self-Compton model we estimate the expected very high energy flux, finding that
in the majority of cases it is within the reach of present generation of
Cherenkov arrays or of the forthcoming CTA.Comment: 6 pages, to appear in the Proceedings of the Conference "High-Energy
Phenomena and Relativistic Outflows V", held in La Plata, 5-8 October 201
On the detectability of Lorentz invariance violation through anomalous multi-TeV -ray spectra of blazars
Cosmic opacity for very high-energy gamma rays ( TeV) due to the
interaction with the extragalactic background light can be strongly reduced
because of possible Lorentz-violating terms in the particle dispersion
relations expected, e.g., in several versions of quantum gravity theories. We
discuss the possibility to use very high energy observations of blazars to
detect anomalies of the cosmic opacity induced by LIV, considering in
particular the possibility to use -- besides the bright and close-by BL Lac Mkn
501 -- extreme BL Lac objects. We derive the modified expression for the
optical depth of rays considering also the redshift dependence and we
apply it to derive the expected high-energy spectrum above 10 TeV of Mkn 501 in
high and low state and the extreme BL Lac 1ES 0229+200. We find that, besides
the nearby and well studied BL Lac Mkn 501 -- especially in high state --,
suitable targets are extreme BL Lac objects, characterized by quite hard TeV
intrinsic spectra likely extending at the energies relevant to detect LIV
features.Comment: 9 pages, 9 figures, accepted by A&
Gamma-ray emission from AGNs
Blazars, radio-loud active galactic nuclei with the relativistic jet closely
aligned with the line of sight, dominate the extragalactic sky observed at
gamma-ray energies, above 100 MeV. We discuss some of the emission properties
of these sources, focusing in particular on the "blazar sequence" and the
interpretative models of the high-energy emission of BL Lac objects.Comment: 8 pages, 4 figures, to appear in the proceedings of the HEPRO II
conference, Buenos Aires, October 26-30 200
X-ray Emission from Extragalactic Jets
This review focuses on the X-ray emission processes of extra-galactic jets on
scales resolvable by the sub arcsec resolution of the Chandra X-ray
Observatory. It is divided into 4 parts. The introductory chapter reviews the
classical problems for jets, as well as those associated directly with the
X-ray emission. Throughout this section, we deal with the dualisms of low
powered radio sources versus high powered radio galaxies and quasars;
synchrotron models versus inverse Compton models; and the distinction between
the relativistic plasma responsible for the received radiation and the medium
responsible for the transport of energy down the jet. The second part collects
the observational and inferred parameters for the currently detected X-ray jets
and attempts to put their relative sizes and luminosities in perspective. In
part 3, we first give the relevant radio and optical jet characteristics, and
then examine the details of the X-ray data and how they can be related to
various jet attributes. The last section is devoted to a critique of the two
non-thermal emission processes and to prospects for progress in our
understanding of jets.Comment: This is a version of a review article to be published (2006 Sep) in
the Annual Reviews of Astronomy and Astrophysics, vol. 44, p. 463. 8 of the
12 figures have been removed from the article and are provided as separate
jpg files to conserve space. There are 38 pages remaining in the text.
Complete postscript and pdf versions are available at:
http://hea-www.harvard.edu/~harris/Xjetreview
Did we observe the supernova shock breakout in GRB 060218?
If the early optical data of GRB 060218 up to 1e5 s are interpreted as the
black-body flux associated with the supernova shock breakout, we can derive
lower limits to the bolometric luminosity and energetics of this black-body
component. These limits are more severe for the very early data that imply
energetics of order of 1e51 erg. These values, puzzlingly large, are rather
independent of the assumed time profile of the emitting surface, provided that
the corresponding radius does not increase superluminally. Another concern is
the luminosity of the black-body component observed in the X-rays, that is
large and appears to be produced by an approximately constant temperature and a
surface area increasing only slowly in time. Although it has been suggested
that the long X-ray black-body duration is consistent with the supernova shock
breakout if anisotropy is assumed, the nearly constant emitting surface
requires some fine tuning, allowing and suggesting an alternative
interpretation, i.e. emission from late dissipation of the fireball bulk
kinetic energy. This in turn requires a small value of the bulk Lorentz factor.Comment: 5 pages, 2 figures, revised version, MNRAS Letters, in pres
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