10,409 research outputs found
Flaring gamma-ray emission from high redshift blazars
High redshift blazars are among the most powerful objects in the Universe.
Although they represent a significant fraction of the extragalactic hard X-ray
sky, they are not commonly detected in gamma-rays. High redshift (z>2) objects
represent <10 per cent of the AGN population observed by Fermi so far, and
gamma-ray flaring activity from these sources is even more uncommon. The
characterization of the radio-to-gamma-ray properties of high redshift blazars
represent a powerful tool for the study of both the energetics of such extreme
objects and the Extragalactic Background Light. We present results of a
multi-band campaign on TXS 0536+145, which is the highest redshift flaring
gamma-ray blazar detected so far. At the peak of the flare the source reached
an apparent isotropic gamma-ray luminosity of 6.6x10^49 erg/s, which is
comparable with the luminosity observed from the most powerful blazars. The
physical properties derived from the multi-wavelength observations are then
compared with those shown by the high redshift population. In addition
preliminary results from the high redshift flaring blazar PKS 2149-306 will be
discussed.Comment: 2014 Fermi Symposium proceedings - eConf C14102.
Radio emission at the centre of the galaxy cluster Abell 3560: evidence for core sloshing?
Previous radio observations of the galaxy cluster A3560 in the Shapley
Concentration showed complex radio emission associated with the brightest
cluster member.To understand its origin we observed it with the GMRT, the VLA
and ATCA at 240 and 610 MHz, 1.28,1.4, 2.3,4.8 and 8.4 GHz, and performed a
detailed morphological and spectral study of the radio emission associated with
the BCG. We also observed the cluster with XMM-Newton and Chandra to derive the
properties of the ICM. The radio emission of the N-E nucleus of the dumb-bell
BCG shows an active radio galaxy, plus aged diffuse emission, which is not
refurbished at present. Our Chandra data show that the radio active nucleus of
the BCG has extended X-ray emission, which we classify as a low-luminosity
corona. A residual image of the XMM-Newton brightness shows the presence of a
spiral-like feature, which we interpret as the signature of gas sloshing. The
presence of a subgroup is clear in the surface brightness residual map, and in
the XMM-Newton temperature analysis. The optical 2D analysis shows substructure
in A3560. A galaxy clump was found at the location of the X-ray subgroup, and
another group is present south of the cluster core, close to the spiral-like
feature. The aged part of the radio emission closely follows the spiral pattern
of the X-ray residual brightness distribution, while the two active radio lobes
are bent in a completely different direction. We conclude that the complex
radio emission associated with the cluster BCG is the result of a minor merger
event in A3560. The aged diffuse emission is strongly affected by the sloshing
motion in the ICM. On the other hand, the bent jets and lobes of the current
radio AGN activity may reflect a complex gas velocity field in the innermost
cluster regions and/or sloshing-induced oscillations in the motion of the cD
galaxy.Comment: 15 pages, 8 figures, 5 tables, A&A in pres
An unlikely radio halo in the low X-ray luminosity galaxy cluster RXC J1514.9-1523
We report the discovery of a giant radio halo in the galaxy cluster RXC
J1514.9-1523 at z=0.22 with a relatively low X-ray luminosity, erg s. This faint, diffuse
radio source is detected with the Giant Metrewave Radio Telescope at 327 MHz.
The source is barely detected at 1.4 GHz in a NVSS pointing that we have
reanalyzed. The integrated radio spectrum of the halo is quite steep, with a
slope \alpha = 1.6 between 327 MHz and 1.4 GHz. While giant radio halos are
common in more X-ray luminous cluster mergers, there is a less than 10%
probability to detect a halo in systems with L_X \ltsim 8 \times 10^{44} erg
s. The detection of a new giant halo in this borderline luminosity
regime can be particularly useful for discriminating between the competing
theories for the origin of ultrarelativistic electrons in clusters.
Furthermore, if our steep radio spectral index is confirmed by future deeper
radio observations, this cluster would provide another example of the recently
discovered population of ultra-steep spectrum radio halos, predicted by the
model in which the cluster cosmic ray electrons are produced by turbulent
reacceleration.Comment: 4 pages, 2 figures - Accepted for publication on A&A Research Note
Method of comparison equations for cosmological perturbations
We apply the method of comparison equations to study cosmological
perturbations during inflation, obtaining the full power spectra of scalar and
tensor perturbations to first and to second order in the slow-roll parameters.
We compare our results with those derived by means of other methods, in
particular the Green's function method and the improved WKB approximation, and
find agreement for the slow-roll structure. The method of comparison equations,
just as the improved WKB approximation, can however be applied to more general
situations where the slow-roll approximation fails.Comment: 20 pages, 10 figure
The galactic magnetic field in the quasar 3C216
Multifrequency polarimetric observations made with the Very Long Baseline
Array of the quasar 3C216 reveal the presence of Faraday rotation measures
(RMs) in excess of 2000 rad/m**2 in the source rest frame, in the arc of
emission located at ~ 140 mas from the core. Rotation measures in the range
-300 - +300 rad/m**2 are detected in the inner 5 mas (~30 parsecs). while the
rotation measures near the core can be explained as due to a magnetic field in
the narrow line region, we favor the interpretation for the high RM in the arc
as due to a ``local'' Faraday screen, produced in a shock where the jet is
deflected by the interstellar medium of the host galaxy. Our results indicate
that a galacit magnetic field of the order of 50 microGauss on a scale greater
than 100 pc must be present in the galactic medium.Comment: 23 pages, 3 tables, 11 figures. To appear on The Astronomical
Journal, November 1999 Issu
Black hole and the adiabatic phase
An open system consisting of a scalar field bound to a Kerr black hole whose
mass () and specific angular momentum () are slowly (adiabatically)
perturbed is considered. The adiabatically induced phase and the conditions for
the validity of the adiabatic approximation are obtained. The effect of closed
cycles in parameter space (, plane) on the energy levels of both stable
and unstable scalar field bound states, together with other quantities of
interest, is illustrated. Lastly it is noted that the black hole wavefunction
will acquire an equal and opposite phase to that of matter thus leading to a
change of its effective action (entropy).Comment: Plain TeX, 12 page
The stripping of a galaxy group diving into the massive cluster A2142
Structure formation in the current Universe operates through the accretion of
group-scale systems onto massive clusters. The detection and study of such
accreting systems is crucial to understand the build-up of the most massive
virialized structures we see today. We report the discovery with XMM-Newton of
an irregular X-ray substructure in the outskirts of the massive galaxy cluster
Abell 2142. The tip of the X-ray emission coincides with a concentration of
galaxies. The bulk of the X-ray emission of this substructure appears to be
lagging behind the galaxies and extends over a projected scale of at least 800
kpc. The temperature of the gas in this region is 1.4 keV, which is a factor of
~4 lower than the surrounding medium and is typical of the virialized plasma of
a galaxy group with a mass of a few 10^13M_sun. For this reason, we interpret
this structure as a galaxy group in the process of being accreted onto the main
dark-matter halo. The X-ray structure trailing behind the group is due to gas
stripped from its original dark-matter halo as it moves through the
intracluster medium (ICM). This is the longest X-ray trail reported to date.
For an infall velocity of ~1,200 km s-1 we estimate that the stripped gas has
been surviving in the presence of the hot ICM for at least 600 Myr, which
exceeds the Spitzer conduction timescale in the medium by a factor of >~400.
Such a strong suppression of conductivity is likely related to a tangled
magnetic field with small coherence length and to plasma microinstabilities.
The long survival time of the low-entropy intragroup medium suggests that the
infalling material can eventually settle within the core of the main cluster.Comment: 11 pages, 7 figures, accepted for publication in A&
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