218 research outputs found
Relativistic protons in the Coma galaxy cluster: first gamma-ray constraints ever on turbulent reacceleration
The Fermi-LAT collaboration recently published deep upper limits to the
gamma-ray emission of the Coma cluster, a cluster that hosts the prototype of
giant radio halos. In this paper we extend previous studies and use a formalism
that combines particle reacceleration by turbulence and the generation of
secondary particles in the intracluster medium to constrain relativistic
protons and their role for the origin of the radio halo. We conclude that a
pure hadronic origin of the radio halo is clearly disfavoured as it would
require magnetic fields that are too strong. For instance G is
found in the cluster center assuming that the magnetic energy density scales
with thermal density, to be compared with G as inferred from
Rotation Measures (RM) under the same assumption. However secondary particles
can still generate the observed radio emission if they are reaccelerated. For
the first time the deep gamma-ray limits allow us to derive meaningful
constraints if the halo is generated during phases of reacceleration of
relativistic protons and their secondaries by cluster-scale turbulence. In this
paper we explore a relevant range of parameter-space of reacceleration models.
Within this parameter space a fraction of model configurations is already ruled
out by current gamma-ray limits, including the cases that assume weak magnetic
fields in the cluster core, G. Interestingly, we also find that
the flux predicted by a large fraction of model configurations that assume a
magnetic field consistent with RM is not far from the limits. This suggests
that a detection of gamma rays from the cluster might be possible in the near
future, provided that the electrons generating the radio halo are secondaries
reaccelerated and the magnetic field in the cluster is consistent with that
inferred from RM.Comment: 22 pages, 12 figures, submitte
Diffuse Radio Emission from Galaxy Clusters
In a growing number of galaxy clusters diffuse extended radio sources have
been found. These sources are not directly associated with individual cluster
galaxies. The radio emission reveal the presence of cosmic rays and magnetic
fields in the intracluster medium (ICM). We classify diffuse cluster radio
sources into radio halos, cluster radio shocks (relics), and revived AGN fossil
plasma sources. Radio halo sources can be further divided into giant halos,
mini-halos, and possible `intermediate' sources. Halos are generally positioned
at cluster center and their brightness approximately follows the distribution
of the thermal ICM. Cluster radio shocks (relics) are polarized sources mostly
found in the cluster's periphery. They trace merger induced shock waves.
Revived fossil plasma sources are characterized by their radio steep-spectra
and often irregular morphologies. In this review we give an overview of the
properties of diffuse cluster radio sources, with an emphasis on recent
observational results. We discuss the resulting implications for the underlying
physical acceleration processes that operate in the ICM, the role of
relativistic fossil plasma, and the properties of ICM shocks and magnetic
fields. We also compile an updated list of diffuse cluster radio sources which
will be available on-line http://galaxyclusters.com. We end this review with a
discussion on the detection of diffuse radio emission from the cosmic web.Comment: To appear in Space Science Reviews, 56 pages, 31 figures; For
associated data see http://galaxyclusters.co
The origin of the diffuse non-thermal X-ray and radio emission in the Ophiuchus cluster of galaxies
We present high resolution 240 and 607 MHz GMRT radio observations,
complemented with 74 MHz archival VLA radio observations of the Ophiuchus
cluster of galaxies, whose radio mini-halo has been recently detected at 1400
MHz. We also present archival Chandra and XMM-Newton data of the Ophiuchus
cluster. Our observations do not show significant radio emission from the
mini-halo, hence we present upper limits to the integrated, diffuse non-thermal
radio emission of the core of the Ophiuchus cluster. The XMM-Newton
observations can be well explained by a two-temperature thermal model with
temperatures of ~=1.8 keV and ~=9.0 keV, respectively, which confirms previous
results that suggest that the innermost central region of the Ophiuchus cluster
is a cooling core. We also used the XMM-Newton data to set up an upper limit to
the (non-thermal) X-ray emission from the cluster.
The combination of available radio and X-ray data has strong implications for
the currently proposed models of the spectral energy distribution (SED) from
the Ophiuchus cluster. In particular, a synchrotron+IC model is in agreement
with the currently available data, if the average magnetic field is in the
range (0.02-0.3) microG. A pure WIMP annihilation scenario can in principle
reproduce both radio and X-ray emission, but at the expense of postulating very
large boost factors from dark matter substructures, jointly with extremely low
values of the average magnetic field. Finally, a scenario where synchrotron and
inverse Compton emission arise from PeV electron-positron pairs (via
interactions with the CMB), can be ruled out, as it predicts a non-thermal soft
X-ray emission that largely exceeds the thermal Bremsstrahlung measured by
INTEGRAL.Comment: Accepted for publication in MNRAS; 13 pages, 8 figures. Includes
minor changes. Abridged abstrac
The search for DM in nearby dSph galaxies with MAGIC: candidates, results and prospects
At present, dwarf spheroidal galaxies satellites of the Milky Way may
represent the best astrophysical objects for dark matter (DM) searches with
gamma-ray telescopes. They present the highest mass-to-light ratios known in
the Universe. Furthermore, many of them are near enough from the Earth to be
able to yield high predicted DM annihilation fluxes that might be observed by
current gamma-ray instruments like MAGIC. The picture has become even better
with the recent discovery of new dwarfs. These new objects are expected to
yield even higher DM annihilation fluxes, since most of them are nearer than
the previously known dwarfs and are even more DM dominated systems. Here a
tentative list of the best candidates is given. The observational results
obtained with MAGIC from the Draco dwarf as well as the observation of other
dwarfs carried out by other Cherenkov telescopes are presented as well.
Finally, we discuss the detection prospects of such kind of objects in the
context of DM searches.Comment: To appear in the proceedings of the 8th UCLA Dark Matter Symposium,
Marina del Rey, USA, 20-22 February 200
MAGIC observation of the GRB 080430 afterglow
6 páginas, 1 figura.-- El Pdf del artículo es la versión pre-print: arXiv:1004.3665v2.-- MAGIC Collaboration: et al.[Context]: Gamma-ray bursts are cosmological sources emitting radiation from the gamma-rays to the radio band. Substantial observational efforts have been devoted to the study of gamma-ray bursts during the prompt phase, i.e. the initial burst of high-energy radiation, and during the long-lasting afterglows. In spite of many successes in interpreting these phenomena, there are still several open key questions about the fundamental emission processes, their energetics and the environment.
[Aims]: Independently of specific gamma-ray burst theoretical recipes, spectra in the GeV/TeV range are predicted to be remarkably simple, being satisfactorily modeled with power-laws, and therefore offer a very valuable tool to probe the extragalactic background light distribution. Furthermore, the simple detection of a component at very-high energies, i.e. at ~100 GeV, would solve the ambiguity about the importance of various possible emission processes, which provide barely distinguishable scenarios at lower energies.
[Methods]: We used the results of the MAGIC telescope observation of the moderate resdhift (z ~ 0.76) GRB 080430 at energies above about 80 GeV, to evaluate the perspective for late-afterglow observations with ground based GeV/TeV telescopes.
[Results]: We obtained an upper limit of F95% CL = 5.5 × 10-11 erg cm-2 s-1 for the very-high energy emission of GRB 080430, which cannot set further constraints on the theoretical scenarios proposed for this object also due to the difficulties in modeling the low-energy afterglow. Nonetheless, our observations show that Cherenkov telescopes have already reached the required sensitivity to detect the GeV/TeV emission of GRBs at moderate redshift (z ≲ 0.8), provided the observations are carried out at early times, close to the onset of their afterglow phase.The support of the German BMBF and MPG,
the Italian INFN and Spanish MICINN is gratefully acknowledged. This work
was also supported by ETH Research Grant TH 34/043, by the Polish MNiSzW
Grant N N203 390834, and by the YIP of the Helmholtz Gemeinschaft.Peer reviewe
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