605 research outputs found
Halo and Relic Sources in Clusters of Galaxies
New images of 7 radio halos and relics, obtained with the Very Large Array at
20 or 90 cm, are presented here. The existence of a cluster-wide radio halo in
the clusters A 665 and CL 0016+16 is confirmed. Both these clusters share the
properties of the other clusters with radio halos, i.e. are luminous in X-rays,
have high temperature, and show recent merger processes. No diffuse sources are
detected in a sample of clusters showing at least a tailed radio galaxy within
300 kpc from the cluster center, indicating that the connection between tailed
radio galaxies and halos is not relevant. For these clusters we give limits to
the surface brightness and to the angular size of possible undetected diffuse
sources.Comment: 16 Figures in separated files. A large ps file with figures
(gg-aug00.ps.gz) in the text is avaialble at
ftp://terra.bo.cnr.it/papers/journals New Astronomy Vol. 5, p.335, in pres
Particle reacceleration in Coma cluster: radio properties and hard X-ray emission
The radio spectral index map of the Coma halo shows a progressive steepening
of the spectral index with increasing radius. Such a steepening cannot be
simply justified by models involving continuous injection of fresh particles in
the Coma halo or by models involving diffusion of fresh electrons from the
central regions. We propose a {\it two phase} model in which the relativistic
electrons injected in the Coma cluster by some processes (starbursts, AGNs,
shocks, turbulence) during a {\it first phase} in the past are systematically
reaccelerated during a {\it second phase} for a relatively long time ( 1
Gyr) up to the present time. We show that for reacceleration time scales of
Gyr this hypothesis can well account for the radio properties of
Coma C. For the same range of parameters which explain Coma C we have
calculated the expected fluxes from the inverse Compton scattering of the CMB
photons finding that the hard X-ray tail discovered by BeppoSAX may be
accounted for by the stronger reacceleration allowed by the model. The
possibility of extending the main model assumptions and findings to the case of
the other radio haloes is also discussed, the basic predictions being
consistent with the observations.Comment: 15 pages, 13 figures, accepted for publication in MNRA
Radio Halo and Relic Candidates from the NRAO VLA Sky Survey
We present the first results of the search of new halo and relic candidates
in the NRAO VLA Sky Survey. We have inspected a sample of 205 clusters from the
X-ray-brightest Abell-type clusters presented by Ebeling et al (1996), and
found 29 candidates. Out of them, 11 clusters are already known from the
literature to contain a diffuse cluster-wide source, while in 18 clusters this
is the first indication of the existence of this type of sources. We classify
these sources as halos or relics according to their location in the cluster
center or periphery, respectively. We find that the occurrence of cluster halos
and relics is higher in clusters with high X-ray luminosity and high
temperature. We also confirm the correlation between the absence of a cooling
flow and the presence of a radio halo at the cluster center.Comment: 25 Pages, 4 Figures included in the text, Figures 3a to 3g in
separated files. New Astronomy in pres
Financial Globalization and Exchange Rates
The founders of the Bretton Woods System sixty years ago were primarily concerned with orderly exchange rateadjustment in a world economy that was characterized by widespread restrictions on international capitalmobility. In contrast, the rapid pace of financial globalization during recent years poses new challenges for theinternational monetary system. In particular, large gross cross-holdings of foreign assets and liabilities meansthat the valuation channel of exchange rate adjustment has grown in importance, relative to the traditional tradebalance channel. Accordingly, this paper empirically explores some of the inter-connections between financialglobalization and exchange rate adjustment and discusses the policy implications.Financial integration, capital flows, external assets and liabilities
A double radio halo in the close pair of galaxy clusters Abell 399 and Abell 401
Radio halos are faint radio sources usually located at the center of merging
clusters of galaxies. These diffuse radio sources are rare, having so far been
found only in about 30 clusters of galaxies, suggesting that particular
conditions are needed to form and maintain them. It is interesting to
investigate the presence of radio halos in close pairs of interacting clusters
in order to possibly clarify their origin in relation to the evolutionary state
of the merger. In this work, we study the case of the close pair of galaxy
clusters A399 and A401. A401 is already known to contain a faint radio halo,
while a hint of diffuse emission in A399 has been suggested based on the NVSS.
To confirm this possibility, we analyzed deeper Very Large Array observations
at 1.4 GHz of this cluster. We find that the central region of A399 is
permeated by a diffuse low-surface brightness radio emission that we classify
as a radio halo with a linear size of about 570 kpc and a central brightness of
0.3 micro-Jy/arcsec^2. Indeed, given their comparatively small projected
distance of about 3 Mpc, the pair of galaxy clusters A401 and A399 can be
considered as the first example of double radio halo system. The discovery of
this double halo is extraordinary given the rarity of these radio sources in
general and given that current X-ray data seem to suggest that the two clusters
are still in a pre-merger state. Therefore, the origin of the double radio halo
is likely to be attributed to the individual merging histories of each cluster
separately, rather than to the result of a close encounter between the two
systems.Comment: 6 pages, 4 figures, accepted for publication in A&
Time Variable Faraday Rotation Measures of 3C-273 and 3C-279
Multifrequency polarimetry with the VLBA confirms the previously reported
time-varying Faraday rotation measure (RM) in the quasar 3C-279. Variability in
the RM and electric vector position angle (EVPA) of the jet component (C4) is
seen making it an unreliable absolute EVPA calibrator. 3C-273 is also shown to
vary its RM structure on 1.5 year time-scales. Variation in the RM properties
of quasars may result from a Faraday screen which changes on time-scales of a
few years, or from the motion of jet components which sample spatial variations
in the screen. A new component emerging from the core of 3C-279 appears to be
starting to sample such a spatial variation. Future monitoring of this
component and its RM properties is suggested as a diagnostic of the narrow line
region in 3C-279. We also present a new method of EVPA calibration using the
VLA Monitoring Program.Comment: Accepted to ApJ Letters. 12 pages, 5 figure
The giant radio halo in Abell 2163
New radio data is presented for the rich cluster Abell 2163. The cluster
radio emission is characterized by the presence of a radio halo, which is one
of the most powerful and extended halos known so far. In the NE peripheral
cluster region, we also detect diffuse elongated emission, which we classify as
a cluster relic. The cluster A2163 is very hot and luminous in X-ray. Its
central region is probably in a highly non relaxed state, suggesting that this
cluster is likely to be a recent merger. The existence of a radio halo in this
cluster confirms that halos are associated with hot massive clusters, and
confirms the connection between radio halos and cluster merger processes. The
comparison between the radio emission of the halo and the cluster X-ray
emission shows a close structural similarity. A power law correlation is found
between the radio and X-ray brightness, with index = 0.64. We also report the
upper limit to the hard X-ray emission, obtained from a BeppoSAX observation.
We discuss the implications of our results.Comment: 7 pages, 8 figures (5 in ps and 3 in gif), Accepted for publication
in Astron. Astrop
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