320 research outputs found
Discovery of diffuse radio emission at the center of the most X-ray-luminous cluster RX J1347.5-1145
We report on new VLA radio observations of the distant cluster RX
J1347.5-1145, which is the most luminous in X-rays. We aim at investigating the
possible presence of diffuse and extended radio emission in this very peculiar
system which shows both a massive cooling flow and merging signatures. New low
resolution (~18 arcsec) VLA radio observations of this cluster are combined
with higher resolution (~2 arcsec) data available in the VLA archive. We
discover the presence of a diffuse and extended (~500 kpc) radio source
centered on the cluster, unrelated to the radio emission of the central AGN.
The properties of the radio source, in particular a) its occurrence at the
center of a massive cooling flow cluster, b) its total size comparable to that
of the cooling region, c) its agreement with the observational trend between
radio luminosity and cooling flow power, indicate that RX J1347.5-1145 hosts a
radio mini-halo. We suggest that the radio emission of this mini-halo, which is
the most distant object of its class discovered up to now, is due to electron
re-acceleration triggered by the central cooling flow. However, we also note
that the morphology of the diffuse radio emission shows an elongation
coincident with the position of a hot subclump detected in X-rays, thus
suggesting that additional energy for the electron re-acceleration might be
provided by the submerger event.Comment: 5 pages, 6 figures, accepted for publication in A&A Letter
The complex galaxy cluster Abell 514: New results obtained with the XMM - Newton satellite
We study the X-ray morphology and dynamics of the galaxy cluster Abell 514.
Also, the relation between the X-ray properties and Faraday Rotation measures
of this cluster are investigated in order to study the connection of magnetic
fields and the intra-cluster medium. We use two combined XMM - Newton pointings
that are split into three distinct observations. The data allow us to evaluate
the overall cluster properties like temperature and metallicity with high
accuracy. Additionally, a temperature map and the metallicity distribution are
computed, which are used to study the dynamical state of the cluster in detail.
Abell 514 represents an interesting merger cluster with many substructures
visible in the X-ray image and in the temperature and abundance distributions.
The new XMM - Newton data of Abell 514 confirm the relation between the X-ray
brightness and the sigma of the Rotation Measure (S_X - sigma_RM relation)
proposed by Dolag et al. (2001).Comment: 9 pages, 13 figures, accepted for publication in A&
A candidate supermassive binary black hole system in the brightest cluster galaxy of RBS 797
The radio source at the center of the cool core galaxy cluster RBS 797
(z=0.35) is known to exhibit a misalignment of its radio jets and lobes
observed at different VLA-scale, with the innermost kpc-scale jets being almost
orthogonal to the radio emission which extends for tens of kpc filling the
X-ray cavities. Gitti et al. suggested that this peculiar radio morphology may
indicate a recurrent activity of the central radio source, where the jet
orientation is changing between the different outbursts due to the effects of
supermassive binary black holes (SMBBHs). We aim at unveiling the nuclear radio
properties of the brightest cluster galaxy (BCG) in RBS 797 and at
investigating the presence of a SMBBH system in its center. We have performed
new high-resolution observations at 5 GHz with the European VLBI Network (EVN),
reaching an angular resolution of 9x5 mas^2 and a sensitivity of 36
microJy/beam. We report the EVN detection of two compact components in the BCG
of RBS 797, with a projected separation of ~77 pc. We can envisage two possible
scenarios: the two components are two different nuclei in a close binary
system, or they are the core and a knot of its jet. Both interpretations are
consistent with the presence of SMBBHs. Our re-analysis of VLA archival data
seems to favor the first scenario, as we detect two pairs of radio jets
misaligned by ~90 degrees on the same kpc scale emanating from the central
radio core. If the two outbursts are almost contemporaneous, this is clear
evidence of the presence of two active SMBHs, whose radio nuclei are unresolved
at VLA resolution. The nature of the double source detected by our EVN
observations in the BCG of RBS 797 can be established only by future sensitive,
multi-frequency VLBI observations. If confirmed, RBS 797 would be the first
SMBBH system observed at medium-high redshift at VLBI resolution. (abridged)Comment: 4 pages, 2 figures, A&A Letter in pres
The mystery of the 'Kite' radio source in Abell 2626: insights from new Chandra observations
We present the results of a new Chandra study of the galaxy cluster A2626.
The radio emission of the cluster shows a complex system of four symmetric arcs
without known correlations with the X-ray emission. The mirror symmetry of the
radio arcs toward the center and the presence of two optical cores in the
central galaxy suggested that they may be created by pairs of precessing radio
jets powered by dual AGNs inside the cD galaxy. However, previous observations
failed to observe the second jetted AGN and the spectral trend due to radiative
age along the radio arcs, thus challenging this interpretation. The new Chandra
observation had several scientific objectives, including the search for the
second AGN that would support the jet precession model. We focus here on the
detailed study of the local properties of the thermal and non-thermal emission
in the proximity of the radio arcs, in order to get more insights into their
origin. We performed a standard data reduction of the Chandra dataset deriving
the radial profiles of temperature, density, pressure and cooling time of the
intra-cluster medium. We further analyzed the 2D distribution of the gas
temperature, discovering that the south-western junction of the radio arcs
surrounds the cool core of the cluster. We studied the X-ray SB and spectral
profiles across the junction, finding a cold front spatially coincident with
the radio arcs. This may suggest a connection between the sloshing of the
thermal gas and the nature of the radio filaments, raising new scenarios for
their origin. A possibility is that the radio arcs trace the projection of a
complex surface connecting the sites where electrons are most efficiently
reaccelerated by the turbulence that is generated by the gas sloshing. In this
case, diffuse emission embedded by the arcs and with extremely steep spectrum
should be most visible at very low radio frequencies.Comment: 7 pages, 7 figures. Accepted for publication on A&
A morphological comparison between giant radio halos and radio mini--halos in galaxy clusters
In this letter we present a morphological comparison between giant radio
halos and radio mini-halos in galaxy clusters based on radio--X-ray luminosity,
P_{1.4}-L_X, and radio luminosity-size, P_{1.4}-R_H, correlations. We report
evidence that P_{1.4}-L_X and P_{1.4}-R_H trends may also exist for
mini--halos: mini--halo clusters share the same region of giant halo clusters
in the (P_{1.4},L_X) plane, whereas they are clearly separated in the
(P_{1.4},R_H) plane. The synchrotron emissivity of mini-halos is found to be
more than 50 times larger than that of giant halos, implying a very efficient
process for their origins. By assuming a scenario of sporadical turbulent
particle re-acceleration for both giant and mini halos, we discuss basic
physical differences between these sources. Regardless of the origin of the
turbulence, a more efficient source of injection of particles, which eventually
takes part in the re-acceleration process, is required in mini-halos, and this
may result from the central radio galaxy or from proton-proton collisions in
the dense cool core regions.Comment: 4 pages, 3 figures, A&A Letter in pres
Heating the hot atmospheres of galaxy groups and clusters with cavities: the relationship between jet power and low-frequency radio emission
We present scaling relations between jet power and radio power measured using
the Giant Metrewave Radio Telescope (GMRT), Chandra and XMM-Newton, for a
sample of 9 galaxy groups combined with the Birzan et al. sample of clusters.
Cavity power is used as a proxy for mechanical jet power. Radio power is
measured at 235 MHz and 1.4 GHz, and the integrated 10 MHz-10 GHz radio
luminosity is estimated from the GMRT 610-235 MHz spectral index. The use of
consistently analysed, high resolution low-frequency radio data from a single
observatory makes the radio powers for the groups more reliable than those used
by previous studies, and the combined sample covers 6-7 decades in radio power
and 5 decades in cavity power. We find a relation of the form Pjet proportional
to Lradio^~0.7 for integrated radio luminosity, with a total scatter of
sigma_Lrad=0.63 and an intrinsic scatter of sigma_i,Lrad=0.59. A similar
relation is found for 235 MHz power, but a slightly flatter relation with
greater scatter is found for 1.4 GHz power, suggesting that low-frequency or
broad band radio measurements are superior jet power indicators. We find our
low-frequency relations to be in good agreement with previous observational
results. Comparison with jet models shows reasonable agreement, which may be
improved if radio sources have a significant low-energy electron population. We
consider possible factors which could bias our results or render them more
uncertain, and find that correcting for such factors in those groups we are
able to study in detail leads to a flattening of the Pjet:Lradio relation.Comment: Accepted for publication in ApJ, 7 pages, 3 figure
A radio minihalo in the extreme cool-core galaxy cluster RXCJ1504.1-0248
Aims. We report the discovery of a radio minihalo in RXCJ1504.1-0248, a
massive galaxy cluster that has an extremely luminous cool core. To date, only
9 radio minihalos are known, thus the discovery of a new one, in one of the
most luminous cool-core clusters, provides important information on this
peculiar class of sources and sheds light on their origin. Methods. The diffuse
radio source is detected using GMRT at 327 MHz and confirmed by pointed VLA
data at 1.46 GHz. The minihalo has a radius of 140 kpc. A Chandra gas
temperature map shows that the minihalo emission fills the cluster cool core
and has some morphological similarities to it, as has been previously observed
for other minihalos. Results. The Chandra data reveal two subtle cold fronts in
the cool core, likely created by sloshing of the core gas, as observed in most
cool-core clusters. Following previous work, we speculate that the origin of
the minihalo is related to sloshing. Sloshing may result in particle
acceleration by generating turbulence and/or amplifying the magnetic field in
the cool core, leading to the formation of a minihalo.Comment: 4 pages, 1 table, 3 color figures. Accepted for publication in A&A
Letter
Apparent high metallicity in 3-4 keV galaxy clusters: the inverse iron-bias in action in the case of the merging cluster Abell 2028
Recent work based on a global measurement of the ICM properties find evidence
for an increase of the iron abundance in galaxy clusters with temperature
around 2-4 keV up to a value about 3 times larger than that typical of very hot
clusters. We have started a study of the metal distribution in these objects
from the sample of Baumgartner et al. (2005), aiming at resolving spatially the
metal content of the ICM. We report here on a 42ks XMM observation of the first
object of the sample, the cluster Abell 2028. The XMM observation reveals a
complex structure of the cluster over scale of 300 kpc, showing an interaction
between two sub-clusters in cometary-like configurations. At the leading edges
of the two substructures cold fronts have been detected. The core of the main
subcluster is likely hosting a cool corona. We show that a one-component fit
for this region returns a biased high metallicity. This inverse iron bias is
due to the behavior of the fitting code in shaping the Fe-L complex. In
presence of a multi-temperature structure of the ICM, the best-fit metallicity
is artificially higher when the projected spectrum is modeled with a single
temperature component and it is not related to the presence of both Fe-L and
Fe-K emission lines in the spectrum. After accounting for the bias, the overall
abundance of the cluster is consistent with the one typical of hotter, more
massive clusters. We caution the interpretation of high abundances inferred
when fitting a single thermal component to spectra derived from relatively
large apertures in 3-4 keV clusters, because the inverse iron bias can be
present. Most of the inferences trying to relate high abundances in 3-4 keV
clusters to fundamental physical processes will likely have to be revised.Comment: 13 pages, 8 figures.Accepted for publication in Astronomy and
Astrophysycs. Minor changes to match published versio
The central FR0 in the sloshing cluster Abell 795: Indications of mechanical feedback from Chandra data
We present a detailed study of the galaxy cluster Abell 795 and of its central Fanaroff-Riley Type 0 (FR0) radio galaxy. From an archival Chandra observation, we found a dynamically disturbed environment with evidences for sloshing of the intracluster medium. We argue that the environment alone cannot explain the compactness of the radio galaxy, as similar conditions are also found around extended sources. We identified a pair of putative X-ray cavities in the proximity of the center: These could have been created in a past outburst of the FR0, and dragged away by the large-scale gas movement. The presence of X-ray cavities associated with a FR0 could open a new window on the study of jet power and feedback properties of this recently discovered class of compact radio galaxies
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