344 research outputs found
Occurrence of radio minihalos in a mass-limited sample of galaxy clusters
We investigate the occurrence of radio minihalos --- diffuse radio sources of
unknown origin observed in the cores of some galaxy clusters --- in a
statistical sample of 58 clusters drawn from the Planck Sunyaev-Zel'dovich
cluster catalog using a mass cut (). We
supplement our statistical sample with a similarly-sized non-statistical sample
mostly consisting of clusters in the ACCEPT X-ray catalog with suitable X-ray
and radio data, which includes lower-mass clusters. Where necessary (for 9
clusters), we reanalyzed the Very Large Array archival radio data to determine
if a mihinalo is present. Our total sample includes all 28 currently known and
recently discovered radio minihalos, including 6 candidates. We classify
clusters as cool-core or non-cool core according to the value of the specific
entropy floor in the cluster center, rederived or newly derived from the
Chandra X-ray density and temperature profiles where necessary (for 27
clusters). Contrary to the common wisdom that minihalos are rare, we find that
almost all cool cores - at least 12 out of 15 (80%) - in our complete sample of
massive clusters exhibit minihalos. The supplementary sample shows that the
occurrence of minihalos may be lower in lower-mass cool-core clusters. No
minihalos are found in non-cool-cores or "warm cores". These findings will help
test theories of the origin of minihalos and provide information on the
physical processes and energetics of the cluster cores.Comment: 34 pages, accepted for publication in ApJ. Added a section
"Definition of a minihalo" and an appendix "Radio size and average surface
brigthtness of minihalos and halos
HESS J1943+213: a non-classical high-frequency-peaked BL Lac object
HESS J1943+213 is an unidentified TeV source that is likely a
high-frequency-peaked BL Lac (HBL) object but also compatible with a pulsar
wind nebula (PWN) nature. Each of these enormously different astronomical
interpretations is supported by some of the observed unusual characteristics.
In order to finally classify and understand this object we took a three-pronged
approach, through time-domain, high angular resolution, and multi-frequency
radio studies. First, our deep time-domain observations with the Arecibo
telescope failed to uncover the putative pulsar powering the proposed PWN. We
conclude with ~70% certainty that HESS J1943+213 does not host a pulsar.
Second, long-baseline interferometry of the source with e-MERLIN at 1.5- and 5-
GHz, shows only a core, a point source at ~ 1 - 100 milli-arcsecond resolution.
Its 2013 flux density is about one-third lower than detected in 2011
observations with similar resolution. This radio variability of the core
strengthens the HBL object hypothesis. More evidence against the PWN scenario
comes, third, from the radio spectrum we compiled. The extended structure
follows a power-law behavior with spectral index alpha = -0.54 +- 0.04 while
the core component is flat spectrum (alpha = -0.03 +- 0.03). In contrast, the
radio synchrotron emission of PWNe predicts a single power-law distribution.
Overall we rule out the PWN hypothesis and conclude the source is a BL Lac
object. The consistently high fraction (70%) of the flux density from the
extended structure then leads us to conclude that HESS J1943+213 must be a
non-classical HBL object.Comment: 8 pages, 4 figures, ApJ submitte
The Curious Case of Abell 2256
Abell 2256 is a rich, nearby (z=0.0594) galaxy cluster that has significant
evidence of merger activity. We present new radio and X-ray observations of
this system. The low-frequency radio images trace the diffuse synchrotron
emission of the Mpc-scale radio halo and relics as well as a number of recently
discovered, more compact, steep spectrum sources. The spectral index across the
relics steepens from the north-west toward the south-east. Analysis of the
spectral index gradients between low and and high-frequencies shows spectral
differences away from the north-west relic edge such that the low-frequency
index is significantly flatter than the high frequency spectral index near the
cluster core. This trend would be consistent with an outgoing merger shock as
the origin of the relic emission. New X-ray data from XMM-Newton reveal
interesting structures in the intracluster medium pressure, entropy and
temperature maps. The pressure maps show an overall low pressure core
co-incident with the radio halo emission, while the temperature maps reveal
multiple regions of cool emission within the central regions of Abell 2256. The
two cold fronts in Abell 2256 both appear to have motion in similar directions.Comment: 4 pages, 3 figures, proceedings of contributed talk at "Non-Thermal
Phenomena in Colliding Galaxy Clusters", held in Nice, 15-18 November 2010.
To be published in Mem. S.A.I
New Detections of Radio Minihalos in Cool Cores of Galaxy Clusters
Cool cores of some galaxy clusters exhibit faint radio minihalos. Their origin is unclear, and their study has been limited by their small number. We undertook a systematic search for minihalos in a large sample of X-ray luminous clusters with high-quality radio data. In this article, we report four new minihalos (A 478, ZwCl 3146,RXJ 1532.9+3021, and A 2204) and five candidates found in the reanalyzed archival Very Large Array observations.The radio luminosities of our minihalos and candidates are in the range of 102325 W Hz1 at 1.4 GHz, which is consistent with these types of radio sources. Their sizes (40160 kpc in radius) are somewhat smaller than those of previously known minihalos. We combine our new detections with previously known minihalos, obtaining a total sample of 21 objects, and briefly compare the cluster radio properties to the average X-ray temperature and the total masses estimated from Planck.We find that nearly all clusters hosting minihalos are hot and massive. Beyond that, there is no clear correlation between the minihalo radio power and cluster temperature or mass (in contrast with the giant radio halos found in cluster mergers, whose radio luminosity correlates with the cluster mass). Chandra X-ray images indicate gas sloshing in the cool cores of most of our clusters, with minihalos contained within the sloshing regions in many of them. This supports the hypothesis that radio-emitting electrons are reaccelerated by sloshing. Advection of relativistic electrons by the sloshing gas may also play a role in the formation of the less extended minihalos
The FRII Broad Line Seyfert 1 Galaxy: PKSJ 1037-2705
In this article, we demonstrate that PKSJ 1037-2705 has a weak accretion flow
luminosity, well below the Seyfert1/QSO dividing line, weak broad emission
lines (BELs) and moderately powerful FRII extended radio emission. It is one of
the few documented examples of a broad-line object in which the time averaged
jet kinetic luminosity, , is larger than the total thermal luminosity
(IR to X-ray) of the accretion flow, . The blazar nucleus dominates
the optical and near ultraviolet emission and is a strong source of hard
X-rays. The strong blazar emission indicates that the relativistic radio jet is
presently active. The implication is that even weakly accreting AGN can create
powerful jets. Kinetically dominated () broad-line objects
provide important constraints on the relationship between the accretion flow
and the jet production mechanism.Comment: To appear in ApJ November 1, 2008, v687n1 issu
High Spatial Resolution X-Ray Spectroscopy of the IC443 Pulsar Wind Nebula and Environs
Deep Chandra ACIS observations of the region around the putative pulsar, CXOU
J061705.3+222127, in the supernova remnant IC443 reveal an
~5-radius ring-like structure surrounding the pulsar and a
jet-like feature oriented roughly north-south across the ring and through the
pulsar's location at 06175.200
+222127.52 (J2000.0 coordinates). The
observations further confirm that (1) the spectrum and flux of the central
object are consistent with a rotation-powered pulsar, (2) the non-thermal
spectrum and morphology of the surrounding nebula are consistent with a pulsar
wind and, (3) the spectrum at greater distances is consistent with thermal
emission from the supernova remnant. The cometary shape of the nebula,
suggesting motion towards the southwest, appears to be subsonic: There is no
evidence either spectrally or morphologically for a bow shock or contact
discontinuity; the nearly circular ring is not distorted by motion through the
ambient medium; and the shape near the apex of the nebula is narrow. Comparing
this observation with previous observations of the same target, we set a 99%
confidence upper limit to the proper motion of CXOU J061705.3+222127 to be less
than 44 mas/yr (310 km/s for a distance of 1.5 kpc), with the best-fit (but not
statistically significant) projected direction toward the west.Comment: Accepted for publication in the Astrophysical Journa
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