801 research outputs found
Chandra Observations of the Disruption of the Cool Core in Abell 133
We present the analysis of a Chandra observation of the galaxy cluster Abell
133, which has a cooling flow core, a central radio source, and a diffuse,
filamentary radio source which has been classified as a radio relic.
The X-ray image shows that the core has a complex structure. The most
prominent feature is a "tongue" of emission which extends from the central cD
galaxy to the northwest and partly overlaps the radio relic. One possibility is
that this tongue is produced by Kelvin-Helmholtz (KH) instabilities through the
interaction between the cold gas around the cD galaxy and hot intracluster
medium. We estimate the critical velocity and time scale for the KH instability
to be effective for the cold core around the cD galaxy. We find that the KH
instability can disrupt the cold core if the relative velocity is >~400 km
s^-1. We compare the results with those of clusters in which sharp, undisrupted
cold fronts have been observed; in these clusters, the low temperature gas in
their central regions has a more regular distribution. In contrast to Abell
133, these cluster cores have longer timescales for the disruption of the core
by the KH instability when they are normalized to the timescale of the cD
galaxy motion. Thus, the other cores are less vulnerable to KH instability.
Another possible origin of the tongue is that it is gas which has been uplifted
by a buoyant bubble of nonthermal plasma that we identify with the observed
radio relic. From the position of the bubble and the radio estimate of the age
of the relic source, we estimate avelocity of ~700 km s^-1 for the bubble. The
structure of the bubble and this velocity are consistent with numerical models
for such buoyant bubbles. (abridged)Comment: 38 pages, 15 figures, accepted for publication in Ap
Emergency Department Pain Management Following Implementation of a Geriatric Hip Fracture Program
Introduction: Over 300,000 patients in the United States sustain low-trauma fragility hip fractures annually. Multidisciplinary geriatric fracture programs (GFP) including early, multimodal pain management reduce morbidity and mortality. Our overall goal was to determine the effects of a GFP on the emergency department (ED) pain management of geriatric fragility hip fractures. Methods: We performed a retrospective study including patients age ≥65 years with fragility hip fractures two years before and two years after the implementation of the GFP. Outcomes were time to (any) first analgesic, use of acetaminophen and fascia iliaca compartment block (FICB) in the ED, and amount of opioid medication administered in the first 24 hours. We used permutation tests to evaluate differences in ED pain management following GFP implementation. Results: We studied 131 patients in the pre-GFP period and 177 patients in the post-GFP period. In the post-GFP period, more patients received FICB (6% vs. 60%; difference 54%, 95% confidence interval [CI] 45–63%; p<0.001) and acetaminophen (10% vs. 51%; difference 41%, 95% CI 32–51%; p<0.001) in the ED. Patients in the post-GFP period also had a shorter time to first analgesic (103 vs. 93 minutes; p=0.04) and received fewer morphine equivalents in the first 24 hours (15mg vs. 10mg, p<0.001) than patients in the pre-GFP period. Conclusion: Implementation of a GFP was associated with improved ED pain management for geriatric patients with fragility hip fractures. Future studies should evaluate the effects of these changes in pain management on longer-term outcomes
The VLA Low-frequency Sky Survey
The Very Large Array (VLA) Low-frequency Sky Survey (VLSS) has imaged 95% of
the 3*pi sr of sky north of declination = -30 degrees at a frequency of 74 MHz
(4 meter wavelength). The resolution is 80" (FWHM) throughout, and the typical
RMS noise level is ~0.1 Jy/beam. The typical point-source detection limit is
0.7 Jy/beam and so far nearly 70,000 sources have been catalogued. This survey
used the 74 MHz system added to the VLA in 1998. It required new imaging
algorithms to remove the large ionospheric distortions at this very low
frequency throughout the entire ~11.9 degree field of view. This paper
describes the observation and data reduction methods used for the VLSS and
presents the survey images and source catalog. All of the calibrated images and
the source catalog are available online (http://lwa.nrl.navy.mil/VLSS) for use
by the astronomical community.Comment: 53 pages, including 3 tables and 15 figures. Has been accepted for
publication in the Astronomical Journa
X-ray and Radio Interactions in the Cores of Cooling Flow Clusters
We present high resolution ROSAT x-ray and radio observations of three
cooling flow clusters containing steep spectrum radio sources at their cores.
All three systems exhibit strong signs of interaction between the radio plasma
and the hot intracluster medium. Two clusters, A133 and A2626, show enhanced
x-ray emission spatially coincident with the radio source whereas the third
cluster, A2052, exhibits a large region of x-ray excess surrounding much of the
radio source. Using 3-D numerical simulations, we show that a perturbed jet
propagating through a cooling flow atmosphere can give rise to amorphous radio
morphologies, particularly in the case where the jet was ``turned off'' and
allowed to age passively. In addition, the simulated x-ray surface brightness
produced both excesses and deficits as seen observationally.Comment: 25 pages, 10 figures, accepted for publication in A
Four Extreme Relic Radio Sources in Clusters of Galaxies
(Abridged) We describe the results of the highest-resolution radio
observations yet made of four relic radio sources in the Abell clusters A13,
A85, A133 and A4038. Our VLA images at 1.4 GHz with 4" resolution show a
remarkable variety of fine structure in the form of spectacular arcs, wisps,
plumes and loops. Their integrated radio flux densities fall very rapidly with
frequency, with power-law slopes between 2.1 and 4.4 near 1.4 GHz The relics
possess linear polarization levels ranging between 2.3 % (A133) and 35 % (A85);
the higher polarization fractions imply a highly ordered magnetic field in the
fine structure and low differential Faraday rotation in the intervening cluster
gas. The optical identification of host galaxies remains problematic. In A85,
A133 and A4038 the travel times for the brightest cluster galaxies are
significantly longer than the modeled ages of the relics and nearby bright
ellipticals provide a better match. Excess X-ray emission in the 0.5 keV-to-2
keV band was found near the relics in A85 and A133. The surface brightness was
too high to be attributed to the inverse-Compton mechanism alone. We found
excellent fits to the broad-band radio spectra using the anisotropic (KGKP)
model of spectral ageing, and we have extended the model to include diffusion
of particles between regions of different field strength (the Murgia-JP, or
MJP, model). The steep radio spectra imply ages for the relics of ~ 10^8 yr, at
the start of which period their radio luminosities would have been ~ 10^25 W/Hz
at 1.4 GHz.Comment: 43 pages, 13 figures, AJ, Sep 2001 (accepted
Discovery of Radio Outbursts in the Active Nucleus of M81
The low-luminosity active galactic nucleus of M81 has been monitored at
centimeter wavelengths since early 1993 as a by-product of radio programs to
study the radio emission from Supernova 1993J. The extensive data sets reveal
that the nucleus experienced several radio outbursts during the monitoring
period. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of
1993 September and lasted for approximately three months; at longer
wavelengths, the maximum flux density decreases, and the onset of the burst is
delayed. These characteristics qualitatively resemble the standard model for
adiabatically expanding radio sources, although certain discrepancies between
the observations and the theoretical predictions suggest that the model is too
simplistic. In addition to the large-amplitude, prolonged variations, we also
detected milder changes in the flux density at 3.6 cm and possibly at 6 cm on
short (less than 1 day) timescales. We discuss a possible association between
the radio activity and an optical flare observed during the period that the
nucleus was monitored at radio wavelengths.Comment: To appear in The Astronomical Journal. Latex, 18 pages including
embedded figures and table
A Chandra Observation of Abell 13: Investigating the Origin of the Radio Relic
We present results from the Chandra X-ray observation of Abell 13, a galaxy
cluster that contains an unusual noncentral radio source, also known as a radio
relic. This is the first pointed X-ray observation of Abell 13, providing a
more sensitive study of the properties of the X-ray gas. The X-ray emission
from Abell 13 is extended to the northwest of the X-ray peak and shows
substructure indicative of a recent merger event. The cluster X-ray emission is
centered on the bright galaxy H of Slee et al. 2001. We find no evidence for a
cooling flow in the cluster. A knot of excess X-ray emission is coincident with
the other bright elliptical galaxy F. This knot of emission has properties
similar to the enhanced emission associated with the large galaxies in the Coma
cluster.
With these Chandra data we are able to compare the properties of the hot
X-ray gas with those of the radio relic from VLA data, to study the interaction
of the X-ray gas with the radio emitting electrons. Our results suggest that
the radio relic is associated with cooler gas in the cluster. We suggest two
explanations for the coincidence of the cooler gas and radio source. First, the
gas may have been uplifted by the radio relic from the cluster core.
Alternatively, the relic and cool gas may have been displaced from the central
galaxy during the cluster merger event.Comment: 11 pages, 9 figures, Accepted for Publication in the Astrophysical
Journal, higher-resolution figures can be found at
http://www.astro.virginia.edu/~amj3r/Abell13
The 74MHz System on the Very Large Array
The Naval Research Laboratory and the National Radio Astronomy Observatory
completed implementation of a low frequency capability on the VLA at 73.8 MHz
in 1998. This frequency band offers unprecedented sensitivity (~25 mJy/beam)
and resolution (~25 arcsec) for low-frequency observations. We review the
hardware, the calibration and imaging strategies, comparing them to those at
higher frequencies, including aspects of interference excision and wide-field
imaging. Ionospheric phase fluctuations pose the major difficulty in
calibrating the array. Over restricted fields of view or at times of extremely
quiescent ionospheric ``weather'', an angle-invariant calibration strategy can
be used. In this approach a single phase correction is devised for each
antenna, typically via self-calibration. Over larger fields of view or at times
of more normal ionospheric ``weather'' when the ionospheric isoplanatic patch
size is smaller than the field of view, we adopt a field-based strategy in
which the phase correction depends upon location within the field of view. This
second calibration strategy was implemented by modeling the ionosphere above
the array using Zernike polynomials. Images of 3C sources of moderate strength
are provided as examples of routine, angle-invariant calibration and imaging.
Flux density measurements indicate that the 74 MHz flux scale at the VLA is
stable to a few percent, and tied to the Baars et al. value of Cygnus A at the
5 percent level. We also present an example of a wide-field image, devoid of
bright objects and containing hundreds of weaker sources, constructed from the
field-based calibration. We close with a summary of lessons the 74 MHz system
offers as a model for new and developing low-frequency telescopes. (Abridged)Comment: 73 pages, 46 jpeg figures, to appear in ApJ
XMM-Newton Observations of A133: A Weak Shock Passing through the Cool Core
We use XMM-Newton observations of the cluster of galaxies A133 to study the
X-ray spectrum of the intracluster medium (ICM). We find a cold front to the
southeast of the cluster core. From the pressure profile near the cold front,
we derive an upper limit to the velocity of the core relative to the rest of
the cluster of <230 km s^-1. Our previous Chandra image of A133 showed a
complex, bird-like morphology in the cluster core. Based on the XMM-Newton
spectra and hardness ratio maps, we argue that the wings of this structure are
a weak shock front. The shock was probably formed outside the core of the
cluster, and may be heating the cluster core. Our Chandra image also showed a
``tongue'' of relatively cool gas extending from the center of the cD to the
center of the radio relic. The XMM-Newton results are consistent with the idea
that the tongue is the gas which has been uplifted by a buoyant radio bubble
including the radio relic to the northwest of the core. Alternatively, the
tongue might result from a cluster merger. The small velocity of the core
suggests that the bubble including the relic has moved by buoyancy, rather than
by motions of the core or the ICM. We do not find clear evidence for nonthermal
X-ray emission from the radio relic. Based on the upper limit on the inverse
Compton emission, we derive a lower limit on the magnetic field in the relic of
B>~1.5\mu G.Comment: 29 pages, ApJ in pres
A Systematic Study of Radio-Induced X-ray Cavities in Clusters, Groups, and Galaxies
We present an analysis of sixteen galaxy clusters, one group and one galaxy
drawn from the Chandra X-ray Observatory's data archive. These systems possess
prominent X-ray surface brightness depressions associated with cavities or
bubbles that were created by interactions between powerful radio sources and
the surrounding hot gas. The minimum energy associated with the cavities ranges
between pV~10^55 erg in galaxies, groups, and poor clusters to pV~10^60 erg in
rich clusters. We evaluate the hypothesis that cooling in the hot gas can be
quenched by energy injected into the surrounding gas by the rising bubbles.
Nearly half of the systems in this study may have instantaneous mechanical
luminosities large enough to balance cooling, at least for a short period of
time, if the cavities are filled with a relativistic gas. We find a trend or
upper envelope in the distribution of central X-ray luminosity versus
instantaneous mechanical luminosity with the sense that the most powerful
cavities are found in the most X-ray luminous systems. Such a trend would be
expected if many of these systems produce bubbles at a rate that scales in
proportion to the cooling rate of the surrounding gas. Finally, we use the
X-ray cavities to measure the mechanical power of radio sources over six
decades of radio luminosity, independently of the radio properties themselves.
We find that the ratio of the instantaneous mechanical (kinetic) luminosity to
the 1.4 GHz synchrotron luminosity ranges from a few to roughly a thousand.
This wide range implies that the 1.4 GHz synchrotron luminosity is an
unreliable gauge of the mechanical power of radio sources.Comment: 10 pages, 3 figures, accepted by ApJ. Correction made to Centaurus
radio flux that affects one table and two figures. Small changes to the text
required by this correction and to reflect the published versio
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