1,136 research outputs found
Evidence for Proportionate Partition Between the Magnetic Field and Hot Gas in Turbulent Cassiopeia A
We present a deep X-ray observation of the young Galactic supernova remnant
Cas A, acquired with the ROSAT High Resolution Imager. This high dynamic range
(232 ks) image reveals low-surface-brightness X-ray structure, which appears
qualitatively similar to corresponding radio features. We consider the
correlation between the X-ray and radio morphologies and its physical
implications. After correcting for the inhomogeneous absorption across the
remnant, we performed a point by point (4" resolution) surface brightness
comparison between the X-ray and radio images. We find a strong (r = 0.75)
log-log correlation, implying an overall relationship of . This is
consistent with proportionate partition (and possibly equipartition) between
the local magnetic field and the hot gas --- implying that Cas A's plasma is
fully turbulent and continuously amplifying the magnetic field.Comment: 8 pages with embedded bitmapped figures, Accepted by ApJ Letters
5/1/9
An X-ray upper limit on the presence of a Neutron Star for the Small Magellanic Cloud and Supernova Remnant 1E0102.2-7219
We present Chandra X-ray Observatory archival observations of the supernova
remnant 1E0102.2-7219, a young Oxygen-rich remnant in the Small Magellanic
Cloud. Combining 28 ObsIDs for 324 ks of total exposure time, we present an
ACIS image with an unprecedented signal-to-noise ratio (mean S/N ~ sqrt(S) ~6;
maximum S/N > 35) . We search within the remnant, using the source detection
software {\sc wavdetect}, for point sources which may indicate a compact
object. Despite finding numerous detections of high significance in both broad
and narrow band images of the remnant, we are unable to satisfactorily
distinguish whether these detections correspond to emission from a compact
object. We also present upper limits to the luminosity of an obscured compact
stellar object which were derived from an analysis of spectra extracted from
the high signal-to-noise image. We are able to further constrain the
characteristics of a potential neutron star for this remnant with the results
of the analysis presented here, though we cannot confirm the existence of such
an object for this remnant.Comment: Accepted to the Astrophysical Journa
A Broadband X-Ray Study of the Supernova Remnant 3C 397
We present an X-ray study of the radio bright supernova remnant (SNR) 3C 397
with ROSAT, ASCA, and RXTE. A central X-ray spot seen with the ROSAT
High-Resolution Imager hints at the presence of a pulsar-powered component, and
gives this SNR a composite X-ray morphology. Combined ROSAT and ASCA imaging
show that the remnant is highly asymmetric, with its hard X-ray emission
peaking at the western lobe. The spectrum of 3C 397 is heavily absorbed, and
dominated by thermal emission with emission lines evident from Mg, Si, S, Ar
and Fe. Single-component models fail to describe the spectrum, and at least two
components are required. We use a set of non-equilibrium ionization (NEI)
models (Borkowski et al. in preparation). The temperatures from the soft and
hard components are 0.2 keV and 1.6 keV respectively. The corresponding
ionization time-scales ( being the pre-shock hydrogen density) are
6 cm s and 6 10 cm s,
respectively. The spectrum obtained with the Proportional Counter Array (PCA)
of RXTE is contaminated by emission from the Galactic ridge, with only
15% of the count rate originating from 3C 397 in the 5-15 keV range. The PCA
spectrum allowed us to confirm the thermal nature of the hard X-ray emission. A
third component originating from a pulsar-driven component is possible, but the
contamination of the source signal by the Galactic ridge did not allow us to
find pulsations from any hidden pulsar. We discuss the X-ray spectrum in the
light of two scenarios: a young ejecta-dominated remnant of a core-collapse SN,
and a middle-aged SNR expanding in a dense ISM. Spatially resolved spectroscopy
(with CHANDRA and XMM) is needed to differentiate between the two scenarios,
and address the nature of the mysterious radio-quiet X-ray hot spot.Comment: 21 pages including 8 figures and 5 tables. Accepted for publication
in the Astrophysical journa
A Bow Shock Nebula Around a Compact X-Ray Source in the Supernova Remnant IC443
We present spectra and high resolution images of the hard X-ray feature along
the southern edge of the supernova remnant IC443. Data from the Chandra X-ray
Observatory reveal a comet-shaped nebula of hard emission, which contains a
softer point source at its apex. We also present 20cm, 6cm, and 3.5cm images
from the Very Large Array that clearly show the cometary nebula. Based on the
radio and X-ray morphology and spectrum, and the radio polarization properties,
we argue that this object is a synchrotron nebula powered by the compact source
that is physically associated with IC443. The spectrum of the soft point source
is adequately but not uniquely fit by a black body model (kT=0.71 +/- 0.08 keV,
L=(6.5 +/- 0.9) * 10^31 erg/s). The cometary morphology of the nebula is the
result of the supersonic motion of the neutron star (V_NS=250 +/- 50 km/s),
which causes the relativistic wind of the pulsar to terminate in a bow shock
and trail behind as a synchrotron tail. This velocity is consistent with an age
of 30,000 years for the SNR and its associated neutron star.Comment: 9 pages, 5 figures, accepted for publication in the ApJ Letter
A Near-Infrared and X-ray Study of W49B: A Wind Cavity Explosion
We present near-infrared narrow-band images of the supernova remnant W49B,
taken with the WIRC instrument on the Hale 200 inch telescope on Mt. Palomar.
The 1.64 micron [Fe II] image reveals a barrel-shaped structure with coaxial
rings, which is suggestive of bipolar wind structures surrounding massive
stars. The 2.12 micron shocked molecular hydrogen image extends 1.9 pc outside
of the [Fe II] emission to the southeast. We also present archival Chandra
data, which show an X-ray jet-like structure along the axis of the [Fe II]
barrel, and flaring at each end. Fitting single temperature X-ray emission
models reveals: an enhancement of heavy elements, with particularly high
abundances of hot Fe and Ni, and relatively metal-rich core and jet regions. We
interpret these findings as evidence that W49B originated inside a wind-blown
bubble (R ~ 5 pc) interior to a dense molecular cloud. This suggests that
W49B's progenitor was a supermassive star, that could significantly shape its
surrounding environment. We also suggest two interpretations for the jet
morphology, abundance variations and molecular hydrogen emission: (1) the
explosion may have been jet-driven and interacting with the molecular cavity
(i.e. a Gamma-ray burst); or (2) the explosion could have been a traditional
supernova, with the jet structure being the result of interactions between the
shock and an enriched interstellar cloud.Comment: 9 pages with embedded figures Accepted by the Astrophysical Journa
Recycling bins, garbage cans or think tanks? Three myths regarding policy analysis institutes
The phrase 'think tank' has become ubiquitous â overworked and underspecified â in the political lexicon. It is entrenched in scholarly discussions of public policy as well as in the 'policy wonk' of journalists, lobbyists and spin-doctors. This does not mean that there is an agreed definition of think tank or consensual understanding of their roles and functions. Nevertheless, the majority of organizations with this label undertake policy research of some kind. The idea of think tanks as a research communication 'bridge' presupposes that there are discernible boundaries between (social) science and policy. This paper will investigate some of these boundaries. The frontiers are not only organizational and legal; they also exist in how the 'public interest' is conceived by these bodies and their financiers. Moreover, the social interactions and exchanges involved in 'bridging', themselves muddy the conception of 'boundary', allowing for analysis to go beyond the dualism imposed in seeing science on one side of the bridge, and the state on the other, to address the complex relations between experts and public policy
Political boundary spanning:politicians at the interface between collaborative governance and representative democracy
Research finds that productive interfaces between collaborative and bureaucratic forms of governance hinges on the extent t
Evidence of X-ray Synchrotron Emission from Electrons Accelerated to 40 TeV in the Supernova Remnant Cassiopeia A
We present the 2-60 keV spectrum of the supernova remnant Cassiopeia A
measured using the Proportional Counter Array and the High Energy X-ray Timing
Experiment on the Rossi X-ray Timing Explorer satellite. In addition to the
previously reported strong emission-line features produced by thermal plasmas,
the broad-band spectrum has a high-energy "tail" that extends to energies at
least as high as 120 keV. This tail may be described by a broken power law that
has photon indices of 1.8 +0.5/-0.6 and 3.04 +0.15/-0.13 and a break energy of
15.9 +0.3/-0.4 keV. We argue that the high-energy component, which dominates
the spectrum above about 10 keV, is produced by synchrotron radiation from
electrons that have energies up to at least 40 TeV. This conclusion supports
the hypothesis that Galactic cosmic rays are accelerated predominantly in
supernova remnants.Comment: 10 pages of text, 3 figures, accepted for Astrophys. J. Letter
Cosmic ray diffusion near the Bohm limit in the Cassiopeia A supernova remnant
Supernova remnants (SNRs) are believed to be the primary location of the
acceleration of Galactic cosmic rays, via diffusive shock (Fermi) acceleration.
Despite considerable theoretical work the precise details are still unknown, in
part because of the difficulty in directly observing nucleons that are
accelerated to TeV energies in, and affect the structure of, the SNR shocks.
However, for the last ten years, X-ray observatories ASCA, and more recently
Chandra, XMM-Newton, and Suzaku have made it possible to image the synchrotron
emission at keV energies produced by cosmic-ray electrons accelerated in the
SNR shocks. In this article, we describe a spatially-resolved spectroscopic
analysis of Chandra observations of the Galactic SNR Cassiopeia A to map the
cutoff frequencies of electrons accelerated in the forward shock. We set upper
limits on the electron diffusion coefficient and find locations where particles
appear to be accelerated nearly as fast as theoretically possible (the Bohm
limit).Comment: 18 pages, 5 figures. Accepted for publication in Nature Physics (DOI
below), final version available week of August 28, 2006 at
http://www.nature.com/nphy
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