712 research outputs found
Setting the Allowable Harvest on National Forests
11 pages (includes illustrations).
Contains references
X-Ray Synchrotron Emitting Fe-Rich Ejecta in SNR RCW 86
Supernova remnants may exhibit both thermal and nonthermal X-ray emission. We
present Chandra observations of RCW 86. Striking differences in the morphology
of X-rays below 1 keV and above 2 keV point to a different physical origin.
Hard X-ray emission is correlated fairly well with the edges of regions of
radio emission, suggesting that these are the locations of shock waves at which
both short-lived X-ray emitting electrons, and longer-lived radio-emitting
electrons, are accelerated. Soft X-rays are spatially well-correlated with
optical emission from nonradiative shocks, which are almost certainly portions
of the outer blast wave. These soft X-rays are well fit with simple thermal
plane-shock models. Harder X-rays show Fe K alpha emission and are well
described with a similar soft thermal component, but a much stronger
synchrotron continuum dominating above 2 keV, and a strong Fe K alpha line.
Quantitative analysis of this line and the surrounding continuum shows that it
cannot be produced by thermal emission from a cosmic-abundance plasma; the
ionization time is too short, as shown both by the low centroid energy (6.4
keV) and the absence of oxygen lines below 1 keV. Instead, a model of a plane
shock into Fe-rich ejecta, with a synchrotron continuum, provides a natural
explanation. This requires that reverse shocks into ejecta be accelerating
electrons to energies of order 50 TeV. We show that maximum energies of this
order can be produced by radiation-limited diffusive shock acceleration at the
reverse shocks.Comment: ApJ, accepted; full resolution images in
http://spider.ipac.caltech.edu/staff/rho/rcw86chandra.p
Constraints On the Diffusive Shock Acceleration From the Nonthermal X-ray Thin Shells In SN1006 NE Rim
Characteristic scale lengths of nonthermal X-rays from the SN1006 NE rim,
which are observed by Chandra, are interpreted in the context of the diffusive
shock acceleration on the assumption that the observed spatial profile of
nonthermal X-rays corresponds to that of accelerated electrons with energies of
a few tens of TeV. To explain the observed scale lengths, we construct two
simple models with a test particle approximation, where the maximum energy of
accelerated electrons is determined by the age of SN1006 (age-limited model) or
the energy loss (energy loss-limited model), and constrain the magnetic field
configuration and the diffusion coefficients of accelerated electrons. When the
magnetic field is nearly parallel to the shock normal, the magnetic field
should be in the range of 20-85 micro Gauss and highly turbulent both in
upstream and downstream, which means that the mean free path of accelerated
electrons is on the order of their gyro-radius (Bohm limit). This situation can
be realized both in the age-limited and energy loss-limited model. On the other
hand, when the magnetic field is nearly perpendicular to the shock normal,
which can exist only in the age-limited case, the magnetic field is several
micro Gauss in the upstream and 14-20 micro Gauss in the downstream, and the
upstream magnetic field is less turbulent than the downstream.Comment: 9 pages, 4 figures, accepted for publication in A&
An Axial Time-of-flight Mass Spectrometer for Upper Atmospheric Measurements
As the âshorelineâ of the Earthâs atmosphere, the mesosphere/lower thermosphere (MLT) region is home to many interesting and important phenomena, the most visible of which are the auroras. Geomagnetic storms, in addition to causing very intense auroral activity, also deposit large amounts of energy into the earthâs ionosphere. Recent analysis of data from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument aboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite suggests that 5.3Îźm emission from vibrationally excited NO is the main method of energy dissipation from energy deposited by geomagnetic storms. Additionally, NO+ has been shown to be the major contributor to geomagnetic storm induced 4.3Îźm nighttime emission.
In order to better physically understand these two large sources of geomagnetic storm energy dissipation, a sounding rocket mission, ROCKet-borne Storm Energetics of Auroral Dosing in the E-region (ROCK-STEADE) is being proposed. The ROCK-STEADE instrument suite consists of several photometers, an interferometer, an IR spectrometer, and two time-of-flight mass spectrometers (TOFMS). The TOFMS will measure the ion and neutral compositions in the atmosphere as the sounding rocket travels through the MLT.
Due to the use of microchannel plate (MCP) detectors in TOFMS, one of the major challenges to making measurements in the MLT is the high ambient pressure. Other challenges and sources of error and background include stray UV photons, scattering of gas molecules from the interior surfaces of the instrument, dissociation of molecules in the bow shock caused by the supersonic rocket flight, and reactive recombination at the surfaces of the instrument. Methods of dealing with these challenges include:
⢠Recent advances in MCP technology allowing MCP operation into the mtorr range
⢠Cooling the front surface of the TOFMS using liquid He to eliminate the bow shock (thus making possible the direct sampling of the ambient atmosphere)
⢠Cryogenically cooling the interior of the instrument to eliminate scattering of gas from instrument walls and therefore also reducing the contribution of reactive recombination
⢠Rigorous error analysis to account for the background contribution of stray U
Young people's uses of celebrity: Class, gender and 'improper' celebrity
This is an Author's Accepted Manuscript of an article published in Discourse: Studies in the Cultural
Politics of Education, 34(1), 2013, copyright Taylor & Francis, available online at:
http://www.tandfonline.com/10.1080/01596306.2012.698865.In this article, we explore the question of how celebrity operates in young people's everyday lives, thus contributing to the urgent need to address celebrity's social function. Drawing on data from three studies in England on young people's perspectives on their educational and work futures, we show how celebrity operates as a classed and gendered discursive device within young people's identity work. We illustrate how young people draw upon class and gender distinctions that circulate within celebrity discourses (proper/improper, deserving/undeserving, talented/talentless and respectable/tacky) as they construct their own identities in relation to notions of work, aspiration and achievement. We argue that these distinctions operate as part of neoliberal demands to produce oneself as a âsubject of valueâ. However, some participants produced readings that show ambivalence and even resistance to these dominant discourses. Young people's responses to celebrity are shown to relate to their own class and gender position.The Arts and Humanities Research Council, the British Academy, the
Economic and Social Research Council, and the UK Resource Centre for
Women in Science Engineering and Technology
Australia Telescope Compact Array Observations of the OH Star Roberts 22: Resolved Images of OH Emission
We have imaged the unusual OH/IR object G 284-0.8 (Roberts 22) in the OH
transitions at 1612, 1665 and 1667 MHz using the Australia Telescope Compact
Array in Narrabri, NSW, Australia. The angular resolution of the OH images we
present here is 6 arcsec (12000 AU at a distance of 2 kpc). We find OH
emission, of extent 1.5 arcsec (2800 AU), located inside the optical bipolar
flow region (size 8 arcsec) discovered by Allen et al. (1980) and imaged
recently by Sahai et al. (1999) with the Hubble Space Telescope. We present
astrometry of Roberts 22, providing registration of the OH and Hubble Space
Telescope images to within ~0.35 arcsec. We describe the OH velocity field in
Roberts 22 and discuss the kinematic properties of the source and its
evolutionary status.Comment: 6 pages, 8 figures, tables. Accepted for AJ, tentatively scheduled
for the May 2001 issu
X-Ray Observations of the supernova remnant G21.5-0.9
We present the analysis of archival X-ray observations of the supernova
remnant (SNR) G21.5-0.9. Based on its morphology and spectral properties,
G21.5-0.9 has been classified as a Crab-like SNR. In their early analysis of
the CHANDRA calibration data, Slane et al. (2000) discovered a
low-surface-brightness, extended emission. They interpreted this component as
the blast wave formed in the supernova (SN) explosion. In this paper, we
present the CHANDRA analysis using a total exposure of ~150 ksec. We also
include ROSAT and ASCA observations. Our analysis indicates that the extended
emission is non-thermal -- a result in agreement with XMM observations. The
entire remnant of radius ~ 2'.5 is best fitted with a power law model with a
photon index steepening away from the center. The total unabsorbed flux in the
0.5-10 keV is 1.1E-10 erg/cm2/s with an 85% contribution from the 40" radius
inner core. Timing analysis of the High-Resolution Camera (HRC) data failed to
detect any pulsations. We put a 16% upper limit on the pulsed fraction. We
derive the physical parameters of the putative pulsar and compare them with
those of other plerions (such as the Crab and 3C 58). G21.5-0.9 remains the
only plerion whose size in X-rays is bigger than in the radio. Deep radio
observations will address this puzzle.Comment: 23 pages including 11 figures and 3 tables; accepted by ApJ June 22,
2001; to appear in Oct 20, 2001 issue of Ap
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