27,795 research outputs found
A spatially resolved plerionic X-ray nebula around PSR B0540-69
We present a high resolution Chandra X-ray observation of PSR B0540-69, the
Crab-like 50 msec pulsar in the Large Magellanic Cloud. We use phase-resolved
imaging to decompose the extended X-ray emission, as expected of a synchrotron
nebula, from the point-like emission of the pulsar. The image of the pulsed
X-ray emission shows a well-defined point-spread function of the observation,
while the resolved nebula has a morphology and size remarkably similar to the
Crab nebula, including evidence for a jet-like feature from PSR B0540-69. The
patchy outer shell, which most likely represents the expanding blast-wave of
the supernova, is reminiscent of that seen in radio. Based on morphology, size,
and energetics there can be little doubt that PSR B0540-69 is an analogous
system to the Crab but located in our neighboring galaxy.Comment: 5 pages with 5 figures, LaTex, emulateapj.sty. Revision based on data
corrected for instrumental artifacts; main result unchanged. This version to
appear in the Astrophysical Journal Letter
Modeling SN 1996cr's X-ray lines at high-resolution: Sleuthing the ejecta/CSM geometry
SN 1996cr, located in the Circinus Galaxy (3.7 Mpc, z ~ 0.001) was
non-detected in X-rays at ~ 1000 days yet brightened to ~ 4 x 10^{39} erg/s
(0.5-8 keV) after 10 years (Bauer et al. 2008). A 1-D hydrodynamic model of the
ejecta-CSM interaction produces good agreement with the measured X-ray light
curves and spectra at multiple epochs. We conclude that the progenitor of SN
1996cr could have been a massive star, M > 30 M_solar, which went from an RSG
to a brief W-R phase before exploding within its ~ 0.04 pc wind-blown shell
(Dwarkadas et al. 2010). Further analysis of the deep Chandra HETG observations
allows line-shape fitting of a handful of bright Si and Fe lines in the
spectrum. The line shapes are well fit by axisymmetric emission models with an
axis orientation ~ 55 degrees to our line-of-sight. In the deep 2009 epoch the
higher ionization Fe XXVI emission is constrained to high lattitudes: the
Occam-est way to get the Fe H-like emission coming from high latitude/polar
regions is to have more CSM at/around the poles than at mid and lower
lattitudes, along with a symmetric ejecta explosion/distribution. Similar
CSM/ejecta characterization may be possible for other SNe and, with
higher-throughput X-ray observations, for gamma-ray burst remnants as well.Comment: 4 pages, 5 figures. To appear in the Proceedings of the Gamma Ray
Bursts 2010 Conference, Annapolis, USA. Editors: McEnery, Racusin, Gehrel
ROSAT HRI Detection of the 16 ms Pulsar PSR J0537-6910 Inside SNR N157B
Based on a deep ROSAT HRI observation, we have detected a pulsed signal in
the 0.1-2 keV band from PSR J0537-6910 --- the recently discovered pulsar
associated with the supernova remnant N157B in the Large Magellanic Cloud. The
measured pulse period 0.01611548182 ms (+- 0.02 ns), Epoch MJD 50540.5, gives a
revised linear spin-down rate of , slightly
greater than the previously derived value. The narrow pulse shape (FWHM = 10%
duty cycle) in the ROSAT band resembles those seen in both XTE and ASCA data (>
2 keV), but there is also marginal evidence for an interpulse. This ROSAT
detection enables us to locate the pulsar at R.A., Dec (J2000) =
. With its uncertainty , this
position coincides with the centroid of a compact X-ray source. But the pulsed
emission accounts for only about 10% of the source luminosity in the 0.1-2 keV band. These results support our previous
suggestions: (1) The pulsar is moving at a high velocity ();
(2) A bow shock, formed around the pulsar, is responsible for most of the X-ray
emission from the source; (3) A collimated outflow from the bow shock region
powers a pulsar wind nebula that accounts for an elongated non-thermal radio
and X-ray feature to the northwest of the pulsar.Comment: 6 pages including 3 figures. To be published in ApJ
Constraints on the Redshift and Luminosity Distributions of Gamma Ray Bursts in an Einstein-de Sitter Universe
Two models of the gamma ray burst population, one with a standard candle
luminosity and one with a power law luminosity distribution, are chi^2-fitted
to the union of two data sets: the differential number versus peak flux
distribution of BATSE's long duration bursts, and the time dilation and energy
shifting versus peak flux information of pulse duration time dilation factors,
interpulse duration time dilation factors, and peak energy shifting factors.
The differential peak flux distribution is corrected for threshold effects at
low peak fluxes and at short burst durations, and the pulse duration time
dilation factors are also corrected for energy stretching and similar effects.
Within an Einstein-de Sitter cosmology, we place strong bounds on the evolution
of the bursts, and these bounds are incompatible with a homogeneous population,
assuming a power law spectrum and no luminosity evolution. Additionally, under
the implied conditions of moderate evolution, the 90% width of the observed
luminosity distribution is shown to be < 10^2, which is less constrained than
others have demonstrated it to be assuming no evolution. Finally, redshift
considerations indicate that if the redshifts of BATSE's faintest bursts are to
be compatible with that which is currently known for galaxies, a standard
candle luminosity is unacceptable, and in the case of the power law luminosity
distribution, a mean luminosity < 10^57 ph s^-1 is favored.Comment: Accepted to the Astrophysical Journal, 18 pages, LaTe
Evaluation of macrophyte control in 38 Florida lakes using triploid grass carp
Florida’s large number of shallow lakes, warm climate and
long growing season have contributed to the development of
excessive growths of aquatic macrophytes that have seriously
interfered with many water use activities. The introduction
of exotic aquatic macrophyte species such as hydrilla (
Hydrilla
verticillata
) have added significantly to aquatic plant problems
in Florida lakes. The use of grass carp (
Ctenopharyngodon
idella
) can be an effective and economical control for aquatic
vegetation such as hydrilla. Early stocking rates (24 to 74
grass carp per hectare of lake area) resulted in grass carp
consumption rates that vastly exceeded the growth rates of
the aquatic plants and often resulted in the total loss of all
submersed vegetation. This study looked at 38 Florida lakes
that had been stocked with grass carp for 3 to 10 years with
stocking rates ranging from < 1 to 59 grass carp per hectare
of lake and 1 to 207 grass carp per hectare of vegetation to
determine the long term effects of grass carp on aquatic macrophyte
communities. The median PAC (percent area coverage)
value of aquatic macrophytes for the study lakes after
they were stocked with grass carp was 14% and the median
PVI (percent volume infested) value of aquatic macrophytes
was 2%. Only lakes stocked with less than 25 to 30 fish per
hectare of vegetation tended to have higher than median
PAC and PVI values. When grass carp are stocked at levels of
> 25 to 30 fish per hectare of vegetation the complete control
of aquatic vegetation can be achieved, with the exception of
a few species of plants that grass carp have extreme difficulty
consuming. If the management goal for a lake is to control
some of the problem aquatic plants while maintaining a
small population of predominately unpalatable aquatic
plants, grass carp can be stocked at approximately 25 to 30
fish per hectare of vegetation
Nonperturbative contributions to the QCD pressure
We summarize the most important arguments why a perturbative description of
finite-temperature QCD is unlikely to be possible and review various
well-established approaches to deal with this problem. Then, using a recently
proposed method, we investigate nonperturbative contributions to the QCD
pressure and other observables (like energy, anomaly and bulk viscosity)
obtained by imposing a functional cutoff at the Gribov horizon. Finally, we
discuss how such contributions fit into the picture of consecutive effective
theories, as proposed by Braaten and Nieto, and give an outline of the next
steps necessary to improve this type of calculation.Comment: 15 pages, 13 figures, uses xcolor.sty; in v2 quality of some figures
has been improved, discussion of other approaches has been extende
Vortex Lattice Inhomogeneity in Spatially Inhomogeneous Superfluids
A trapped degenerate Bose gas exhibits superfluidity with spatially
nonuniform superfluid density. We show that the vortex distribution in such a
highly inhomogeneous rotating superfluid is nevertheless nearly uniform. The
inhomogeneity in vortex density, which diminishes in the rapid-rotation limit,
is driven by the discrete way vortices impart angular momentum to the
superfluid. This effect favors highest vortex density in regions where the
superfluid density is most uniform (e.g., the center of a harmonically trapped
gas). A striking consequence of this is that the boson velocity deviates from a
rigid-body form exhibiting a radial-shear flow past the vortex lattice.Comment: 5 RevTeX pgs,2 figures, published versio
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