343 research outputs found
The mass and energy budget of Cassiopeia A
Further analysis of X-ray spectroscopy results recently obtained from the MOS
CCD cameras on-board XMM-Newton provides a detailed description of the hot and
cool X-ray emitting plasma in Cas A. Measurement of the Doppler broadening of
the X-ray lines is consistent with the expected ion velocities, ~1500 km/s
along the line of sight, in the post shock plasma. Assuming a constant total
pressure throughout the remnant we estimate the total remnant mass as 10 Msun
and the total thermal energy as 7E43 J. We derive the differential mass
distribution as a function of ionisation age for both X-ray emitting
components. This distribution is consistent with a hot component dominated by
swept up mass heated by the primary shock and a cool component which are
ablated clumpy ejecta material which were and are still being heated by
interaction with the preheated swept up material. We calculate a balanced mass
and energy budget for the supernova explosion giving 1E44 J in ejected mass;
approximately 0.4 Msun of the ejecta were diffuse with an initial rms velocity
of 15000 km/s while the remaining ~1.8 Msun were clumpy with an initial rms
velocity of ~2400 km/s. Using the Doppler velocity measurements of the X-ray
spectral lines we can project the mass into spherical coordinates about the
remnant. This provides quantitative evidence for mass and energy beaming in the
supernova explosion. The mass and energy occupy less than 4.5 sr (<40 % of the
available solid angle) around the remnant and 64 % of the mass occurs in two
jets within 45 degrees of a jet axis. We calculate a swept up mass of 7.9 Msun
in the emitting plasma and estimate that the total mass lost from the
progenitor prior to the explosion could be as high as ~20 Msun.Comment: 8 pages, 7 figures, submitted to Astronomy & Astrophysic
A universal GRB photon energy-peak luminosity relation
The energetics and emission mechanism of GRBs are not well understood. Here
we demonstrate that the instantaneous peak flux or equivalent isotropic peak
luminosity, L_iso ergs s^-1, rather than the integrated fluence or equivalent
isotropic energy, E_iso ergs, underpins the known high-energy correlations.
Using new spectral/temporal parameters calculated for 101 bursts with redshifts
from BATSE, BeppoSAX, HETE-II and Swift we describe a parameter space which
characterises the apparently diverse properties of the prompt emission. We show
that a source frame characteristic-photon-energy/peak luminosity ratio, K_z,
can be constructed which is constant within a factor of 2 for all bursts
whatever their duration, spectrum, luminosity and the instrumentation used to
detect them. The new parameterization embodies the Amati relation but indicates
that some correlation between E_peak and E_iso follows as a direct mathematical
inference from the Band function and that a simple transformation of E_iso to
L_iso yields a universal high energy correlation for GRBs. The existence of K_z
indicates that the mechanism responsible for the prompt emission from all GRBs
is probably predominantly thermal.Comment: Submitted to Ap
Medium-term performance and maintenance of SUDS:a case-study of Hopwood Park Motorway Service Area, UK
One of the main barriers to implementing SUDS is concern about performance and maintenance costs since there are few well-documented case-studies. This paper summarizes studies conducted between 2000 and 2008 of the performance and maintenance of four SUDS management trains constructed in 1999 at the Hopwood Park Motorway Service Area, central England. Assessments were made of the wildlife value and sedimentation in the SUDS ponds, the hydraulic performance of the coach park management train, water quality in all management trains, and soil/sediment composition in the grass filter strip, interceptor and ponds. Maintenance procedures and costs were also reviewed. Results demonstrate the benefits of a management train approach over individual SUDS units for flow attenuation, water treatment, spillage containment and maintenance. Peak flows, pond sediment depth and contaminant concentrations in sediment and water decreased through the coach park management train. Of the 2007 annual landscape budget of £15,000 for the whole site, the maintenance costs for SUDS only accounted for £2,500 compared to £4,000 for conventional drainage structures. Furthermore, since sediment has been attenuated in the management trains, the cost of sediment removal after the recommended period of three years was only £554 and, if the design is not compromised, less frequent removal will be required in future
Interpretation of UV Absorption Lines in SN1006
We present a theoretical interpretation of the broad silicon and iron UV
absorption features observed with the Hubble Space Telescope in the spectrum of
the Schweizer-Middleditch star behind the remnant of Supernova 1006. These
features are caused by supernova ejecta in SN1006. We propose that the
redshifted SiII2 1260 A feature consists of both unshocked and shocked SiII.
The sharp red edge of the line at 7070 km/s indicates the position of the
reverse shock, while its Gaussian blue edge reveals shocked Si with a mean
velocity of 5050 km/s and a dispersion of 1240 km/s, implying a reverse shock
velocity of 2860 km/s. The measured velocities satisfy the energy jump
condition for a strong shock, provided that all the shock energy goes into
ions, with little or no collisionless heating of electrons. The line profiles
of the SiIII and SiIV absorption features indicate that they arise mostly from
shocked Si. The total mass of shocked and unshocked Si inferred from the SiII,
SiIII and SiIV profiles is M_Si = 0.25 \pm 0.01 Msun on the assumption of
spherical symmetry. Unshocked Si extends upwards from 5600 km/s. Although there
appears to be some Fe mixed with the Si at lower velocities < 7070 km/s, the
absence of FeII absorption with the same profile as the shocked SiII suggests
little Fe mixed with Si at higher (before being shocked) velocities. The column
density of shocked SiII is close to that expected for SiII undergoing steady
state collisional ionization behind the reverse shock, provided that the
electron to SiII ratio is low, from which we infer that most of the shocked Si
is likely to be of a fairly high degree of purity, unmixed with other elements.
We propose that the ambient interstellar density on the far side of SN1006 is
anomalously low compared to the density around the rest of the remnant. ThisComment: 24 pages, with 8 figures included. Accepted for publication in the
Astrophysical Journa
The XMM-Newton view of the Crab
Aims. We discuss the current X-ray view of the Crab Nebula and Pulsar, summarising our analysis of observations of the source with the EPIC-pn camera on board the XMM-Newton observatory. Different modes of EPIC-pn were combined in order to yield a complete scenario of the spectral properties of the Crab resolved in space and time (pulse phase). In addition we give a description of the special
EPIC-pn Burst mode and guidance for data reduction in that mode.
Methods. We analysed spectra for the nebula and pulsar separately in the 0.6−12.0 keV energy band. All data were processed with the
SAS 6.0.0 XMM-Newton Scientific Analysis System package; models were fitted to the data with XSPEC 11. The high time resolution of EPIC-pn in its Burst mode (7 μs) was used for a phase resolved analysis of the pulsar spectrum, after determination of the period with epoch folding techniques. Data from the SmallWindow mode were processed and corrected for pile-up allowing for spectroscopy
simultaneously resolved in space and time.
Results. The spatial variation of the spectrum over the entire region of the Crab shows a gradual spectral softening from the inner pulsar region to the outer nebula region with a variation in photon index, Γ, from 2.0 to 2.4. Pulse phase resolved spectroscopy of the Crab Pulsar reveals a phase dependent modulation of the photon index in form of a significant hardening of the spectrum in the
inter-peak phase from Γ = 1.7 during the pulse peak to Γ = 1.5
X-ray spectral imaging and Doppler mapping of Cassiopeia A
A detailed X-ray spectral analysis of Cas A using a deep exposure from the
EPIC-MOS cameras on-board XMM-Newton is presented. Spectral fitting was
performed on a 15x15 grid of 20"x20" pixels using a two component
non-equilibrium ionisation model (NEI) giving maps of ionisation age,
temperature, interstellar column density, abundances and Doppler velocities.
The abundances of Si, S, Ar and Ca are strongly correlated. The abundance
ratios are consistent with the nucleosynthesis yield from the collapse of a 12
Msun progenitor. The abundance ratios Ne/Si, Mg/Si, Fe/Si and Ni/Si are very
variable and distinctly different from S/Si, Ar/Si and Ca/Si, in line with the
current explosive nucleosynthesis models. The ionisation age and temperature of
both NEI components varies considerably over the remnant. Accurate
determination of these parameters yield reliable Doppler velocities for both
components. The data are consistent with a plasma velocity of 2600 km/s at the
shock radius of 153" implying a primary shock velocity of 4000+/-500 km/s. The
Si-K and S-K line emission from the cool component is confined to a relatively
narrow shell with radius 100-150". This component is almost certainly ejecta
material which has been heated by a combination of the reverse shock and
heating of ejecta clumps as they plough through the medium which has been
pre-heated by the primary shock. The Fe-K line emission is expanding faster and
spans a radius range 110-170". The bulk of the Fe emission is confined to two
large clumps and it is likely that these too are the result of ablation from
ejecta bullets rather swept up circumstellar medium.Comment: 10 pages, 11 figures, submitted to Astronomy and Astrophysic
1SXPS: A deep Swift X-ray Telescope point source catalog with light curves and spectra
We present the 1SXPS (Swift-XRT Point Source) catalog of 151,524 X-ray
point-sources detected by the Swift-XRT in 8 years of operation. The catalog
covers 1905 square degrees distributed approximately uniformly on the sky. We
analyze the data in two ways. First we consider all observations individually,
for which we have a typical sensitivity of ~3e-13 erg/cm2/s (0.3--10 keV). Then
we co-add all data covering the same location on the sky: these images have a
typical sensitivity of ~9e-14 erg/cm2/s (0.3--10 keV). Our sky coverage is
nearly 2.5 times that of 3XMM-DR4, although the catalog is a factor of ~1.5
less sensitive. The median position error is 5.5" (90% confidence), including
systematics. Our source detection method improves on that used in previous XRT
catalogs and we report >68,000 new X-ray sources. The goals and observing
strategy of the Swift satellite allow us to probe source variability on
multiple timescales, and we find ~30,000 variable objects in our catalog. For
every source we give positions, fluxes, time series (in four energy bands and
two hardness ratios), estimates of the spectral properties, spectra and
spectral fits for the brightest sources, and variability probabilities in
multiple energy bands and timescales.Comment: 27 pages, 19 figures; accepted for publication in ApJS. The
accompanying website, http://www.swift.ac.uk/1SXPS is live; the Vizier entry
should be available shortl
Confinement of relativistic electrons in a magnetic mirror en route to a magnetized relativistic pair plasma
The Unexpected Role of Evolving Longitudinal Electric Fields in Generating Energetic Electrons in Relativistically Transparent Plasmas
Superponderomotive-energy electrons are observed experimentally from the
interaction of an intense laser pulse with a relativistically transparent
target. For a relativistically transparent target, kinetic modeling shows that
the generation of energetic electrons is dominated by energy transfer within
the main, classically overdense, plasma volume. The laser pulse produces a
narrowing, funnel-like channel inside the plasma volume that generates a field
structure responsible for the electron heating. The field structure combines a
slowly evolving azimuthal magnetic field, generated by a strong laser-driven
longitudinal electron current, and, unexpectedly, a strong propagating
longitudinal electric field, generated by reflections off the walls of the
funnel-like channel. The magnetic field assists electron heating by the
transverse electric field of the laser pulse through deflections, whereas the
longitudinal electric field directly accelerates the electrons in the forward
direction. The longitudinal electric field produced by reflections is 30 times
stronger than that in the incoming laser beam and the resulting direct laser
acceleration contributes roughly one third of the energy transferred by the
transverse electric field of the laser pulse to electrons of the
super-ponderomotive tail
Longitudinal Ion Acceleration from High-Intensity Laser Interactions with Underdense Plasma
Longitudinal ion acceleration from high-intensity (I ~ 10^20 Wcm^-2) laser
interactions with helium gas jet targets (n_e ~ 0.04 n_c) have been observed.
The ion beam has a maximum energy for He^2+ of approximately 40 MeV and was
directional along the laser propagation path, with the highest energy ions
being collimated to a cone of less than 10 degrees. 2D particle-in-cell
simulations have been used to investigate the acceleration mechanism. The time
varying magnetic field associated with the fast electron current provides a
contribution to the accelerating electric field as well as providing a
collimating field for the ions. A strong correlation between the plasma density
and the ion acceleration was found. A short plasma scale-length at the vacuum
interface was observed to be beneficial for the maximum ion energies, but the
collimation appears to be improved with longer scale-lengths due to enhanced
magnetic fields in the ramp acceleration region.Comment: 18 pages, 6 figure
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