6,364 research outputs found
A survey of the UK benefit system
This paper describes all the main benefits in the UK system, giving details of rates and allowances, as well as numbers and types of claimants and levels of expenditure
The Kinematics of Kepler's Supernova Remnant as revealed by Chandra
I determine the expansion of the supernova remnant of SN1604 (Kepler's
supernova) based on archival Chandra ACIS-S observations made in 2000 and 2006.
The measurements were done in several distinct energy bands, and were made for
the remnant as a whole, and for six individual sectors. The average expansion
parameter indicates that the remnant expands as , but there
are significant differences in different parts of the remnant: the bright
northwestern part expands as , whereas the rest of the
remnant's expansion shows an expansion . The latter is
consistent with an explosion in which the outer part of the ejecta has a
negative power law slope for density () of , or with
an exponential density profile(). The expansion
parameter in the southern region, in conjunction with the shock radius,
indicate a rather low value (<5E50 erg) for the explosion energy of SN1604 for
a distance of 4 kpc. An higher explosion energy is consistent with the results,
if the distance is larger.
The filament in the eastern part of the remnant, which is dominated by X-ray
synchrotron radiation seems to mark a region with a fast shock speed , corresponding to a shock velocity of v= 4200 km/s, for a distance to
SN1604 of 4 kpc. This is consistent with the idea that X-ray synchrotron
emission requires shock velocities in excess of ~2000 km/s.
The X-ray based expansion measurements reported are consistent with results
based on optical and radio measurements, but disagree with previous X-ray
measurements based on ROSAT and Einstein observations.Comment: Accepted for publication in ApJ. This new version is the accepted
version, which differs mainly in the discussion sectio
Evolution of Magnetic Fields in Supernova Remnants
Supernova remnants (SNR) are now widely believed to be a source of cosmic
rays (CRs) up to an energy of 1 PeV. The magnetic fields required to accelerate
CRs to sufficiently high energies need to be much higher than can result from
compression of the circumstellar medium (CSM) by a factor 4, as is the case in
strong shocks. Non-thermal synchrotron maps of these regions indicate that
indeed the magnetic field is much stronger, and for young SNRs has a dominant
radial component while for old SNRs it is mainly toroidal. How these magnetic
fields get enhanced, or why the field orientation is mainly radial for young
remnants, is not yet fully understood. We use an adaptive mesh refinement MHD
code, AMRVAC, to simulate the evolution of supernova remnants and to see if we
can reproduce a mainly radial magnetic field in early stages of evolution. We
follow the evolution of the SNR with three different configurations of the
initial magnetic field in the CSM: an initially mainly toroidal field, a
turbulent magnetic field, and a field parallel to the symmetry axis. Although
for the latter two topologies a significant radial field component arises at
the contact discontinuity due to the Rayleigh-Taylor instability, no radial
component can be seen out to the forward shock. Ideal MHD appears not
sufficient to explain observations. Possibly a higher compression ratio and
additional turbulence due to dominant presence of CRs can help us to better
reproduce the observations in future studies.Comment: 5 pages, 3 figures. To appear in conference proceedings of "Magnetic
Fields in the Universe II" (2008), RevMexA
Multiwavelength Signatures of Cosmic Ray Acceleration by Young Supernova Remnants
An overview is given of multiwavelength observations of young supernova
remnants, with a focus on the observational signatures of efficient cosmic ray
acceleration. Some of the effects that may be attributed to efficient cosmic
ray acceleration are the radial magnetic fields in young supernova remnants,
magnetic field amplification as determined with X-ray imaging spectroscopy,
evidence for large post-shock compression factors, and low plasma temperatures,
as measured with high resolution optical/UV/X-ray spectroscopy. Special
emphasis is given to spectroscopy of post-shock plasma's, which offers an
opportunity to directly measure the post-shock temperature. In the presence of
efficient cosmic ray acceleration the post-shock temperatures are expected to
be lower than according to standard equations for a strong shock. For a number
of supernova remnants this seems indeed to be the case.Comment: Invited review, to appear in the proceedings of "4th Heidelberg
International Symposium on High Energy Gamma-Ray Astronomy 2008
Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and non-ideal polymers
An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and
non-adsorbing polymers, that takes polymer non-ideality into account through a
repulsive stepfunction pair potential between polymers, is studied with grand
canonical Monte Carlo simulations and density functional theory. Simulation
results validate previous theoretical findings for the shift of the bulk fluid
demixing binodal upon increasing strength of polymer-polymer repulsion,
promoting the tendency to mix. For increasing strength of the polymer-polymer
repulsion, simulation and theory consistently predict the interfacial tension
of the free colloidal liquid-gas interface to decrease significantly for fixed
colloid density difference in the coexisting phases, and to increase for fixed
polymer reservoir packing fraction.Comment: 10 pages, 4 figure
Effects of Neutral Hydrogen on Cosmic Ray Precursors in Supernova Remnant Shock Waves
Many fast supernova remnant shocks show spectra dominated by Balmer lines.
The H profiles have a narrow component explained by direct excitations
and a thermally Doppler broadened component due to atoms that undergo charge
exchange in the post-shock region. However, the standard model does not take
into account the cosmic-ray shock precursor, which compresses and accelerates
plasma ahead of the shock. In strong precursors with sufficiently high
densities, the processes of charge exchange, excitation and ionization will
affect the widths of both narrow and broad line components. Moreover, the
difference in velocity between the neutrals and the precursor plasma gives rise
to frictional heating due to charge exchange and ionization in the precursor.
In extreme cases, all neutrals can be ionized by the precursor.
In this paper we compute the ion and electron heating for a wide range of
shock parameters, along with the velocity distribution of the neutrals that
reach the shock. Our calculations predict very large narrow component widths
for some shocks with efficient acceleration, along with changes in the broad-
to-narrow intensity ratio used as a diagnostic for the electron-ion temperature
ratio. Balmer lines may therefore provide a unique diagnostic of precursor
properties. We show that heating by neutrals in the precursor can account for
the observed H narrow component widths, and that the acceleration
efficiency is modest in most Balmer line shocks observed thus far.Comment: 9 pages, 3 figure
Characterizing the non-thermal emission of Cas A
We report on our analysis of the 1 Ms Chandra observation of the supernova
remnant Cas A in order to localize, characterize and quantify its non-thermal
X-ray emission. More specifically, we investigated whether the X-ray
synchrotron emission from the inside of the remnant is from the outward shock,
but projected toward the inner ring, or from the inner shell. We tackle this
problem by employing a Lucy-Richardson deconvolution technique and measuring
spectral indices in the 4.2-6 keV band.
We show that most of the continuum emission is coming from an inner ring that
coincides with the location of the reverse shock. This inner ring includes
filaments, whose X-ray emission has been found to be dominated by X-ray
synchrotron emission. The X-ray emission from these filaments, both at the
forward shock and from the inner ring, have relatively hard spectra with
spectral index > -3.1. The regions emitting hard X-ray continuum contribute
about 54% of the total X-ray emission in the 4.2-6 keV. This is lower than
suggested by extrapolating the hard X-ray spectrum as measured by BeppoSAX-PDS
and INTEGRAL. This can be reconciled by assuming a gradual steepening of the
spectrum toward higher energies. We argue that the X-ray synchrotron emission
is mainly coming from the Western part of the reverse shock. The reverse shock
in the West is almost at rest in our observation frame, corresponding to a
relatively high reverse shock velocity of ~6000 km/s in the frame of the freely
expanding ejecta.Comment: Accepted for publication in ApJ, high resolution figures will appear
in the official pape
Small-Scale X-ray Variability in the Cassiopeia A Supernova Remnant
A comparison of X-ray observations of the Cassiopeia A supernova remnant
taken in 2000, 2002, and 2004 with the Chandra ACIS-S3 reveals the presence of
several small scale features (<= 10 arcsec) which exhibit significant intensity
changes over a 4 year time frame. Here we report on the variability of six
features, four of which show count rate increases from ~ 10% to over 90%, and
two which show decreases of ~ 30% -- 40%. While extracted 1-4.5 keV X-ray
spectra do not reveal gross changes in emission line strengths, spectral fits
using non-equilibrium ionization, metal-rich plasma models indicate increased
or decreased electron temperatures for features showing increasing or
decreasing count rates, respectively. Based on the observed count rate changes
and the assumption that the freely expanding ejecta has a velocity of ~ 5000
km/s at the reverse shock front, we estimate the unshocked ejecta to have
spatial scale variations of 0.02 - 0.03 pc, which is consistent with the X-ray
emitting ejecta belonging to a more diffuse component of the supernova ejecta
than that seen in the optically emitting ejecta, which have spatial scales ~
0.001 pc.Comment: 9 pages, 8 figures, to be published in Astronomical Journa
The relation between post-shock temperature, cosmic-ray pressure and cosmic-ray escape for non-relativistic shocks
Supernova remnants are thought to be the dominant source of Galactic cosmic
rays. This requires that at least 5% of the available energy is transferred to
cosmic rays, implying a high cosmic-ray pressure downstream of supernova
remnant shocks. Recently, it has been shown that the downstream temperature in
some remnants is low compared to the measured shock velocities, implying that
additional pressure support by accelerated particles is present.
Here we use a two-fluid thermodynamic approach to derive the relation between
post-shock fractional cosmic-ray pressure and post-shock temperature, assuming
no additional heating beyond adiabatic heating in the shock precursor and with
all non-adiabatic heating occurring at the subshock. The derived relations show
that a high fractional cosmic-ray pressure is only possible, if a substantial
fraction of the incoming energy flux escapes from the system. Recently a shock
velocity and a downstream proton temperature were measured for a shock in the
supernova remnant RCW 86. We apply the two-fluid solutions to these
measurements and find that the the downstream fractional cosmic-ray pressure is
at least 50% with a cosmic-ray energy flux escape of at least 20%. In general,
in order to have 5% of the supernova energy go into accelerating cosmic rays,
on average the post-shock cosmic-ray pressure needs to be 30% for an effective
cosmic-ray adiabatic index of 4/3.Comment: 9 pages, 6 color figures. This is updated with a corrected figure 5a
and 5b, reflecting an ApJ erratu
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