79 research outputs found
A Comparison of X-ray and Optical Emission in Cassiopeia A
Broadband optical and narrowband Si XIII X-ray images of the young Galactic
supernova remnant Cas A obtained over several decades are used to investigate
spatial and temporal correlations on both large and small scales. The data
consist of optical and near infrared ground-based and Hubble Space Telescope
images taken between 1951 and 2011, and X-ray images from Einstein, ROSAT, and
Chandra taken between 1979 and 2013. We find weak spatial correlations between
the remnant's emission features on large scales, but several cases of good
optical/X-ray correlations on small scales for features which have brightened
due to recent interaction with the reverse shock. We also find instances where:
(i) a time delay is observed between the appearance of a feature's optical and
X-ray emissions, (ii) displacements of several arcseconds between a feature's
X-ray and optical emission peaks and, (iii) regions showing no corresponding
X-ray or optical emissions. To explain this behavior, we propose a
inhomogeneous model for Cas A's ejecta consisting of small, dense optically
emitting knots (n ~ 10^(2-3)/cm^(3)) and a much lower density (n ~ 0.1 -
1/cm^(3)) diffuse X-ray emitting component often spatially associated with
optical emission knots. The X-ray emitting component is sometimes linked to
optical clumps through shock induced mass ablation generating trailing material
leading to spatially offset X-ray/optical emissions. A range of ejecta
densities can also explain the observed X-ray/optical time delays since the
remnant's 5000 km/s reverse shock heats dense ejecta clumps to temperatures
around 3x10^4 K relatively quickly which then become optically bright while
more diffuse ejecta become X-ray bright on longer timescales. Highly
inhomogeneous ejecta as proposed here for Cas A may help explain some of the
X-ray/opticalfeatures seen in other young core collapse SN remnants.Comment: 31 pages, 21 figures. Submitted to the Astrophysical Journal. Please
contact the corresponding author for higher resolution postscript versions of
the figures ([email protected]
Super-luminous X-ray Emission from the Interaction of Supernova Ejecta with Dense Circumstellar Shells
For supernova powered by the conversion of kinetic energy into radiation due
to the interactions of the ejecta with a dense circumstellar shell, we show
that there could be X-ray analogues of optically super-luminous SNe with
comparable luminosities and energetics. We consider X-ray emission from the
forward shock of SNe ejecta colliding into an optically-thin CSM shell, derive
simple expressions for the X-ray luminosity as a function of the circumstellar
shell characteristics, and discuss the different regimes in which the shock
will be radiative or adiabatic, and whether the emission will be dominated by
free-free radiation or line-cooling. We find that even with normal supernova
explosion energies of 10^51 erg, there exists CSM shell configurations that can
liberate a large fraction of the explosion energy in X-rays, producing
unabsorbed X-ray luminosities approaching 10^44 erg/s events lasting a few
months, or even 10^45 erg/s flashes lasting days. Although the large column
density of the circumstellar shell can absorb most of the flux from the initial
shock, the most luminous events produce hard X-rays that are less susceptible
to photoelectric absorption, and can counteract such losses by completely
ionizing the intervening material. Regardless, once the shock traverses the
entire circumstellar shell, the full luminosity could be available to
observers.Comment: Submitted to MNRAS. 12 pages, 4 figure
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
Proper Motions and Brightness Variations of Nonthermal X-Ray Filaments in the Cassiopeia a Supernova Remnant
We present Chandra ACIS X-ray observations of the Galactic supernova remnant Cassiopeia A taken in 2007 December. Combining these data with previous archival Chandra observations taken in 2000, 2002, and 2004, we estimate the remnant\u27s forward shock velocity at various points around the outermost shell to range between 4200 and 5200 ± 500 km sâ1. Using these results together with previous analyses of Cas A\u27s X-ray emission, we present a model for the evolution of Cas A and find that it\u27s expansion is well fit by a Ïej â r â(7â9) ejecta profile running into a circumstellar wind. We further find that while the position of the reverse shock in this model is consistent with that measured in the X-rays, in order to match the forward shock velocity and radius we had to assume that ~ 30% of the explosion energy has gone into accelerating cosmic rays at the forward shock. The new X-ray images also show that brightness variations can occur for some forward shock filaments like that seen for several nonthermal filaments seen projected in the interior of the remnant. Spectral fits to exterior forward shock filaments and interior nonthermal filaments show that they exhibit similar spectra. This together with similar flux variations suggests that interior nonthermal filaments might be simply forward shock filaments seen in projection and not located at the reverse shock as has been recently proposed
A CR-hydro-NEI Model of Multi-wavelength Emission from the Vela Jr. Supernova Remnant (SNR RX J0852.0-4622)
Based largely on energy budget considerations and the observed cosmic-ray
(CR) ionic composition, supernova remnant (SNR) blast waves are the most likely
sources of CR ions with energies at least up to the "knee" near 3 PeV. Shocks
in young shell-type TeV-bright SNRs are surely producing TeV particles, but the
emission could be dominated by ions producing neutral pion-decay emission or
electrons producing inverse-Compton gamma-rays. Unambiguously identifying the
GeV-TeV emission process in a particular SNR will not only help pin down the
origin of CRs, it will add significantly to our understanding of the diffusive
shock acceleration (DSA) mechanism and improve our understanding of supernovae
and the impact SNRs have on the circumstellar medium. In this study, we
investigate the Vela Jr. SNR, an example of TeV-bright non-thermal SNRs. We
perform hydrodynamic simulations coupled with non-linear DSA and
non-equilibrium ionization near the forward shock (FS) to confront currently
available multi-wavelength data. We find, with an analysis similar to that used
earlier for SNR RX J1713.7-3946, that self-consistently modeling the thermal
X-ray line emission with the non-thermal continuum in our one-dimensional model
strongly constrains the fitting parameters, and this leads convincingly to a
leptonic origin for the GeV-TeV emission for Vela Jr. This conclusion is
further supported by applying additional constraints from observation,
including the radial brightness profiles of the SNR shell in TeV gamma-rays,
and the spatial variation of the X-ray synchrotron spectral index. We will
discuss implications of our models on future observations by the
next-generation telescopes.Comment: 12 pages, 10 figures, to appear at the Astrophysical Journa
The Role of Diffusive Shock Acceleration on Nonequilibrium Ionization in Supernova Remnant Shocks II: Emitted Spectra
We present a grid of nonequilibrium ionization models for the X-ray spectra
from supernova remnants undergoing efficient diffusive shock acceleration. The
calculation follows the hydrodynamics of the blast wave as well as the
time-dependent ionization of the plasma behind the shock. The ionization state
is passed to a plasma emissivity code to compute the thermal X-ray emission,
which is combined with the emission from nonthermal synchrotron emission to
produce a self-consistent model for the thermal and nonthermal emission from
cosmic-ray dominated shocks. We show how plasma diagnostics such as the
G'-ratio of He-like ions, defined as the ratio of the sum of the
intercombination, forbidden, and satellite lines to the resonance line, can
vary with acceleration efficiency, and discuss how the thermal X-ray emission,
when the time-dependent ionization is not calculated self-consistently with the
hydrodynamics, can differ from the thermal X-ray emission from models which do
account for the hydrodynamics. Finally we compare the thermal X-ray emission
from models which show moderate acceleration (~ 35%) to the thermal X-ray
emission from test-particle models.Comment: 17 pages, 12 figures. accepted for publication in the Astrophysical
Journa
Cooling of the Cassiopeia A neutron star and the effect of diffusive nuclear burning
The study of how neutron stars cool over time can provide invaluable insights
into fundamental physics such as the nuclear equation of state and
superconductivity and superfluidity. A critical relation in neutron star
cooling is the one between observed surface temperature and interior
temperature. This relation is determined by the composition of the neutron star
envelope and can be influenced by the process of diffusive nuclear burning
(DNB). We calculate models of envelopes that include DNB and find that DNB can
lead to a rapidly changing envelope composition which can be relevant for
understanding the long-term cooling behavior of neutron stars. We also report
on analysis of the latest temperature measurements of the young neutron star in
the Cassiopeia A supernova remnant. The 13 Chandra observations over 18 years
show that the neutron star's temperature is decreasing at a rate of 2-3 percent
per decade, and this rapid cooling can be explained by the presence of a proton
superconductor and neutron superfluid in the core of the star.Comment: 7 pages, 7 figures; to appear in the AIP Conference Proceedings of
the Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the
Era of Gravitational Wave Astronomy (January 3-7, 2019, Xiamen, China
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