697 research outputs found
High-Resolution X-ray Spectroscopy of SNR 1987A: Chandra LETG and HETG Observations in 2007
We present an extended analysis of the deep Chandra LETG and HETG
observations of the supernova remnant 1987A (SNR 1987A) carried out in 2007.
The global fits to the grating spectra show that the temperature of the X-ray
emitting plasma in the slower shocks in this system has remained stable for the
last three years, while that in the faster shocks has decreased. This
temperature evolution is confirmed by the first light curves of strong X-ray
emission lines and their ratios. On the other hand, bulk gas velocities
inferred from the X-ray line profiles are too low to account for the post-shock
plasma temperatures inferred from spectral fits. This suggests that the X-ray
emission comes from gas that has been shocked twice, first by the blast wave
and again by shocks reflected from the inner ring of SNR 1987A. A new model
that takes these considerations into account gives support to this physical
picture.Comment: 36 pages, 10 figures, Accepted for publication in Ap
Chandra observations of SN 1987A: the soft X-ray light curve revisited
We report on the present stage of SN 1987A as observed by the Chandra X-ray
Observatory. We reanalyze published Chandra observations and add three more
epochs of Chandra data to get a consistent picture of the evolution of the
X-ray fluxes in several energy bands. We discuss the implications of several
calibration issues for Chandra data. Using the most recent Chandra calibration
files, we find that the 0.5-2.0 keV band fluxes of SN 1987A have increased by
~6 x 10 ^-13 erg s^-1 cm^-2 per year since 2009. This is in contrast with our
previous result that the 0.5-2.0 keV light curve showed a sudden flattening in
2009. Based on our new analysis, we conclude that the forward shock is still in
full interaction with the equatorial ring.Comment: Accepted for publication by ApJ, 7 pages, 5 figure
Remediation of Chlorinated Solvent Plumes Using In-Situ Air Sparging—A 2-D Laboratory Study
In-situ air sparging has evolved as an innovative technique for soil and groundwater remediation impacted with volatile organic compounds (VOCs), including chlorinated solvents. These may exist as non-aqueous phase liquid (NAPL) or dissolved in groundwater. This study assessed: (1) how air injection rate affects the mass removal of dissolved phase contamination, (2) the effect of induced groundwater flow on mass removal and air distribution during air injection, and (3) the effect of initial contaminant concentration on mass removal. Dissolved-phase chlorinated solvents can be effectively removed through the use of air sparging; however, rapid initial rates of contaminant removal are followed by a protracted period of lower removal rates, or a tailing effect. As the air flow rate increases, the rate of contaminant removal also increases, especially during the initial stages of air injection. Increased air injection rates will increase the density of air channel formation, resulting in a larger interfacial mass transfer area through which the dissolved contaminant can partition into the vapor phase. In cases of groundwater flow, increased rates of air injection lessened observed downward contaminant migration effect. The air channel network and increased air saturation reduced relative hydraulic conductivity, resulting in reduced groundwater flow and subsequent downgradient contaminant migration. Finally, when a higher initial TCE concentration was present, a slightly higher mass removal rate was observed due to higher volatilization-induced concentration gradients and subsequent diffusive flux. Once concentrations are reduced, a similar tailing effect occurs
Chandra Observations of Shock Kinematics in Supernova Remnant 1987A
We report the first results from deep X-ray observations of the SNR 1987A
with the Chandra LETG. Temperatures inferred from line ratios range from 0.1 -
2 keV and increase with ionization potential. Expansion velocities inferred
from X-ray line profiles range from 300 - 1700 km/s, much less than the
velocities inferred from the radial expansion of the radio and X-ray images. We
can account for these observations with a scenario in which the X-rays are
emitted by shocks produced where the supernova blast wave strikes dense
protrusions of the inner circumstellar ring, which are also responsible for the
optical hot spots.Comment: 12 pages, 3 figures, accepted for publication in ApJ
Observing Supernova 1987A with the Refurbished Hubble Space Telescope
Observations with the Hubble Space Telescope (HST), conducted since 1990, now
offer an unprecedented glimpse into fast astrophysical shocks in the young
remnant of supernova 1987A. Comparing observations taken in 2010 using the
refurbished instruments on HST with data taken in 2004, just before the Space
Telescope Imaging Spectrograph failed, we find that the Ly-a and H-a lines from
shock emission continue to brighten, while their maximum velocities continue to
decrease. We observe broad blueshifted Ly-a, which we attribute to resonant
scattering of photons emitted from hotspots on the equatorial ring. We also
detect NV~\lambda\lambda 1239,1243 A line emission, but only to the red of
Ly-A. The profiles of the NV lines differ markedly from that of H-a, suggesting
that the N^{4+} ions are scattered and accelerated by turbulent electromagnetic
fields that isotropize the ions in the collisionless shock.Comment: Science, accepted. Science Express, 02 Sept 2010. 5 figures.
Supporting online material can be found at
http://www.sciencemag.org/cgi/content/full/sci;science.1192134/DC
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