925 research outputs found
Spectra of cosmic X-ray sources
X-ray measurements provide the most direct probes of astrophysical environments with temperatures exceeding one million K. Progress in experimental research utilizing dispersive techniques (e.g., Bragg and grating spectroscopy) is considerably slower than that in areas utilizing photometric techniques, because of the relative inefficiency of the former for the weak X-ray signals from celestial sources. As a result, the term "spectroscopy" as applied to X-ray astronomy has traditionally satisfied a much less restrictive definition (in terms of resolving power) than it has in other wavebands. Until quite recently, resolving powers of order unity were perfectly respectable, and still provide (in most cases) the most useful spectroscopic data. In the broadest sense, X-ray photometric measurements are spectroscopic, insofar as they represent samples of the overall electromagnetic continua of celestial objects
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
Ionization of Carbon and Nitrogen in The Intercloud Medium
If the intercloud medium in the solar neighborhood was suddenly ionized less than 106 years ago, the subsequent relaxation of the gas can account for the very low observed column density of C III and N III yet still leave a significant fraction ( \u270.05) of ionized hydrogen and of C I in a cold (T ≈ 20° K) intercloud medium. The OAO-C Copernicus observations in the direction of X Sco are consistent with cosmic abundances and a uniform-density medium, except for C III whose predicted column density from the C m ionization zone surrounding the star is a factor ~1Q greater than the observed result
Notes on Recent Cases
Notes on recent cases by J. S. Angelino, Marc Wonderlin, W. S. McCray, John P. Berscheid, J. J. Canty, J. J. Lyons, R. C. Kuehl, D. M. Donahue, M. E. McGcogehgan, G. L. Housley, Thomas J. Jones, Jr., and F. Earl Lamboley
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
Supplementing tuberculosis surveillance with automated data from health maintenance organizations.
Data collected by health maintenance organizations (HMOs), which provide care for an increasing number of persons with tuberculosis (TB), may be used to complement traditional TB surveillance. We evaluated the ability of HMO-based surveillance to contribute to overall TB reporting through the use of routinely collected automated data for approximately 350,000 HMO members. During approximately 1.5 million person-years, 45 incident cases were identified in either HMO or public health department records. Eight (18%) confirmed cases had not been identified by the public health department. The most useful screening criterion (sensitivity of 89% and predictive value positive of 30%) was dispensing of two or more TB drugs. Pharmacy dispensing information routinely collected by many HMOs appears to be a useful adjunct to traditional TB surveillance, particularly for identifying cases without positive microbiologic results that may be missed by traditional public health surveillance methods
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
A Chandra View of The Morphological And Spectral Evolution of Supernova Remnant 1987A
We present an update on the results of our monitoring observations of the
X-ray remnant of supernova (SN) 1987A with the {\it Chandra X-Ray Observatory}.
As of 2002 December, we have performed a total of seven observations of SN
1987A. The high angular resolution images from the latest data reveal
developments of new X-ray bright spots in the northwestern and the southwestern
portions of the remnant as well as changes on the eastern side. The latest
0.5-2 keV band flux ( 6 10 ergs cm
s) is four times brighter than three years earlier. The overall X-ray
emission is primarily from the blast wave shock with 2.4 keV. As
the blast wave approaches the dense circumstellar material, the contribution
from the decelerated slow shock ( 0.22 keV) to the observed X-ray
emission is becoming significant. The increase of this slow shock contribution
over the last two years is particularly noticeable in the western half of the
remnant. These results indicate that the shock front is now reaching the main
body of the inner circumstellar ring. Based on the best-fit two-shock spectral
model, we derive approximate densities of the X-ray-emitting regions (
235 cm for the fast shock and 7500 cm for the
slow shock). We obtain an upper limit on the observed X-ray luminosity of any
embedded point source ( 1.5 10 ergs s) in the
210 keV band. The X-ray remnant continues to expand linearly at a rate of
4167 km s.Comment: 22 pages (ApJ preprint style), 7 Figures, Accepted by ApJ (scheduled
on July 20, 2004), for high-quality Fig 1 and Fig 2, please contact
[email protected]
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