135 research outputs found
Interpretation of the Center-Filled Emission from the Supernova Remnant W44
(Abridged) We have investigated two evolutionary scenarios advanced to
explain the centrally-brightened X-ray morphology of the supernova remnant
(SNR) W44: (1) a model involving the slow thermal evaporation of clouds
engulfed by a supernova blast wave as it propagates though a clumpy
interstellar medium (ISM), and (2) a hydrodynamical simulation of a blast wave
propagating through a homogeneous ISM, including the effects of radiative
cooling. Both models can have their respective parameters tuned to approximate
the remnant's morphology. The mean temperature of the hot plasma in W44 (~0.9
keV) as determined by our nonequilibrium ionization X-ray spectral analysis
provides the essential key to discriminate between these scenarios. Based on
the size (using the well established distance of 3 kpc) and temperature of W44,
the dynamical evolution predicted by the cloud evaporation model gives an age
for the SNR of merely 6500 yr. We argue that, because this age is inconsistent
with the characteristic age (approx. 20000 yr) of the associated PSR 1853+01,
this model cannot provide the explanation for the center-filled morphology. We
favor the radiative-phase shock model since it can reproduce both the
morphology and age of W44 assuming reasonable values for the initial explosion
energy in the range 0.7E51 to 0.9E51 ergs and the ambient ISM density of
between 3 and 4 cm**-3.Comment: 31 pages, including 4 postscript figs, LaTeX, accepted by Ap.
X-Ray Observations of the supernova remnant G21.5-0.9
We present the analysis of archival X-ray observations of the supernova
remnant (SNR) G21.5-0.9. Based on its morphology and spectral properties,
G21.5-0.9 has been classified as a Crab-like SNR. In their early analysis of
the CHANDRA calibration data, Slane et al. (2000) discovered a
low-surface-brightness, extended emission. They interpreted this component as
the blast wave formed in the supernova (SN) explosion. In this paper, we
present the CHANDRA analysis using a total exposure of ~150 ksec. We also
include ROSAT and ASCA observations. Our analysis indicates that the extended
emission is non-thermal -- a result in agreement with XMM observations. The
entire remnant of radius ~ 2'.5 is best fitted with a power law model with a
photon index steepening away from the center. The total unabsorbed flux in the
0.5-10 keV is 1.1E-10 erg/cm2/s with an 85% contribution from the 40" radius
inner core. Timing analysis of the High-Resolution Camera (HRC) data failed to
detect any pulsations. We put a 16% upper limit on the pulsed fraction. We
derive the physical parameters of the putative pulsar and compare them with
those of other plerions (such as the Crab and 3C 58). G21.5-0.9 remains the
only plerion whose size in X-rays is bigger than in the radio. Deep radio
observations will address this puzzle.Comment: 23 pages including 11 figures and 3 tables; accepted by ApJ June 22,
2001; to appear in Oct 20, 2001 issue of Ap
Study of the Composite Supernova Remnant MSH 11-62
We present the analysis of the X-ray data collected during an observation of
the supernova remnant (SNR) MSH 11-62 by the Advanced Satellite for Cosmology
and Astrophysics (ASCA). We show that MSH 11-62 is a composite remnant whose
X-ray emission comes from two distinct contributions. Nonthermal, synchrotron
emission, localized to a region of radius (~~)3' (consistent with a point
source) dominates the total flux above 2 keV. A second contribution comes from
a thermal component, extended up to a radius of (~~)6' and detected only at
energies below 2keV. The spatial and spectral analysis imply the presence of a
neutron star losing energy at a rate of about (10**36 - 10**37) ergs/s. No
pulsed emission is detected and we set a limit on the pulsed fraction of 10%.
This is consistent with the lack of a radio pulsar in the remnant, which may
indicate that the pulsed emission from the rapidly rotating compact object that
should be powering the synchrotron nebula is beamed and our viewing direction
is unfavorable. In either event, the central neutron star deposits much of its
spin-down energy into the surrounding synchrotron nebula where, through direct
imaging with broadband satellites such as ASCA, it is possible to study the
energetics and evolution of the compact remnant.Comment: 30 pages, including 5 figures, Latex. To appear in ApJ (May 20, 1998
issue, Vol. 499.
A New ASCA and ROSAT Study of the Supernova Remnant: G272.2-3.2
G272.2-3.2 is a supernova remnant (SNR) characterized by an apparent
centrally brightened X-ray morphology and thermally dominated X-ray emission.
Because of this combination of Sedov-type (thermal emission) and non-Sedov type
(non-shell like morphology) features, the remnant is classified as a ``thermal
composite'' SNR. This class of remnant is still poorly understood due in part
to the difficulties in modeling accurately all the physical conditions which
shape the emission morphology. In this paper we present a combined analysis of
data from the ASCA and ROSAT satellites coupled with previous results at other
wavelengths. We find that the X-ray emission from G272.2-3.2 is best described
by a non-equilibrium ionization (NEI) model with a temperature around 0.70 keV,
an ionization timescale of 3200 cm^-3 yr and a relatively high column density
(NH about 10^22 atoms/cm^2). We look into the possible explanations for the
apparent morphology of G272.2-3.2 using several models (among which both cloud
evaporation and thermal conduction models). For each of the models considered
we examine all the implications on the evolution of G272.2-3.2.Comment: 26 pages, 8 figures. Accepted for publication in Ap
Six Years of Chandra Observations of Supernova Remnants
We present a review of the first six years of Chandra X-ray Observatory
observations of supernova remnants. From the official "first-light" observation
of Cassiopeia A that revealed for the first time the compact remnant of the
explosion, to the recent million-second spectrally-resolved observation that
revealed new details of the stellar composition and dynamics of the original
explosion, Chandra observations have provided new insights into the supernova
phenomenon. We present an admittedly biased overview of six years of these
observations, highlighting new discoveries made possible by Chandra's unique
capabilities.Comment: 82 pages, 28 figures, for the book Astrophysics Update
Neglected Tropical Diseases outside the Tropics
Neglected Tropical Diseases (NTDs) have been targeted due to their prevalence and the burden of disease they cause globally, but there has been no significant focus in the literature on the subject of NTDs as a group in immigrants and travelers, and no specific studies on the emerging phenomenon of imported NTDs. We present the experience of a Tropical Medicine Unit in a major European city, over a 19-year period, describing and comparing NTDs diagnosed amongst immigrants, travelers and travelers visiting friends and relatives (VFRs). NTDs were diagnosed outside tropical areas and occurred more frequently in immigrants, followed by VFR travelers and then by other travelers. The main NTDs diagnosed in immigrants were onchocerciasis, Chagas disease and ascariasis; most frequent NTDs in travelers were schistosomiasis, onchocerciasis and ascariasis, and onchocerciasis and schistosomiasis in VFRs. Issues focusing on modes of transmission outside endemic areas and how eradication programs for some NTDs in endemic countries may have an impact in non-tropical Western countries by decreasing disease burden in immigrants, are addressed. Adherence to basic precautions such as safe consumption of food/water and protection against arthropod bites could help prevent many NTDs in travelers
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