250 research outputs found
Chandra Observations of the Crab-like Supernova Remnant G21.5-0.9
Chandra observations of the Crab-like supernova remnant G21.5-0.9 reveal a
compact central core and spectral variations indicative of synchrotron burn-off
of higher energy electrons in the inner nebula. The central core is slightly
extended, perhaps indicating the presence of an inner wind-shock nebula
surrounding the pulsar. No pulsations are observed from the central region,
yielding an upper limit of ~40% for the pulsed fraction. A faint outer shell
may be the first evidence of the expanding ejecta and blast wave formed in the
initial explosion, indicating a composite nature for G21.5-0.9.Comment: 4 pages, 2 figures, formatted with emulateapj, submitted to ApJ
Maternal feeding behaviour and young children's dietary quality: A cross-sectional study of socially disadvantaged mothers of two-year old children using the Theory of Planned Behaviour
Background: Having breakfast, eating food 'cooked from scratch' and eating together as a family have health and psychosocial benefits for young children. This study investigates how these parentally determined behaviours relate to children's dietary quality and uses a psychological model, the Theory of Planned Behaviour (TPB), to investigate socio-cognitive predictors of these behaviours in socially disadvantaged mothers of young children in Scotland. Method: Three hundred mothers of children aged 2 years (from 372 invited to participate, 81% response rate), recruited via General Practitioners, took part in home-based semi-structured interviews in a cross-sectional survey of maternal psychological factors related to their children's dietary quality. Regression analyses examined statistical predictors of maternal intentions and feeding behaviours. Results: Mothers of children with poorer quality diets were less likely than others to provide breakfast every day, cook from 'scratch' and provide 'proper sit-down meals'. TPB socio-cognitive factors (intentions, perceived behavioural control) significantly predicted these three behaviours, and attitudes, norms, and perceived behavioural control significantly predicted mothers' intentions, with medium to large effect sizes. Conclusions: Interventions to improve young children's dietary health could benefit from a focus on modifying maternal motivations and attitudes in attempts to improve feeding behaviours
Charges on Strange Quark Nuggets in Space
Since Witten's seminal 1984 paper on the subject, searches for evidence of
strange quark nuggets (SQNs) have proven unsuccessful. In the absence of
experimental evidence ruling out SQNs, the validity of theories introducing
mechanisms that increase their stability should continue to be tested. To
stimulate electromagnetic SQN searches, particularly space searches, we
estimate the net charge that would develop on an SQN in space exposed to
various radiation baths (and showers) capable of liberating their less strongly
bound electrons, taking into account recombination with ambient electrons. We
consider, in particular, the cosmic background radiation, radiation from the
sun, and diffuse galactic and extragalactic -ray backgrounds. A
possible dramatic signal of SQNs in explosive astrophysical events is noted.Comment: CitationS added, new subsection added, more discussion, same
numerical result
Guiding the Way to Gamma-Ray Sources: X-ray Studies of Supernova Remnants
Supernova remnants have long been suggested as a class of potential
counterparts to unidentified gamma-ray sources. The mechanisms by which such
gamma-rays can arise may include emission from a pulsar associated with a
remnant, or a variety of processes associated with energetic particles
accelerated by the SNR shock. Imaging and spectral observations in the X-ray
band can be used to identify properties of the remnants that lead to gamma-ray
emission, including the presence of pulsar-driven nebulae, nonthermal X-ray
emission from the SNR shells, and the interaction of SNRs with dense
surrounding material.Comment: 16 pages, 11 figures, To appear in the proceedings of the workshop:
"The Nature of the Unidentified Galactic Gamma-Ray Sources" held at INAOE,
Mexico, October 2000, (A.Carraminana, O. Reiner and D. Thompson, eds.
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.
Discovery of Fast-Moving X-Ray--Emitting Ejecta Knots in the Oxygen-Rich Supernova Remnant Puppis A
We report on the discovery of fast-moving X-ray--emitting ejecta knots in the
Galactic Oxygen-rich supernova remnant Puppis A from XMM-Newton observations.
We find an X-ray knotty feature positionally coincident with an O-rich
fast-moving optical filament with blue-shifted line emission located in the
northeast of Puppis A. We extract spectra from northern and southern regions of
the feature. Applying a one-component non-equilibrium ionization model for the
two spectra, we find high metal abundances relative to the solar values in both
spectra. This fact clearly shows that the feature originates from metal-rich
ejecta. In addition, we find that line emission in the two regions is
blue-shifted. The Doppler velocities derived in the two regions are different
with each other, suggesting that the knotty feature consists of two knots that
are close to each other along the line of sight. Since fast-moving O-rich
optical knots/filaments are believed to be recoiled metal-rich ejecta, expelled
to the opposite direction against the high-velocity central compact object, we
propose that the ejecta knots disclosed here are also part of the recoiled
material.Comment: 16 pages, 3 figure
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
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