2,120 research outputs found
Probing X-ray burst -- accretion disk interaction in low mass X-ray binaries through kilohertz quasiperiodic oscillations
The intense radiation flux of Type I X-ray bursts is expected to interact
with the accretion flow around neutron stars. High frequency quasiperiodic
oscillations (kHz QPOs), observed at frequencies matching orbital frequencies
at tens of gravitational radii, offer a unique probe of the innermost disk
regions. In this paper, we follow the lower kHz QPOs, in response to Type I
X-ray bursts, in two prototypical QPO sources, namely 4U 1636-536 and 4U
1608-522, as observed by the Proportional Counter Array of the Rossi X-ray
Timing Explorer. We have selected a sample of 15 bursts for which the kHz QPO
frequency can be tracked on timescales commensurable with the burst durations
(tens of seconds). We find evidence that the QPOs are affected for over ~200 s
during one exceptionally long burst and ~100 s during two others (although at a
less significant level), while the burst emission has already decayed to a
level that would enable the pre-burst QPO to be detected. On the other hand,
for most of our burst-kHz QPO sample, we show that the QPO is detected as soon
as the statistics allow and in the best cases, we are able to set an upper
limit of ~20 s on the recovery time of the QPO. This diversity of behavior
cannot be related to differences in burst peak luminosity. We discuss these
results in the framework of recent findings that accretion onto the neutron
star may be enhanced during Type I X-ray bursts. The subsequent disk depletion
could explain the disappearance of the QPO for ~100 s, as possibly observed in
two events. However, alternative scenarios would have to be invoked for
explaining the short recovery timescales inferred from most bursts. Clearly the
combination of fast timing and spectral information of Type I X-ray bursts
holds great potential in the study of the dynamics of the inner accretion flow
around neutron stars.Comment: 8 pages, 9 figures, appears in Astronomy & Astrophysics, Volume 567,
id.A80, published 07/201
Galactic center at very high-energies
Employing data collected during the first 25 months' observations by the
Fermi-LAT, we describe and subsequently seek to model the very high energy
(>300 MeV) emission from the central few parsecs of our Galaxy. We analyze the
morphological, spectral and temporal characteristics of the central source,
1FGL J1745.6-2900. Remarkably, the data show a clear, statistically significant
signal at energies above 10 GeV, where the Fermi-LAT has an excellent angular
resolution comparable to the angular resolution of HESS at TeV energies, which
makes meaningful the joint analysis of the Fermi and HESS data. Our analysis
does not show statistically significant variability of 1FGL J1745.6-2900. Using
the combination of Fermi data on 1FGL J1745.6-2900 and HESS data on the
coincident, TeV source HESS J1745-290, we show that the spectrum of the central
gamma-ray source is inflected with a relatively steep spectral region matching
between the flatter spectrum found at both low and high energies. We seek to
model the gamma-ray production in the inner 10 pc of the Galaxy and examine, in
particular, cosmic ray (CR) proton propagation scenarios that reproduce the
observed spectrum of the central source. We show that a model that instantiates
a transition from diffusive propagation of the CR protons at low energy to
almost rectilinear propagation at high energies (given a reasonable
energy-dependence of the assumed diffusion coefficient) can well explain the
spectral phenomenology. In general, however, we find considerable degeneracy
between different parameter choices which will only be broken with the addition
of morphological information that gamma-ray telescopes cannot deliver given
current angular resolution limits.We argue that a future analysis done in
combination with higher-resolution radio continuum data holds out the promise
of breaking this degeneracy.Comment: submitted to Ap
Assessing the Feasibility of Cosmic-Ray Acceleration by Magnetic Turbulence at the Galactic Center
The presence of relativistic particles at the center of our galaxy is
evidenced by the diffuse TeV emission detected from the inner
of the Galaxy. Although it is not yet entirely clear whether the origin of the
TeV photons is due to hadronic or leptonic interactions, the tight correlation
of the intensity distribution with the distribution of molecular gas along the
Galactic ridge strongly points to a pionic-decay process involving relativistic
protons. In earlier work, we concluded that point-source candidates, such as
the supermassive black hole Sagittarius A* (identified with the HESS source
J1745-290), or the pulsar wind nebulae dispersed along the Galactic plane,
could not account for the observed diffuse TeV emission from this region.
Motivated by this result, we consider here the feasibility that the cosmic rays
populating the Galactic Center (GC) region are accelerated in situ by magnetic
turbulence. Our results indicate that even in a highly conductive environment,
this mechanism is efficient enough to energize protons within the intercloud
medium to the \ga TeV energies required to produce the HESS emission.Comment: Accepted for publication in Ap
Prenatal diagnosis and abortion for congenital abnormalities: is it ethical to provide one without the other?
This target article considers the ethical implications of providing prenatal diagnosis (PND) and antenatal screening services to detect fetal abnormalities in jurisdictions that prohibit abortion for these conditions. This unusual health policy context is common in the Latin American region. Congenital conditions are often untreated or under-treated in developing countries due to limited health resources, leading many women/couples to prefer termination of affected pregnancies. Three potential harms derive from the provision of PND in the absence of legal and safe abortion for these conditions: psychological distress, unjust distribution of burdens between socio-economic classes, and financial burdens for families and society. We present Iran as a comparative case study where recognition of these ethical issues has led to the liberalization of abortion laws for fetuses with thalassemia. We argue that physicians, geneticists and policymakers have an ethical and professional duty of care to advocate for change in order to ameliorate these harms. Keywords: abortion, fetal abnormality, Latin America, prenatal diagnosis, reproductive ethics, womenThis article was written by Dr Ainsley Newson during the time of her employment with the University of Bristol, UK (2006-2012). Self-archived in the Sydney eScholarship Repository with permission of Bristol University, Sept 2014
Reconsidering the origin of the X-ray emission lines in GRB 011211
We reanalyze the XMM--Newton data of GRB 011211 showing that the spectral
features, interpreted by Reeves et al. (2002, 2003) as due thermal emission
from a collisionally ionized plasma, can be also reproduced by a reflection
model (with ionization parameter ). We discuss the implications
of this interpretation, estimating the total mass required in the simplified
case of a funnel geometry. We conclude that a moderate clumping of the
reprocessing material (corresponding to a filling factor of the order of ) is required. Finally we show that, if this interpretation is correct,
a bright quasi--thermal component is expected in the optical--UV band
(containing about 90% of the luminosity of the illuminating continuum), whose
presence can be used to test the reflection model.Comment: revised version accepted for publication by A&
High Energy Cosmic-ray Diffusion in Molecular Clouds: A Numerical Approach
The propagation of high-energy cosmic rays through giant molecular clouds
constitutes a fundamental process in astronomy and astrophysics. The diffusion
of cosmic-rays through these magnetically turbulent environments is often
studied through the use of energy-dependent diffusion coefficients, although
these are not always well motivated theoretically. Now, however, it is feasible
to perform detailed numerical simulations of the diffusion process
computationally. While the general problem depends upon both the field
structure and particle energy, the analysis may be greatly simplified by
dimensionless analysis. That is, for a specified purely turbulent field, the
analysis depends almost exclusively on a single parameter -- the ratio of the
maximum wavelength of the turbulent field cells to the particle gyration
radius. For turbulent magnetic fluctuations superimposed over an underlying
uniform magnetic field, particle diffusion depends on a second dimensionless
parameter that characterizes the ratio of the turbulent to uniform magnetic
field energy densities. We consider both of these possibilities and parametrize
our results to provide simple quantitative expressions that suitably
characterize the diffusion process within molecular cloud environments. Doing
so, we find that the simple scaling laws often invoked by the high-energy
astrophysics community to model cosmic-ray diffusion through such regions
appear to be fairly robust for the case of a uniform magnetic field with a
strong turbulent component, but are only valid up to TeV particle
energies for a purely turbulent field. These results have important
consequences for the analysis of cosmic-ray processes based on TeV emission
spectra associated with dense molecular clouds.Comment: Accepted for publication in The Astrophysical Journa
Broadband X-ray spectral analysis of the Seyfert 1 galaxy GRS 1734-292
We discuss the broadband X-ray spectrum of GRS 1734-292 obtained from
non-simultaneous XMM-Newton and NuSTAR observations, performed in 2009 and
2014, respectively. GRS1734-292 is a Seyfert 1 galaxy, located near the
Galactic plane at . The NuSTAR spectrum ( keV) is dominated by
a primary power-law continuum with and a high-energy
cutoff keV, one of the lowest measured by NuSTAR in a
Seyfert galaxy. Comptonization models show a temperature of the coronal plasma
of keV and an optical depth, assuming a slab
geometry, or a similar temperature and
assuming a spherical geometry. The 2009 XMM-Newton
spectrum is well described by a flatter intrinsic continuum
() and one absorption line due to Fe\textsc{XXV}
K produced by a warm absorber. Both data sets show a modest iron
K emission line at keV and the associated Compton reflection, due
to reprocessing from neutral circumnuclear material
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