438 research outputs found
The decline and fall of GRS 1915+105: the end is nigh?
The Galactic microquasar GRS 1915+105 has been in a continuous state of outburst since 1992, over 20 times longer than any other black hole X-ray transient. Assuming that the outburst is powered via accretion of an irradiated gaseous disc, we calculate how the predicted outburst duration varies according to the efficiency of the self-irradiation mechanism. At least one current model leads to the conclusion that the end of the outburst is imminent. The timing of the decline of GRS 1915+105, whenever it arrives, will be an excellent discriminator of the self-irradiation mechanism in X-ray transients, allowing us to infer the fraction of the disc that is heated by the incident X-rays and the magnitude of the mass loss rate in the form of a wind
Hydrodynamic modelling of accretion flows
In the proceedings of this, and of several recent close binary conferences,
there have been several contributions describing smoothed particle
hydrodynamics simulations of accretion disks. It is apposite therefore to
review the numerical scheme itself with emphasis on its advantages for disk
modelling, and the methods used for modelling viscous processes.Comment: 3 pages, to appear in proceedings of IAU Colloquium 194: Compact
binaries in the galaxy and beyon
Optimal Uncertainty Quantification
We propose a rigorous framework for Uncertainty Quantification (UQ) in which
the UQ objectives and the assumptions/information set are brought to the
forefront. This framework, which we call \emph{Optimal Uncertainty
Quantification} (OUQ), is based on the observation that, given a set of
assumptions and information about the problem, there exist optimal bounds on
uncertainties: these are obtained as values of well-defined optimization
problems corresponding to extremizing probabilities of failure, or of
deviations, subject to the constraints imposed by the scenarios compatible with
the assumptions and information. In particular, this framework does not
implicitly impose inappropriate assumptions, nor does it repudiate relevant
information. Although OUQ optimization problems are extremely large, we show
that under general conditions they have finite-dimensional reductions. As an
application, we develop \emph{Optimal Concentration Inequalities} (OCI) of
Hoeffding and McDiarmid type. Surprisingly, these results show that
uncertainties in input parameters, which propagate to output uncertainties in
the classical sensitivity analysis paradigm, may fail to do so if the transfer
functions (or probability distributions) are imperfectly known. We show how,
for hierarchical structures, this phenomenon may lead to the non-propagation of
uncertainties or information across scales. In addition, a general algorithmic
framework is developed for OUQ and is tested on the Caltech surrogate model for
hypervelocity impact and on the seismic safety assessment of truss structures,
suggesting the feasibility of the framework for important complex systems. The
introduction of this paper provides both an overview of the paper and a
self-contained mini-tutorial about basic concepts and issues of UQ.Comment: 90 pages. Accepted for publication in SIAM Review (Expository
Research Papers). See SIAM Review for higher quality figure
Periodic Variability During the X-ray Decline of 4U 1636-53
We report the onset of a large amplitude, statistically significant
periodicity (~46 d) in the RXTE/ASM data of the prototype X-ray burster 4U
1636-53, the X-ray flux of which has been gradually declining over the last
four years. This behaviour is remarkably similar to that observed in the
neutron star LMXB KS 1731-260, which is a long-term transient. We also report
on an INTEGRAL/IBIS observation of 4U 1636-53 during its decline phase, and
find that the hard X-ray flux (20-100 keV) indicates an apparent
anti-correlation with soft X-rays (2-12 keV). We argue that 4U 1636-53 is
transiting from activity to quiescence, as occurred in KS 1731-260. We also
suggest that the variability during the X-ray decline is the result of an
accretion rate variability related to the X-ray irradiation of the disc.Comment: 6 pages. Accepted by MNRA
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