75 research outputs found
Modelling spectral and timing properties of accreting black holes: the hybrid hot flow paradigm
The general picture that emerged by the end of 1990s from a large set of
optical and X-ray, spectral and timing data was that the X-rays are produced in
the innermost hot part of the accretion flow, while the optical/infrared (OIR)
emission is mainly produced by the irradiated outer thin accretion disc. Recent
multiwavelength observations of Galactic black hole transients show that the
situation is not so simple. Fast variability in the OIR band, OIR excesses
above the thermal emission and a complicated interplay between the X-ray and
the OIR light curves imply that the OIR emitting region is much more compact.
One of the popular hypotheses is that the jet contributes to the OIR emission
and even is responsible for the bulk of the X-rays. However, this scenario is
largely ad hoc and is in contradiction with many previously established facts.
Alternatively, the hot accretion flow, known to be consistent with the X-ray
spectral and timing data, is also a viable candidate to produce the OIR
radiation. The hot-flow scenario naturally explains the power-law like OIR
spectra, fast OIR variability and its complex relation to the X-rays if the hot
flow contains non-thermal electrons (even in energetically negligible
quantities), which are required by the presence of the MeV tail in Cyg X-1. The
presence of non-thermal electrons also lowers the equilibrium electron
temperature in the hot flow model to <100 keV, making it more consistent with
observations. Here we argue that any viable model should simultaneously explain
a large set of spectral and timing data and show that the hybrid
(thermal/non-thermal) hot flow model satisfies most of the constraints.Comment: 26 pages, 13 figures. To be published in the Space Science Reviews
and as hard cover in the Space Sciences Series of ISSI - The Physics of
Accretion on to Black Holes (Springer Publisher
An overview of jets and outflows in stellar mass black holes
In this book chapter, we will briefly review the current empirical
understanding of the relation between accretion state and and outflows in
accreting stellar mass black holes. The focus will be on the empirical
connections between X-ray states and relativistic (`radio') jets, although we
are now also able to draw accretion disc winds into the picture in a systematic
way. We will furthermore consider the latest attempts to measure/order jet
power, and to compare it to other (potentially) measurable quantities, most
importantly black hole spin.Comment: Accepted for publication in Space Science Reviews. Also to appear in
the Space Sciences Series of ISSI - The Physics of Accretion on to Black
Holes (Springer Publisher
Radio emission and jets from microquasars
To some extent, all Galactic binary systems hosting a compact object are
potential `microquasars', so much as all galactic nuclei may have been quasars,
once upon a time. The necessary ingredients for a compact object of stellar
mass to qualify as a microquasar seem to be: accretion, rotation and magnetic
field. The presence of a black hole may help, but is not strictly required,
since neutron star X-ray binaries and dwarf novae can be powerful jet sources
as well. The above issues are broadly discussed throughout this Chapter, with a
a rather trivial question in mind: why do we care? In other words: are jets a
negligible phenomenon in terms of accretion power, or do they contribute
significantly to dissipating gravitational potential energy? How do they
influence their surroundings? The latter point is especially relevant in a
broader context, as there is mounting evidence that outflows powered by
super-massive black holes in external galaxies may play a crucial role in
regulating the evolution of cosmic structures. Microquasars can also be thought
of as a form of quasars for the impatient: what makes them appealing, despite
their low number statistics with respect to quasars, are the fast variability
time-scales. In the first approximation, the physics of the jet-accretion
coupling in the innermost regions should be set by the mass/size of the
accretor: stellar mass objects vary on 10^5-10^8 times shorter time-scales,
making it possible to study variable accretion modes and related ejection
phenomena over average Ph.D. time-scales. [Abridged]Comment: 28 pages, 13 figures, To appear in Belloni, T. (ed.): The Jet
Paradigm - From Microquasars to Quasars, Lect. Notes Phys. 794 (2009
The balance of power: accretion and feedback in stellar mass black holes
In this review we discuss the population of stellar-mass black holes in our
galaxy and beyond, which are the extreme endpoints of massive star evolution.
In particular we focus on how we can attempt to balance the available accretion
energy with feedback to the environment via radiation, jets and winds,
considering also possible contributions to the energy balance from black hole
spin and advection. We review quantitatively the methods which are used to
estimate these quantities, regardless of the details of the astrophysics close
to the black hole. Once these methods have been outlined, we work through an
outburst of a black hole X-ray binary system, estimating the flow of mass and
energy through the different accretion rates and states. While we focus on
feedback from stellar mass black holes in X-ray binary systems, we also
consider the applicability of what we have learned to supermassive black holes
in active galactic nuclei. As an important control sample we also review the
coupling between accretion and feedback in neutron stars, and show that it is
very similar to that observed in black holes, which strongly constrains how
much of the astrophysics of feedback can be unique to black holes.Comment: To be published in Haardt et al. Astrophysical Black Holes. Lecture
Notes in Physics. Springer 201
Self-help interventions for anxiety disorders: An overview.
Anxiety disorders are highly prevalent and are associated with a marked impairment in quality of life and a huge economic cost to society. Unfortunately, a considerable number of people who struggle with anxiety do not seek or receive adequate treatment. Self-help interventions have been proposed to constitute a relatively cheap, effective, efficient, and low-threshold intervention for anxiety disorders. This paper offers a critical discussion of their advantages and disadvantages and the evidence for their effectiveness. We conclude that guided self-help can play a major role in mental health care for patients with anxiety disorders. However, several research questions need to be answered before broad-scale dissemination is possible. The Internet will continue to play a prominent role in the further development of this field of research and clinical practice
'Disc-jet' coupling in black hole X-ray binaries and active galactic nuclei
In this chapter I will review the status of our phenomenological
understanding of the relation between accretion and outflows in accreting black
hole systems. This understanding arises primarily from observing the relation
between X-ray and longer wavelength (infrared, radio) emission. The view is
necessarily a biased one, beginning with observations of X-ray binary systems,
and attempting to see if they match with the general observational properties
of active galactic nuclei.Comment: 28 pages, 15 figures, To appear in Belloni, T. (ed.): The Jet
Paradigm - From Microquasars to Quasars, Lect. Notes Phys. 794 (2009
Essential medicines for emergency care in Africa
Objectives: Essential medicines lists (EMLs) are efficient means to ensure access to safe and effective medications. The WHO has led this initiative, generating a biannual EML since 1977. Nearly all countries have implemented national EMLs based on the WHO EML. Although EMLs have given careful consideration to many public health priorities, they have yet to comprehensively address the importance of medicines for treating acute illness and injury. Methods: We undertook a multi-step consensus process to establish an EML for emergency care in Africa. After a review of existing literature and international EMLs, we generated a candidate list for emergency care. This list was reviewed by expert clinicians who ranked the medicines for overall inclusion and strength of recommendation. These medications and recommendations were then evaluated by an expert group. Medications that reached consensus in both the online survey and expert review were included in a draft emergency care EML, which underwent a final in-person consensus process. Results: The final emergency care EML included 213 medicines, 25 of which are not in the 2017 WHO EML but were deemed essential for clinical practice by regional emergency providers. The final EML has associated recommendations of desirable or essential, and is subdivided by facility level. Thirty-nine medicines were recommended for basic facilities, an additional 96 for intermediate facilities (e.g. district hospitals), and an additional 78 for advanced facilities (e.g. tertiary centres). Conclusion: The 25 novel medications not currently on the WHO EML should be considered by planners when making rational formularies for developing emergency care systems. It is our hope that these resource-stratified lists will allow for easier implementation, and will be a useful tool for practical expansion of emergency care delivery in Africa. Keywords: Essential medicines, Emergency care, Africa, Health syste
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