The value function of an asymptotic exit-time optimal control problem

We consider a class of exit--time control problems for nonlinear systems with a nonnegative vanishing Lagrangian. In general, the associated PDE may have multiple solutions, and known regularity and stability properties do not hold. In this paper we obtain such properties and a uniqueness result under some explicit sufficient conditions. We briefly investigate also the infinite horizon problem

Radio-loudness in black hole transients: evidence for an inclination effect

Accreting stellar-mass black holes appear to populate two branches in a radio:X-ray luminosity plane. We have investigated the X-ray variability properties of a large number of black hole low-mass X-ray binaries, with the aim of unveiling the physical reasons underlying the radio-loud/radio-quiet nature of these sources, in the context of the known accretion-ejection connection. A reconsideration of the available radio and X-ray data from a sample of black hole X-ray binaries confirms that being radio-quiet is the more normal mode of behaviour for black hole binaries. In the light of this we chose to test, once more, the hypothesis that radio loudness could be a consequence of the inclination of the X-ray binary. We compared the slope of the `hard-line' (an approximately linear correlation between X-ray count rate and rms variability, visible in the hard states of active black holes), the orbital inclination, and the radio-nature of the sources of our sample. We found that high-inclination objects show steeper hard-lines than low-inclination objects, and tend to display a radio-quiet nature (with the only exception of V404 Cyg), as opposed to low-inclination objects, which appear to be radio-loud(er). While in need of further confirmation, our results suggest that - contrary to what has been believed for years - the radio-loud/quiet nature of black-hole low mass X-ray binaries might be an inclination effect, rather than an intrinsic source property. This would solve an important issue in the context of the inflow-outflow connection, thus providing significant constraints to the models for the launch of hard-state compact jets.Comment: 16 pages, 6 figures, accepted for pubblication on MNRA

Collaborative semantic web browsing with Magpie

Web browsing is often a collaborative activity. Users involved in a joint information gathering exercise will wish to share knowledge about the web pages visited and the contents found. Magpie is a suite of tools supporting the interpretation of web pages and semantically enriched web browsing. By automatically associating an ontology-based semantic layer to web resources, Magpie allows relevant services to be invoked as well as remotely triggered within a standard web browser. In this paper we describe how Magpie trigger services can provide semantic support to collaborative browsing activities

Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1

NGC 55 ULX1 is a bright Ultraluminous X-ray source located 1.78 Mpc away. We analysed a sample of 20 Swift observations, taken between 2013 April and August, and two Chandra observations taken in 2001 September and 2004 June. We found only marginal hints of a limited number of dips in the light curve, previously reported to occur in this source, although the uncertainties due to the low counting statistics of the data are large. The Chandra and Swift spectra showed clearly spectral variability which resembles those observed in other ULXs. We can account for this spectral variability in terms of changes in both the normalization and intrinsic column density of a two-components model consisting of a blackbody (for the soft component) and a multicolour accretion disc (for the hard component). We discuss the possibility that strong outflows ejected by the disc are in part responsible for such spectral changes.Comment: 9 pages, 6 figure; accepted to be published on MNRA

“Between a Rock and a Hard Place”: Australia’s Mandatory Detention of Asylum Seekers

For fourteen years Australia has detained asylum seekers arriving unlawfully in its territory. It also intercepts asylum seekers arriving in the territorial waters, detaining them in third countries and preventing them from seeking refugee status in Australia (the “Pacific Solution”). This paper traces the development of the policy, its current implementation, the justification employed by the government for maintaining it, and its legality under international law. On examination of these issues, it is evident that the justification for the mandatory policy is flawed; that the costs of the policy – in terms of the physical and mental well-being of asylum seekers themselves, and the social and financial impact on the Australian community – are too great, and it puts Australia in breach of its obligations under international law. However, the government has not canvassed alternatives to the mandatory detention policy and has no intention of changing it. This leaves asylum seekers who enter Australian territory unlawfully literally and figuratively between a “rock and hard place.”Depuis 14 ans l’Australie a eu pour politique de détenir les demandeurs d’asile qui arrivent illégalement sur son territoire. Il intercepte aussi les demandeurs d’asile qui pénètrent dans ses eaux territoriales, les détenant dans des pays tiers et les empêchant de revendiquer le statut de réfugié en Australie (la « solution du Pacifique », comme on l’appelle). Cet article retrace le développement de cette politique, la manière dont elle est présentement mise en oeuvre, les raisons évoquées par le gouvernement pour justifier son maintien, et sa légalité au regard du droit international. Lorsqu’on examine ces questions de près, il devient clair que la justification offerte pour cette politique obligatoire comporte des lacunes ; que les coûts de la politique – en terme du bien-être physique et mental des demandeurs d’asile, aussi bien qu’en terme de l’impact social et financier sur la société australienne – sont trop élevés, et qu’elle place l’Australie en position de violation de ses obligations vis-à-vis du droit international. Cependant, le gouvernement n’a pas exploré les alternatives possibles à la politique de détention obligatoire et n’a aucunement l’intention de la changer. Ainsi, les demandeurs d’asile qui entrent illégalement sur le territoire australien se retrouvent quasiment coincés