On the gravitational collapse in anti-de Sitter space-time

We study the semiclassical evolution of a self-gravitating thick shell in Anti-de Sitter space-time. We treat the matter on the shell as made of quantized bosons and evaluate the back-reaction of the loss of gravitational energy which is radiated away as a non-adiabatic effect. A peculiar feature of anti-de Sitter is that such an emission also occurs for large shell radius, contrary to the asymptotically flat case.Comment: Changed to Elsevier style, 9 pages, 1 figure. Minor typos corrected. Accepted for publication in Phys. Lett.

The progenitor of a type Ia supernova with a short delay time?

HD49798/RXJ0648.0-4418 is the only known X-ray binary composed of a hot subdwarf and a massive white dwarf (M=1.28+/-0.05 Msun). This system, with an orbital period of 1.55 days, is the outcome of a common envelope evolution, most likely of a pair of stars with initial masses of about 8-10 Msun. When the hot subdwarf, currently in a He-burning phase, will expand again and fill its Roche-lobe, the enhanced mass transfer can rapidly bring the already massive white dwarf above the Chandrasekhar limit. The possible final fate, either a Type Ia supernova explosion or an accretion induced collapse, is particularly interesting in view of the high rotational velocity of this star, which has the shortest spin period (13 s) observed in a white dwarf.Comment: 4 pages, to appear in Proceedings of IAU Symp. 281, Binary Paths to Type Ia Supernovae Explosions, ed. R. Di Stefano and M. Ori

Recent Results on the Anomalous X-ray Pulsars

The ''Anomalous X-ray Pulsars'' (AXPs) are a small group of X-ray pulsars characterized by periods in the 5-10 s range and by the absence of massive companion stars. There are now 7 possible members of this class of objects. We review recent observational results on their X-ray spectra, spin period evolution, and searches for orbital motion and discuss the implications for possible models.Comment: 4 pages, 1 figures. To appear in The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE, Nuclear Physics B Proceedings Supplements, L. Scarsi, H. Bradt, P. Giommi and F. Fiore (eds.), Elsevier Science B.

Discovery of X-rays from the supernova remnant G0.9+0.1

During the $BeppoSAX$ survey of the Galactic Center region, we have discovered X-ray emission from the central region of the supernova remnant G0.9+0.1. The high interstellar absorption (N_H about 3 times 10^{23} cm^-2) is consistent with a distance of order of 10 kpc and, correspondingly, an X-ray luminosity of about 10^{35} erg s^{-1}. Although we cannot completely rule out a thermal origin of the X-ray emission, its small angular extent (radius of about 2'), the good fit with a power law, the presence of a flat spectrum radio core, and the estimated SNR age of a few thousand years, favour the interpretation in terms of synchrotron emission powered by a young, energetic pulsar.Comment: 4 pages, 1 figure. Uses espcrc2.sty (included). To appear in The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE, Nuclear Physics B Proceedings Supplements, L. Scarsi, H. Bradt, P. Giommi and F. Fiore (eds.), Elsevier Science B.

Stability of 1+1 dimensional causal relativistic viscous hydrodynamics

The stability of the 1+1 dimensional solution of Israel-Stewart theory is investigated. Firstly, the evolution of the temperature and the ratio of the bulk pressure over the equilibrium pressure of the background is explored. Then the stability with linear perturbations is studied by using the Lyapunov direct method. It shows that the shear viscosity may weaken the instability induced by the large peak of bulk viscosity around the phase transition temperature $T_c$.Comment: 18 pages, 4 figures, 1 table; to be published in Nuclear Physics

Thermodynamics of a collapsing shell in an expanding Universe

We describe the quasi-static collapse of a radiating, spherical shell of matter in de Sitter space-time using a thermodynamical formalism. It is found that the specific heat at constant area and other thermodynamical quantities exhibit singularities related to phase transitions during the collapse.Comment: 9 pages, 2 figures, accepted for publication in Phys. Lett.

Intensity-hardness correlation and deep infrared observation of the anomalous X-ray pulsar 1RXS J170849-400910

We report here on X-ray and IR observations of the Anomalous X-ray Pulsar 1RXS J170849-400910. First, we report on new XMM-Newton, Swift-XRT and Chandra observations of this AXP, which confirm the intensity-hardness correlation observed in the long term X-ray monitoring of this source by Rea et al. (2005). These new X-ray observations show that the AXP flux is rising again, and the spectrum hardening. If the increase in the source intensity is indeed connected with the glitches and a possible bursting activity, we expect this source to enter in a bursting active phase around 2006-2007. Second, we report on deep IR observations of 1RXS J170849-400910, taken with the VLT-NACO adaptive optics, showing that there are many weak sources consistent with the AXP position. Neither star A or B, previously proposed by different authors, might yet be conclusively recognised as the IR counterpart of 1RXS J170849-400910. Third, using Monte Carlo simulations, we re-address the calculation of the significance of the absorption line found in a phase-resolved spectrum of this source by Rea et al. (2003), and interpreted as a resonant scattering cyclotron feature.Comment: 7 pages, 5 color figures; Astrophysics & Space Science, in press ("Isolated Neutron Stars"; London, UK

Bulk viscosity driving the acceleration of the Universe

The possibility that the present acceleration of the universe is driven by a kind of viscous fluid is exploited. At background level this model is similar to the generalized Chaplygin gas model (GCGM). But, at perturbative level, the viscous fluid exhibits interesting properties. In particular the oscillations in the power spectrum that plagues the GCGM are not present. Possible fundamental descriptions for this viscous dark energy are discussed.Comment: Latex file, 8 pages, 3 eps figure