Real time localization of Gamma Ray Bursts with INTEGRAL

The INTEGRAL satellite has been successfully launched in October 2002 and has recently started its operational phase. The INTEGRAL Burst Alert System (IBAS) will distribute in real time the coordinates of the GRBs detected with INTEGRAL. After a brief introduction on the INTEGRAL instruments, we describe the main IBAS characteristics and report on the initial results. During the initial performance and verification phase of the INTEGRAL mission, which lasted about two months, two GRBs have been localized with accuracy of about 2-4 arcmin. These observations have allowed us to validate the IBAS software, which is now expected to provide quick (few seconds delay) and precise (few arcmin) localization for about 10-15 GRBs per year.Comment: 6 pages, latex, 3 figures, submitted to Adv. Sp. Res., Proceedings of the 34th COSPAR Scientific Assembly, Houston, 10-19 October 200

Two and a half years of GRB localizations with the INTEGRAL Burst Alert System

We review the results on Gamma-ray Bursts obtained during the first two and a half years of operations of the INTEGRAL Burst Alert System (IBAS). In many cases GRB coordinates have been distributed with an unprecedented combination of accuracy (3 arcmin) and speed (20-30 s). The resulting rapid follow-ups at other wavelengths, including sensitive XMM-Newton and Swift observations, have led to several interesting results.Comment: A shorter version of this paper will be published in the Proceedings of the 4th Workshop "Gamma-Ray Bursts in the Afterglow Era", Roma, 2004 October 18-22, eds. L. Piro, L. Amati, S. Covino, and B. Gendre. Il Nuovo Cimento, in pres

On the magnetic fields of Be/X-ray pulsars in the Small Magellanic Cloud

We explore the possibility to explain the properties of the Be/X-ray pulsars observed in the Small Magellanic Cloud within the magnetic levitation accretion scenario. This implies that their X-ray emission is powered by a wind-fed accretion onto a neutron star (NS) which captures matter from a magnetized stellar wind. The NS in this case is accreting matter from a non-keplerian magnetically levitating disc (ML-disc) which is surrounding its magnetosphere. This allows us to explain the observed periods of the pulsars in terms of spin equilibrium without the need of invoking dipole magnetic fields outside the usual range ~ 10^11- 10^13 G inferred from cyclotron features of Galactic high mass X-ray binaries. We find that the equilibrium period of a NS, under certain conditions, depends strongly on the magnetization of the stellar wind of its massive companion and, correspondingly, on the magnetic field of the massive companion itself. This may help to explain why similar NSs in binaries with similar properties rotate with different periods yielding a large scatter of periods of the accretion-powered pulsar observed in SMC and our galaxy.Comment: 6 pages, 1 figure, Published in MNRAS 454, 3760-3765 (2015

News on the X-ray emission from hot subdwarf stars

In latest years, the high sensitivity of the instruments on-board the XMM-Newton and Chandra satellites allowed us to explore the properties of the X-ray emission from hot subdwarf stars. The small but growing sample of X-ray detected hot subdwarfs includes binary systems, in which the X-ray emission is due to wind accretion onto a compact companion (white dwarf or neutron star), as well as isolated sdO stars, in which X-rays are probably due to shock instabilities in the wind. X-ray observations of these low-mass stars provide information which can be useful for our understanding of the weak winds of this type of stars and can lead to the discovery of particularly interesting binary systems. Here we report the most recent results we have recently obtained in this research area.Comment: 8 pages, 3 figures. To appear in the Proceedings of the 8th Meeting on Hot Subdwarf Stars and Related Objects, 9-15 July 2017, Cracow, Poland. Eds. A. Baran, A. E. Lynas-Gray, Open Astronomy, in pres

Swift monitoring of the massive X-ray binary SAX J0635.2+0533

SAX J0635.2+0533 is a binary pulsar with a very short pulsation period ($P$ = 33.8 ms) and a high long-term spin down ($\dot P$ $>$ 3.8$\times10^{-13}$ s s$^{-1}$), which suggests a rotation-powered (instead of an accretion-powered) nature for this source. While it was discovered at a flux level around 10$^{-11}$ erg cm$^{-2}$ s$^{-1}$, between 2003 and 2004 this source was detected with XMM-Newton at an average flux of about 10$^{-13}$ erg cm$^{-2}$ s$^{-1}$; moreover, the flux varied of over one order of magnitude on time scales of a few days, sometimes decreasing below $3\times10^{-14}$ erg cm$^{-2}$ s$^{-1}$. Since both the rotation-powered and the accretion-powered scenarios have difficulties to explain these properties, the nature of SAX J0635.2+0533 is still unclear. Here we report on our recent long-term monitoring campaign on SAX J0635.2+0533 carried out with Swift and on a systematic reanalysis of all the RXTE observations performed between 1999 and 2001. We found that during this time interval the source remained almost always active at a flux level above 10$^{-12}$ erg cm$^{-2}$ s$^{-1}$.Comment: 8 pages, 6 figures, 2 tables. Accepted for publication in Astronomy & Astrophysic