riassunto2

Speaker_	Zane , Silvia
Talk_	X-ray Dim Isolated Neutron Stars: A review of the latest timing and spectral properties
Abstract_	In the last few years considerable observational resources have been devoted to study the faint thermal emission from neutron stars and to search for features in their spectrum. Particularly interesting targets are seven peculiar sources, discovered by ROSAT in the last decade, which are associated with radio silent isolated neutron stars (XDINSs in the following) and are located within a few hundred parsecs, close enough to be studied with the latest X-ray orbiting observatories. These sources are characterized by clean thermal emission at energies of a few hundred eVs without any trace of contamination from a surrounding supernova remnant or magnetospheric activity. Therefore they represent important targets to study the direct emission from the neutron star surface layers, which is the only source of information on the physical conditions of the star. Surprisingly, when deep observations have been carried out with the new generation satellites, the first results have been negative: no features have been found whatsoever in the spectrum of the brightest object (the prototype of the class, RXJ1856.5-3754), not even after 500 ksec of Chandra observations. However, subsequent Chandra and XMM-Newton observations revealed that all the other members of the XDINSs class do exhibit spectral features. Broad absorption features appear at energies ~200-700 eV and are usually interpreted either as atomic transitions of bound species (hydrogen or heavier elements) in a strong magnetic field or as proton cyclotron resonances. In both cases, the inferred magnetic field is quite high, in the 10^13-10^14 G range. Perhaps most surprising of all was the recent discovery that the spectrum of RXJ0720 has evolved over the last four years from a featureless blackbody into a spectrum that exhibits a broad gaussian absorption line with energy ~270 eV and width ~60 eV. In addition, the X-ray temperature increased and the inferred emission area has decreased. Since this discovery, we are monitoring the source and we find evidence for a cyclic behaviour, most likely do to precession over a ~7years period. In this talk I will review the current status of XDINSs observations, I will present some of our more recent results (including the analysis of the spectral changes in RXJ0720 and a confirmation of the proper motion of RXJ 1605) and I will discuss some of the proposed interpretations for the observed phenomena.