Ultraluminous X-ray Sources Isolated Neutron Stars White Dwarfs in Interacting Binaries: Hydrogen Burning and Winds Accretion in Classical Novae

The new generation of X-ray satellites has boosted X-ray astronomy to unprecedented levels in the last decade. The future will bring marvellous spectroscopic capabilities to study even extreme physical phenomena, like gravity around black-holes. Even before the planned high throughput missions, smaller scale missions, like Symbol-X, will extend imaging to higher energy (10-80 keV), widening the horizons of X-ray astronomy.

At INAF-Padova X-ray astronomy focuses on compact objects, especially black holes in ultraluminous X-ray sources, isolated neutron stars and white dwarfs in accreting and hydrogen burning systems. We investigate the basic physical parameters (mass, radius, spin, surface magnetic field) and the evolution and fate of these systems.

Ultraluminous X-ray sources (ULXs) are extragalactic, non-nuclear X-ray sources that appear to belong to a given galaxy, so that the distance is known, but are extremely luminous. As a matter of fact the luminosity of ULXs is orders of magnitude above the Eddington limit for a few solar masses. The object powering the X-ray luminosity may be a black hole of ~100 solar masses. We are leading a national project, funded by a PRIN-INAF grant, aimed at tackling the most challenging questions on these puzzling sources. The planned activity in this area will concentrate on the measurement of the orbital period of ULX binary systems, the investigation of the correlation between the X-ray spectral and timing properties of ULXs, the comparison of theoretical models of ULX binary systems with observations and the exploration of alternative scenarios for the formation of their compact remnants.

Our research on X-ray dim isolated neutron stars has led to the first identification of the seventh member of this class of objects. We have shown evidence of a periodicity in the X-ray flux and an absorption feature in the X-ray spectrum, yielding an estimate of the surface magnetic field consistent with that of magnetars. Recently we reported the very first detection of a candidate optical counterpart for this object. In the future, we plan to compare the light curves and spectra of this unique class of objects with theoretical models.

We also study the extremely hot white dwarfs in post-outburst novae. The amazing recurrent nova RS Ophiuchi was observed and monitored at all wavelengths by all facilities in the world during its 2006 outburst. We obtained the first high resolution X-ray spectra of both the white dwarf and the nova wind, with the first emission lines spectra of the ejecta. This research is continuing with new observations of other classical novae. One important aim is to understand their X-ray variability and periodic oscillations. Thanks to these observations, we are beginning to gain insight also in the influence of white dwarf rotation and magnetic field in the nova outburst and secular evolution.

We are also engaged in X-ray population studies in nearby galaxies. Our aim is to follow the possible path of supersoft X-ray sources towards SN Ia evolution. In order to address issues such as: "Does hydrogen burning continue at extremely high rates on secular time scales, or do strong winds from the binary systems prevent accumulation of sufficient mass for the supernova?", we observe the frequency and evolution of close binary supersoft X-ray sources in external galaxies. We also monitor the sources for several years, as we recently did on Andromeda.