Final Report Summary - EXTREMEPHYSICS (The slowest accreting neutron stars and black holes: New ways to probe fundamental physics)
X-ray binaries harbor a neutron star or black hole accreting matter from a companion star. Since their first discovery in the late 1960’s, such systems have been studied intensively and have taught us a lot about the accretion processes at work in those systems and also about the extreme physical processes relevant in those systems (strong gravitational fields, ultra-dense matter, super high magnetic field strengths). Nevertheless, many questions remain. However, most of our information and understanding has been obtained using rather bright systems that were easily detectable and studied. In the last decade, a new class of sub-luminous accreting neutron stars and black holes has been identified. The underlying physical processes which give rise to those very faint X-ray luminosities are not understood, but it is clear that those systems provide new windows into the accretion processes at work in X-ray binaries, as well as probes into the extreme physical processes related to neutron star and black holes.
Unfortunately, their faint X-ray luminosities and consequently their rather low X-ray fluxes at Earth, implies that those systems are difficult to find and the quality of the data has been very limited compared to that obtained for the brighter systems. The goal of this project is to significantly increase the number of known systems and to obtain high quality observational data, in X-rays but also at other wavelengths (primarily optical and near-infrared), aiming to elucidate the nature of these enigmatic objects. Utilizing dedicated programs on several of the X-ray satellites currently in orbit, we have significantly increased the total number of known very-faint systems and systematically obtained high quality X-ray and optical/near-infrared data of many individual sources.
With the funding provided by the ERC starting grant a research group (consisting of the PI, a PhD student and a postdoc) could be formed which studied the properties of the slowest accretors. Using the obtained data sets it was found that black holes behave differently at low accretion rates than neutron stars, mainly because the neutron star becomes visible. Since black holes do not have surfaces, this component is not present in them. This gives additional support that black holes have event horizons. The fact that we see so clearly the neutron star surface appearing allows for new ways to study the neutron star by using this surface emission. In addition, the behavior of thermonuclear X-ray bursts on the surface of the accreting neutron stars was different from that predicted (as extrapolated from the faster accreting systems) leading to new theories of this phenomena, specifically tailored to the very low accretion rates in our systems. Moreover, new insights in the crust structure (only partly replaced by accreted matter) of the neutron stars in those systems has resulted in new theoretical venues to study the heating and cooling of accreting neutron stars. Finally, the multi-wavelenght studies have shown that at the lowest accretion rate the accretion flow geometry changes significantly leading to new input to the accretion models.
Unfortunately, their faint X-ray luminosities and consequently their rather low X-ray fluxes at Earth, implies that those systems are difficult to find and the quality of the data has been very limited compared to that obtained for the brighter systems. The goal of this project is to significantly increase the number of known systems and to obtain high quality observational data, in X-rays but also at other wavelengths (primarily optical and near-infrared), aiming to elucidate the nature of these enigmatic objects. Utilizing dedicated programs on several of the X-ray satellites currently in orbit, we have significantly increased the total number of known very-faint systems and systematically obtained high quality X-ray and optical/near-infrared data of many individual sources.
With the funding provided by the ERC starting grant a research group (consisting of the PI, a PhD student and a postdoc) could be formed which studied the properties of the slowest accretors. Using the obtained data sets it was found that black holes behave differently at low accretion rates than neutron stars, mainly because the neutron star becomes visible. Since black holes do not have surfaces, this component is not present in them. This gives additional support that black holes have event horizons. The fact that we see so clearly the neutron star surface appearing allows for new ways to study the neutron star by using this surface emission. In addition, the behavior of thermonuclear X-ray bursts on the surface of the accreting neutron stars was different from that predicted (as extrapolated from the faster accreting systems) leading to new theories of this phenomena, specifically tailored to the very low accretion rates in our systems. Moreover, new insights in the crust structure (only partly replaced by accreted matter) of the neutron stars in those systems has resulted in new theoretical venues to study the heating and cooling of accreting neutron stars. Finally, the multi-wavelenght studies have shown that at the lowest accretion rate the accretion flow geometry changes significantly leading to new input to the accretion models.