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Exploitation of the connections from X-ray binaries to active galactic nuclei

Final Report Summary - XRBGAL (Exploitation of the connections from X-ray binaries to active galactic nuclei)

Accretion onto compact astrophysical objects like black holes or neutron stars is the most efficient source of power in the Universe. It is not only responsible for the powerful quasars in distant galaxies, but also for Gamma-ray bursts and the stellar X-ray binaries. In these systems, matter is flowing from the surrounding space of the compact object to the object. While the matter is falling into the potential well of the object, energy is liberated that can be observed on earth.

Basic accretion theory, at least in the case of black holes, should be scale invariant. If everything is described in units of the radius of the black hole, the system should be oblivious of the central mass. In reality however, the observed properties seem to be different and observers name objects of different masses differently (e.g. stellar and supermassive black holes). In recent years a number of approaches have been started to unify all observable properties in a 'unified model' of accretion.

This project had two main directions:
- Firstly, this project was aimed to refine and reinforce the unified models of all accreting sources in a unified model.
- Secondly, the results should be applied in the context of Galaxy evolution and formation.

Prior to the start of this project we have found scaling relations that directly connect accreting stellar and supermassive black holes. A correlation between two observables in one class of objects (e.g. the radio and the X-ray luminosity of X-ray binaries) can usually be extended to the other class by including the mass as a third parameter. Instead of a simple correlation, one has a plane in the space given by the mass and the two observables. Examples are the so called fundamental plane of black hole activity and the variability plane of accreting black holes. It is not only possible to connect both classes of black holes using observables, but also their evolution seems to have many similarities.

The project in the context of a Marie Curie Reintegration grant was planned for a timeframe of 3 years. However, the early departure of the researcher has cut the project time to under a year. Thus, not all scientific goals of the project could be reached. The main focus of the project had been on the first objectives, namely the reinforcement of the unified models of accretion.

One key result was the direct detection of inefficient accretion flows in X-ray binaries (XRBs). It is usually thought that the difference between an XRB in its hard and it soft state is that the accretion flow in the hard state is inefficient while it is efficient in the soft state. It is however hard to estimate the accretion rate of a source, as one can only observe the emission from the object and not directly the amount of matter flowing towards the compact object. By studying the light-curves of decaying outbursts of XRBs, we were able to show that there is a distinctive break at the time of the state transition in the light-curve. This is fully consistent with the underlying theory and further support the idea that hard state objects do show inefficient accretion.

We were furthermore able to show the accreting white dwarfs also share many observational similarities with X-ray binaries. It had long been thought that one class of accreting white dwarfs, the cataclysmic variables, do not show radio jets, which are usually observed in XRBs. We have now shown that also nova-like cataclysmic variables, do emit radio emission, most likely originating in a jet. Finally, in collaboration with a PhD student co-supervised by the researcher we were able to calculate mass limits for the central black hole in the globular cluster NGC 6338 using the scaling relations which are at the heart of this project.

In the proposal of the Marie Curie Reintegration grant we requested funding for traveling to conferences and scientific collaborations. Thanks to the funding we were able to communicate the findings to the larger community.