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  Current Research Highlight :: February 2005 all highlights

Hysteresis in spectral state transitions of accreting black holes

Scientists at the Max-Planck-Institute for Astrophysics describe a further step in understanding the flow of matter towards objects of very high concentration of mass. These compact objects are known as "black holes", with gravitation so dominant that all matter and radiation fall into them if close enough.

Fig. 1: Schematic view of accretion in hard and soft state.

Fig. 2: Schematic lightcurve of an X-ray nova outburst

Fig. 3: Evaporation rates from accretion disk in hard and soft state as function of the distance from the black hole (values measured in Eddington rates and Schwarzschild radius)

Of great interest for the researchers are the properties of the matter flow towards a black hole (black holes of stellar mass or supermassive black holes in the centers of galaxies). Due to its angular momentum the infalling gas spirals inward. Close to the compact object this can happen either in form of a thin accretion disk or as a more spherical hot flow (Fig.1). Farther away from the center always the former one is present. We here discuss the accretion flows observed for low-mass X-ray binaries, close binary stars where one is a black hole or a neutron star. The spectrum of the observed X-ray light is different for the two accretion modes: (1) a very hard spectrum (up to 100 keV) originates from the very energetic particles of hot flow or (2) a soft spectrum (a few keV) which is radiated from the much less hot thin accretion disk. The radiation from the region close to the black hole dominates the spectrum.

A special feature of the observed spectra is the change between the two types. During the long lasting "quiescent" phases of the binaries the spectrum is hard. This means that at a certain distance from the black hole the disk accretion which is commonly the mode further out, turns into the hot 'coronal" flow (the word "corona" is used in comparison with the solar corona, where we recognise the hot flow as a crown around the sun during a solar eclipse). Taking into account the physics of the interaction between disk and corona we can determine which amount of gas evaporates from the thin cool disk to the coronal hot flow and at which distance from the black hole finally all matter has entered the hot flow. This distance depends on the mass flow rate in the cool disk. The lower this mass flow rate is the larger is the distance where the the thin cool disk is truncated. The coronal evaporation reaches a maximum at a certain distance from the black hole. If the mass flow rate in the accretion disk is larger than this maximum the disk can no longer be completely evaporated and instead contimues all the way to the central black hole finally carrying nearly all of the accreting mass again. The radiation is now from the cool inner disk surface and the spectrum becomes soft. Therefore with increasing and decreasing mass flow as occuring during an X-ray novae outburst (Fig. 2) the accretion mode changes and simultanously the spectrum from hard to soft and back to a hard one.

If now we follow the lightcurve during an outburst we see that these changes hard-soft and soft-hard do not occur at the same luminosity or mass accretion rate. Instead the second transition occurs at a luminosity lower by a factor of about 3 to 5. Where does this unexpected difference come from? The surprising phenomenon can now be understood: The rate of mass evaporation depends on the radiation from further in, either hard from the hot flow at the time of the change from hard to soft state, or soft from the much cooler thin disk in the inner region at the time of the reverse change, as shown in Fig.3.

F. Meyer, E. Meyer-Hofmeister, B.F. Liu

Further information:

E. Meyer-Hofmeister, F. Meyer, B.F. Liu, Astronomy & Astrophysics, March 2005, volume 432, Issue 1, pp.181-187 (linkPfeilExtern.gifastro-ph/0411145)

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