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Frozen accretion and spectacular X-ray bursts from the black hole in our Galactic Center

Researchers of the Max Planck Institute for Astrophysics suggest that a very cold inactive accretion disk is present in the center of Our Galaxy. The revolutionary idea explains two outstanding mysteries of 'our' black hole at once: why there seems to be no accretion now and also what produces frequent and strong X-ray flares.

star passing through a disk

Figure 1: 2D Hydrodynamical simulations of a star passing through an inactive cold disk. The star is modeled as a rigid sphere centered at x,z = 0,0 with radius of 1011 cm. The left image shows the gas density, with red representing unshocked disk material, and yellow the shocked gas. The star is moving along the z-axis in the upward direction, drilling a narrow hole in the disk. Velocity vectors are shown on the right.

expected flares and eclipses

Figure 2: Examples of star's orbits and the expected flares and eclipses. The star moving on the ABCD track hits the disk in points C and D where intense X-ray and weaker near infra-red flares will be emitted. The coordinates of four such points (e.g. CD and EF) will define the plane of the disk rotation. Eclipses in points A and B will constrain the disk size. (From Nayakshin & Sunyaev 2003).

The center of our Galaxy hosts a super massive black hole that is about 3 million times more massive than our Sun ( Schoedel et al. 2002; see also  T. Ensslin highlight contribution).

A decade long mystery is why the black hole radiates only about 0.0001 of the luminosity that is expected  from the accretion of hot gas that is observed to be around it. MPA researchers suggest that there exists a cold inactive disk around the black hole, a remnant of past vigorous accretion. While the disk is too cold (about a 100 K) to be directly visible, it is much more massive than the hot gas seen in X-rays. According to the new research, when the hot gas comes into physical contact with the cold disk, the heat is rapidly drained from the hot gas via thermal conduction. The hot flow effectively gets frozen, and settles onto the inactive disk at large distances from the black hole. Since the hot gas is prevented from falling into the black hole's potential well, little X-ray radiation is emitted (Nayakshin 2003.

The second mystery of our Galactic Center are bursts of X-ray radiation that happen roughly once per day and can be more than 100 times brighter than the black hole quiescent emission. These X-ray flares were discovered two years ago (Baganoff et al. 2001 ) and are so unique that no reasonable explanation was givenup to now. However the inactive disk hypothesis made by the MPA researchers offers a very natural and easily testable model for the flares. The point is that there are thousands of stars in the innermost part of our Galaxy (see the movie on the MPE infrared group's home page). These stars rotate with extraordinary high velocities around the super massive black hole. Generally speaking, these stars will hit the inactive disk twice per orbit. While passing through the disk, the stars create a shock wave which emits X-ray radiation (Fig. 1). The properties of flares expected from such events are very similar to that actually observed (Nayakshin & Sunyaev 2003). In addition, the model shows that similar star-disk encounters yield flares that are too weak to observe in more distant Galactic centers, explaining why these X-ray flares have been observed only from our Galactic Center.

The MPA researchers further suggest that observations of such flares and expected eclipses (see Fig. 2) in the future will allow us to determine the inactive disk properties (i.e. mass, size, plane and direction of rotation). This information should yield crucial clues on the history of black hole formation in our Galaxy. In addition, since our Galactic Center is presumed to be similar to that of many other inactive Galaxies, the discovery of the inactive disk in our Galaxy could signal a change in the paradigm of the accretion processes in all the inactive Galaxies.

Sergei Nayakshin

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