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Does the black hole in the center of our Galaxy rotate clockwise?

New research at the Max-Planck-Institute for Astrophysics proposes a connection between the senses of rotation of the accretion disk around a black hole and that of its emitted radio emission.  Following the new model they should rotate retrograde with respect to each other.  Since the circular polarisation of radio emission can be measured, this would allow for the first time to determine the rotational sense of a black hole. The model is supported by observations of SgrA*, the black hole in the centre of our own Galaxy.

Figure 1: The linearly polarised radio emission of energetic electrons within the back side magnetic field lines (green arrow indicats the plane of linear polarisation) converts partly into circular polarisation during its journey throught the foreground magnetic fields. This conversion is due to the fact that radio emission, with linear polarisation parallel to the magnetic field lines, propagates slower than radio emission, with perpendicular linear polarisation. Our polarised radio emission has perpendicular and parallel components, which oscillate in phase. Caused by the different propagation speeds they get out of phase. The combination of the out of phase osscilating components produces a circular oscillation of the electric field, as sketched.

For 20 years now astronomer have been observing that the radio emission of the 2.6 Million solar masses heavy SgrA* is rotating counter-clockwise.  This is suprising, since this rotational sense is nearly the only constant quantity in the violent events around the black hole.  The accretion disk, a gaseous disk, rotates around the black hole within roughly one hour and therewhile launches radio-emitting jets. The luminosity of the jets, and also their linear and circular polarisation vary strongly.

The new model is based on two assumptions that are motivated by these observations. First, there is a constant mechanism which explains the persistent rotational sense of the radio emission. Second, this mechanism works under strongly varying conditions. In contrast to previous models, this approach does not assume that the magnetic fields within the jets have a permanent polarity (for over 20 years in the case of SgrA*, which would be surprising because of the rapid variations of physical quantities of that system within hours). The new mechanism works even if the magnetic fields change their polarity. It is based on the assumption, that - due to the rotation of the accretion disk - the magnetic fields are twisted.

The radiation of the jets is modified by the twisted magnetic fields in a way that the oscillations of the electric fields are partly changed: Some fraction of the linear polarisation is converted into circular polarisation. Figure 1 shows that the circular polarisation rotates retrograde with respect to the accretion disk.

This implies in the case of SgrA* that its accretion disk rotates clockwise. With this model the 20 year constant rotational sense of the circular polarisation can be explained. It only depends on the rotational sense of the accretion disk, which should change very rarely. The rotation of the accretion disk is likely due to winds of a group of young, hot stars, which orbit around SgrA*. Observations show that they also orbit clockwise around the black hole (see figure 2).

If additional support for the proposed mechanism could be found,  the still unobservable rotational sense of accretion disks around black holes can be determined observationally by measurements of their circular polarized radio emission.

Torsten Enßlin


Star cluster around SgrA*

Figure 2: Star cluster around SgrA* (image from Gentzel 2000). The arrows indicate the directions of individual stars. The blue marked stars are young, hot stars. Their winds are beliefed to feed SgrA* with gas. Since the stars rotate preferentially clockwise, the accretion disk formed from their winds should rotate in the same sense - clockwise, as also predicted by the model.

Molecular cloud around SgrA*

Figure 3: Molecular cloud in the centre of the Galaxy (blue: radio by Yusef-Zadek & Morris. red: FIR by Phillip, Zykla, & Mezger). SgrA* and the central star cluster is located in the cloud's centre. The rotation of the cloud is - as that of the Milkey Way - retrograde with respect to the direction of the young, hot stars and the rotational sense of SgrA* as derived from the circular polarisation.

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