Wednesday, June 27, 2007

Relativistic Jets from GRMHD

For some time now it has been known that rotating black holes can produce extremely powerful jets of gamma rays and very high energy particles that shoot out along their axes of rotation. This phenomena is believed to be the power source for active galatic nuclei, pulsars, quasars, a number of other phenomena ending in -sars, and possibly some types of gamma ray bursts. The mechanism for creating these jets, however, remained mysterious. To complicate matters, whatever process is powering these jets appears to be extremely efficient in converting gravitational potential energy from the in-falling material into these high energy streams of photons and particles. So how does a black hole with an accretion disk swirling around it efficiently shoot jets out of its poles?

Nothing like this jumps out of either the linearized magnetohydrodynamics equations or the Einstein equations, so people knew that it had to be some kind of funky non-linear effect, but was it from the MHD equations, the Einstein equations, or some combination of the two? Since the jets were always created by in-falling plasma it seemed safe to say that MHD was involved. And because these jets were only observed around black holes, it seemed pretty reasonable to assume that GR played a role in creating these jets, but the real question was whether it was just the non-linear GR effects that were needed or was this something to do with the event horizon. As Joe mentioned in his post, there is reason to believe that event horizons are trickier than we generally expect, so could these jets be some sort of mechanism for the black hole to deal with matter trying to pass it's event horizon?

Well, the debate now appears to be settled. Researchers at MIT and the University of Wisconsin have published evidence for relativistic jets coming out of a binary star system with an accreting neutron star (pictured on the left). Since neutron stars obviously don't have event horizons the jets must be produced by the non-linearity in the GRMHD (General Relativistic Magnetohydrodynamics) equations alone and not by some trick of the event horizon.

As a side note, they also found that both the overall energy in the jets and the efficiency of the process decreased in the case of the neutron star, perhaps signaling that the effect scales with the gravitational field strength (i.e. curvature). That gives numerical people a good idea of where to look in the non-linear terms for the driving mechanism. GO NUMERICAL METHODS!!!

You can look at the pre-print or read the ScienceDaily summary for more details.

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