Friday, March 04, 2016

More black holes

Although IceCube didn't have a clear signal to report that was coincident with the LIGO event, the Fermi gamma ray telescope did, sort of. They weren't pointing at the area at the time, and their resolution isn't so good off-axis. Still, seeing something at that moment and in that direction is very interesting.

The thing is, the LIGO event is supposed to have come from 2 merging black holes. When those giant eggbeaters started swirling round each other, you'd expect them to have splattered the accretion disk all over the kitchen, so to speak. There should have been nothing left around them.

So what caused the flash of gamma rays?

I asked Markus about what happens when black holes merge if one is charged, and he pointed me to an article by someone whose general relativity fu is much better than mine, who had a paper out 4 days after the announcement.

Black holes have mass, spin, and charge--and nothing else. You could think of feeding a black hole a diet of nothing but electrons. The result would be a charged black hole. After you fed it enough electrons you wouldn't be able to stuff any more in--the electrostatic repulsion would be too huge, bigger than the black hole's gravity. That maximum charge is obviously pretty big--I won't bother with numbers.

Zhang's paper points out that if one of the black holes was charged, its rapid orbit would twist up magnetic flux lines, and the merger would cause a pulse of radio waves. If the field was intense enough, you'd get gamma rays. Alas, the charge required is gigantic: about 1/10,000 of the maximum charge.

It is pretty hard to see how anything would accumulate that much charge. Over time, if your star is positively charged it will attract electrons, and if negatively charged will attract protons--and things will even out. (Having two fast black holes might change things a little--if there were enough matter left to work with.)

Must be something else.

No comments: