Sunday, February 14, 2016

Fermi saw gamma rays 0.4 seconds after the LIGO merger

Update: Nude Socialist quotes Avi Loeb of Harvard who said in a new preprint that the gamma rays may indicate that the merging couple of black holes could have been twin babies inside a mother's belly and the mother was a very massive star with the weight around 100 Suns. I sympathize with that.
Ted has informed us about a fascinating new paper by the Fermi gamma-ray telescope previously known as Glast,
Fermi GBM Observations of LIGO Gravitational Wave event GW150914 (arXiv; NASA copy)
Some 0.4 seconds after the gravitational wave from the black hole merger detected as gravitational waves in LIGO, Fermi saw a signal of gamma rays above \(50\keV\) with the false positive probability of 0.2%.

Note that 0.4 seconds is \(10^{-17}\) times 1.3 billion years. So if you believe that the gamma-ray and gravitational-wave signals came from the same event, this represents a test of the equality between the speed of light and the speed of gravitational waves whose relative precision is approximately \(10^{-17}\).

The directions of the gamma rays (right, dark red) and the direction of the gravitational waves (left, dark red) seem nontrivially compatible.

The timing and direction seems to agree so well that I am willing to think that the gamma rays came from the region of the LIGO black hole merger. Note that only 1 gamma ray burst is observed every day. Do you expect the September 14th GRB to agree with the LIGO signal within 0.4 seconds if they have nothing to do with each other? I don't.

How were these gamma rays produced? How many?

In the energies per photon between \(1\keV\) and \(10\MeV\), Fermi estimates – assuming that the source coincides with LIGO's – the power to be about \(2\times 10^{42}\) watts, plus minus a factor of two (multiplicatively). The peak power emitted in gravitational waves was some \(10^{48}\) watts – plus minus an order of magnitude or so.

So it's enough if one millionth of the energy deposited to the gravitational waves was converted to gamma rays. I find it totally plausible. The space around the black hole was probably rather empty and clean. But there could be some matter and the merging black holes could have accelerated lots of gas and plasma which emitted some gamma rays.

What do you think? Note that the prevailing wisdom among astrophysicists is that stellar black holes should have no disk of ordinary matter around them. But I don't really know their reasoning and whether it makes any sense.

What bothers me is that I don't quite see a reason why the black hole merger should escalate the emission of the gamma rays relatively to more mundane accretion around individual black holes (either the two initial ones or the single final one). Well, maybe what happens in between the two black holes around 0:23 of the merger video, when they're really getting merged, is probably more extreme than any accretion etc. at more generic moments. And maybe the gamma ray emissions only began when the black holes were already merged into one (as the timing suggests) and this giant was sweeping the matter around it in the vicinity of a greater event horizon.

If you have access to Fermi's events, you may also look for the moments on October 12th and December 26th (8:21-11:46; 1:29-4:55, respectively) when LIGO made the other detections known to the TRF readers. Didn't Fermi observe some bursts at those moments as well?

LIGO has brought us the ability to hear the Universe but the combination of hearing and seeing (in gamma rays, in this case) could make the story even more exciting.

See also Brian Koberlein's text in Forbes.

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