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Mysterious Gamma-Ray Burst May Be Linked To Gravitational Wave Find (latimes.com)
mdsolar quotes a report from Los Angeles Times: After a decades-long search, scientists announced early this year that they had detected gravitational waves probably coming from the merger of two black holes back in September. Now, a team of scientists using NASA's Fermi Gamma-ray Space Telescope say they spotted a brief flash of gamma rays that occurred less than half a second after that long-sought gravitational wave signal. The gamma-ray outburst, described at the American Physical Society's April meeting in Salt Lake City, has not been definitively linked to that first gravitational wave signal, and scientists weren't able to pinpoint its exact origin -- just that they came from the same general area. But if other astronomers begin to find a similar pattern, the results do raise the intriguing possibility that such high-energy events might not be quite as 'invisible' as we thought. The first gravitational wave signal rolled through the Laser Interferometer Gravitational-wave Observatory on Sept. 14, hitting the Louisiana detector first and then the one in Washington state seven milliseconds later, telling researchers that the signal must have come from the southern hemisphere.
This page has a possible explanation: http://www.universetoday.com/1...
They say that the two black holes formed inside a giant star, and collapsed. This created both the gamma ray burst and the gravitational wave. Then the light needed to travel through the star's matter. While the gravitational wave can travel with the speed of light in vacuum, the light requires more time, that's where the .4 seconds delay comes from.
They say also that in order for there being a gamma ray burst, there needs to be matter close to the colliding black holes. So this rules out a pair of black holes that orbited each other for a long time, because they would have cleared out the region.
Disclaimer: I am no physicist, I just read the papers and quote them :)
There is this paper which does some speculation for what could have been the cause: http://arxiv.org/pdf/1602.0473...
It says that the "hydrogen envelope" (probably most of the star), would be required to be ejected before the merger, otherwise it would choke the whole gamma ray burst already, meaning that we wouldn't have detected detect a gamma ray burst the first place if there was a hydrogen envelope. Probably something would have happened what happens right now in the sun, the "envelope" would convert the gamma ray radiation to lower frequency radiation, like visible light.
So you might be right with most of the star being already gone when the event happened.
The paper also tries to find an explanation on the cause for the delay:
"For a progenitor star in the mass range [of 100 to 1000 solar masses], most of the observed 0.4 s delay can be accounted for by the neutrino cooling timescale or by the extra time it takes the GRB to jet to cross the star relative to GWs for a jet Lorentz factor of [gamma around 4-7]".
I admit I can't really understand what they said here. The "neutrino cooling" is most likely cooling by neutrino radiation.
But all references to neutrino cooling I could find in the paper indicate that this cooling is happening before the two black holes form, as a requirement for their formation, and not as an event between the BH merger and the GRB emission.
Also I'm not sure what this has to do with a jet. I mean jets are these matter streams that leave an object at relativistic speeds (therefore also the Lorentz factor mention). But it is very unlikely that for the first GW we detect (GWs get emitted into all directions) we also detect a GRB emission which only gets to us because of a jet being targeted at us (something that affects only a tiny fraction of the star's sky), isn't it?
So no, it isn't the speed of light that is an issue here, as an S-wave that originated in just the right place could ripple across the surface much FASTER than light. It is almost certainly the lack of coincidence with detectors designed and utilized to detect S-waves, ruling out this sort of explanation.
Expanding on this a bit: for the S-wave to appear at both detectors with a plausible speed-of-light separation (i.e. a few milliseconds), it must have originated from a quite specific zone (somewhere on a hyperboloid running through the Earth, within a thickness of a few tens of metres). If an earthquake occurs somewhere in the Earth, it has maybe a 1-in-100,000 chance of being in that zone.
If there were tens of thousands of suitable earthquakes per month, then this might still be a plausible explanation: it just so happened that one of these earthquakes hit the sweet spot to appear on both detectors within an interval resembling a light-speed travel delay. But that's not the case. After their standard processing of the data (which is meant to cut out earthquakes, instrumental problems, etc.), this burst was the *only* event which appeared on both detectors with this magnitude. So that leaves us with two possibilities:
(1) An earthquake not only slipped through their processing (which is plausible), but also happened to occur in just such a position so as to fake a gravitational-wave signal, despite 1-to-100,000 odds against this (which is less plausible).
(2) Gravitational waves.