I just learned something cool: 0.4 seconds after LIGO saw those gravitational waves on 14 September 2015, a satellite named Fermi detected a burst of X-rays!
• V. Connaughton et al, Fermi GBM observations of LIGO gravitational wave event GW150914.
It lasted one second. It was rather weak (for such things). The photons emitted ranged from 50 keV to 10 MeV in energy, with a peak around 3.5 MeV. The paper calls this event a ‘hard X-ray source’. Wikipedia says photons with an energy over 100 keV deserve the name gamma rays, while those between 10 keV and 100 keV are ‘hard X-rays’. So, maybe this event deserves to be maybe a gamma ray burst. I suppose it’s all just a matter of semantics: it’s not as if there’s any sharp difference between a highly energetic X-ray and a low-energy gamma ray.
Whatever you call it, this event does not appear connected with other previously known objects. It’s hard to tell exactly where it happened. But its location is consistent with what little we know about the source of the gravitational waves.
If this X-ray burst was caused by the same event that created the gravitational waves, that would be surprising. Everyone assumed the gravitational waves were formed by two large black holes that had been orbiting each other for millions or billions of years, slowly spiraling down. In this scenario we don’t expect much electromagnetic radiation when the black holes finally collide.
Perhaps those expectations are wrong. Or maybe—just maybe—both the gravitational waves and X-rays were formed during the collapse of a single very large star! That’s what typically causes gamma ray bursts—we think. But it’s not at all typical—as far as we know—for a large star to form two black holes when it collapses! And that’s what we’d need to get that gravitational wave event: two black holes, which then spiral down and merge into one!
Here’s an analysis of the issue:
• Abraham Loeb, Electromagnetic counterparts to black hole mergers detected by LIGO.
As he notes, the collapsing star would need to have an insane amount of angular momentum to collapse into a dumb-bell shape and form two black holes, each roughly 30 times the mass of our Sun, which then quickly spiral down and collide.
Furthermore, as Tony Wells pointed to me, the lack of neutrinos argues against the idea that this event involved a large collapsing star:
• ANTARES collaboration, High-energy neutrino follow-up search of Gravitational wave event GW150914 with ANTARES and IceCube.
To add to the muddle, another satellite devoted to observing gamma rays, called INTEGRAL, did not see anything:
It will take a while to sort this out.
But luckily, the first gravitational wave burst seen by LIGO was not the only one! Dennis Overbye of the New York Times writes:
Shortly after the September event, LIGO recorded another, weaker signal that was probably also from black holes, the team said. According to Dr. Weiss, there were at least four detections during the first LIGO observing run, which ended in January. The second run will begin this summer. In the fall, another detector, Advanced Virgo, operated by the European Gravitational Observatory in Italy, will start up. There are hopes for more in the future, in India and Japan.
So we will know more soon!
For more on Fermi: