Did Fermi Detect LIGO’s Merging Black Holes?

NASA’s Fermi Gamma-ray Space Telescope might have detected a burst from the same merging black holes that emitted the gravitational waves LIGO detected. Or not.

An artist's impression of the Fermi Gamma-Ray Space Telescope.
Credit: NASA/General Dynamics

Back in February, the LIGO team announced it had detected the unmistakable signal of gravitational waves from two black holes as they merged into one. When the news spread, scientists scrambled to see whether they had recorded the event other ways, too — not as spacetime ripples, but as photons. They dug through archived observations taken around the moment on September 14, 2015, when the gravitational wave signal wobbled LIGO’s two sites.

At face value it was a fool’s errand: astronomers didn’t expect the black holes to have set off any kind of light show. That’s because any emission would have to come from gas, and merging black holes of these masses (a few tens of solar masses) should have swept up all surrounding material during their prolonged fatal approach. In other words, merging stellar-mass black holes don’t wear gas tutus.

Yet on the same day of LIGO’s announcement, scientists with NASA’s Fermi Gamma-ray Space Telescope posted a paper to the preprint server arXiv, reporting that Fermi had seen something — a weak, 1-second burst, just 0.4 second after the LIGO event.

The flash’s spectrum looks like that of a short gamma-ray burst (GRB), which is what it sounds like: a burst of gamma rays. The short breed of GRB lasts for less than 2 seconds, probably the result of colliding neutron stars or (more rarely) black holes. Astronomers have seen the afterglow from a neutron star crash before.

But the September 2015 flash was far weaker than the run-of-the-mill short GRB. It was so weak, in fact, that it didn’t trigger the space telescope’s onboard alert system. That’s partly because the burst went off “underneath” the spacecraft; Fermi essentially detected it with peripheral vision. It also doesn’t show up in data from the European Space Agency’s International Gamma-Ray Astrophysics Laboratory (Integral) spacecraft. But the latter doesn’t faze Valerie Connaughton (Universities Space Research Association) and her colleagues, who calculate in their paper that Integral only detects half of the weak, short gamma-ray bursts that Fermi does.

The LIGO and Fermi signals come from the same part of the sky — but “same part of the sky” is a big region, because both observatories are bad at pinpointing where a signal comes from. LIGO constrained the merged black hole’s location to a long arc in the heavens, but that arc covers something like 600 square degrees. That’s equivalent to the celestial territory spanned by the constellation Orion (if you leave out the raised club). Fermi's view overlapped about 200 square degrees of that (more like the span of Cassiopeia’s “W”), as shown in the video below.

The team has identified no alternative source for the Fermi flash. The options, as they stand, are basically

1. the Fermi signal isn’t real (it's an equipment hiccup or a chance background fluctuation);
2. the Fermi signal is real, but it’s a coincidence that it came from the same part of sky that the LIGO signal did; and
3. the Fermi signal is real and it’s from the same cosmic collision that created the gravitational waves.

The team estimates that there’s only a 0.2% chance Fermi would have detected a signal so soon after LIGO’s. That probability doesn’t take into account whether the Fermi signal is real or its location on the sky, Connaughton says — it's only based on the timing.

Astronomers are nothing if not optimists. Abraham Loeb (Harvard), known for his out-of-the-box thinking, suggested soon after the Fermi team reported their find that both signals could come from the death of a huge star with the mass of more than 100 Suns. This hypothetical star, formed when two smaller stars merged, would have died in a catastrophic collapse. If its core broke into two clumps which then became two black holes, Loeb suggests, those black holes could be the ones that merged — which would explain why there was gas around to feed the flash. (The merged stars’ cores might also never have united in the first place, but he thinks it’s harder to generate a GRB this way.) The team hasn’t advocated this theory or any other yet.

However, astronomers do see GRBs from neutron star collisions, which will also produce gravitational waves that are detectable by LIGO and the near-operational Virgo interferometer in Europe. That’s why astronomers are excited about the possibilities, and why they’re actively discussing how Fermi and LIGO can work together. Even if this pair of signals turns out to be coincidence, others won’t.