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.
"Obviously the question that everyone has is, are our observation and LIGO's observations coming from the same object — and we cannot say definitively right now," Adam Goldstein of the NASA Marshall Space Flight Center in Alabama said at a briefing. "But this will likely soon be answered in the next couple of years."
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.
The Fermi space telescope also picked up an odd signal coming from the same direction. The spacecraft observes about 70% of the sky at the same time, allowing it to pick up on sudden and brief events that might be missed otherwise. The problem in this case is that the signal appears to essentially have come from beneath the camera and at a sharp angle – not an ideal viewing situation.
Gamma rays are extremely high-energy bursts of light, higher in energy than even X-rays, that are thought to be produced by the highest-energy phenomena in the universe, from supernovas to pulsars. The Fermi telescope picks up X-rays and gamma rays whose energies range from 8,000 to 40 million electron volts; the energy from the visible light, the kind that we can see, falls around a mere 2 to 3 electron volts.
The scientists say there's less than a 0.2% chance that the two events are merely coincidental. However, some researchers do think they're unrelated because binary black hole mergers aren't thought to trigger gamma rays. The hot gas swirling around them, which would normally produce this high-energy light, should have all been blown away at this late stage in their lives.
In that case, the gamma rays could potentially be from something else, such as a neutron star merger. Regardless, they added, the timing of the electromagnetic waves (the gamma rays) and the gravitational waves seems to work out.
"Even if these were neutron star mergers, you wouldn't expect electromagnetic signals to happen until the merger happened," Judith Racusin of NASA's Goddard Space Flight Center in Maryland said at the briefing. "And the gravitational waves, of course, you're seeing just before, during the end spiral."
For now, the relationship between the two will remain unclear – unless astronomers and physicists begin to find more of these curiously timed signals.