Feb 24 2016
LIGO detected gravitational waves from two merging black holes, shown here in this artist's conception. The Fermi space telescope detected a burst of gamma rays 0.4 seconds later. New research suggests that the burst occurred because the two black holes lived and died inside a single, massive star. Credit: Swinburne Astronomy Productions
On September 14, 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) discovered gravitational waves from the merger of two black holes, 29 and 36 times the total mass of the Sun. This kind of occurrence is usually expected to be dark, however the Fermi Space Telescope spotted a gamma-ray burst instantly after LIGO's signal. Recent research indicates that the black holes must have been living in a single, gigantic star whose death led to the gamma-ray burst.
It's the cosmic equivalent of a pregnant woman carrying twins inside her belly.
Avi Loeb, Astrophysicist, Harvard-Smithsonian Center for Astrophysics (CfA).
Usually when a gigantic star approaches the end of its life term, its core disintegrates into a black hole. If the star spins very fast there is the possibility of the core expanding and taking the shape of a dumbbell and then breaking into two clumps, each developing its own black hole.
A gigantic star often develops as an outcome of the fusion of two tiny stars. The merged star is expected to spin rapidly as the two tiny stars would have quickly rotated around each other in as they spiraled together.
After the black hole is developed, the outer envelope of the star swiftly moved inside. To power the gamma-ray burst and the gravitational wave event, the twin black holes must have originated very near each other, followed by an initial separation of order, and then merged within a few minutes. The newly developed black hole then took in the infalling matter, consuming a large amount of material and then powering a flow of matter that outwardly blasted in order to generate the gamma-ray burst.
Fermi identified this burst just 0.4 seconds after the gravitational waves were detected by LIGO, in the very same common area of the sky. The signal was not confirmed by the European INTEGRAL gamma-ray satellite.
Even if the Fermi detection is a false alarm, future LIGO events should be monitored for accompanying light irrespective of whether they originate from black hole mergers. Nature can always surprise us.
Avi Loeb, Astrophysicist, Harvard-Smithsonian Center for Astrophysics (CfA).
Detection of more gamma-ray bursts occurring from gravitational wave events will provide technique for determining cosmic distances as well as the extension of the universe. Astronomers can control the cosmological parameters by identifying the afterglow of a gamma-ray burst, determining its redshift, and finally comparing it to the distance-independent measurement from LIGO. "Astrophysical black holes are much simpler than other distance indicators, such as supernovae, since they are fully defined just by their mass and spin," says Loeb.
This is an agenda-setting paper that will likely stimulate vigorous follow-up work, in the crucial period after the initial LIGO discovery, where the challenge is to fathom its full implications. If history is any guide, the 'multi-messenger' approach advocated by Loeb, using both gravitational waves and electromagnetic radiation, again promises deeper insight into the physical nature of the remarkable LIGO source.
Volker Bromm, University of Texas at Austin
This study will be published in The Astrophysical Journal Letters and is also available online.