LIGO's twin black holes may have been born inside single star

Washington: Adding a new dimension to the historic gravitational waves discovery, new research suggests that the two black holes in focus may have resided inside a single, massive star whose death generated the gamma-ray burst. On September 14 last year, the Laser Interferometer Gravitational-wave Observatory (LIGO) in the US detected gravitational waves from the merger of two black holes 29 and 36 times the mass of the Sun.

Such an event is expected to be dark but the space observatory Fermi Gamma Ray Space Telescope in low-Earth orbit detected a gamma-ray burst just a fraction of a second after LIGO's signal. Fermi detected the burst just 0.4 seconds after LIGO detected gravitational waves, and from the same general area of the sky. However, the European INTEGRAL gamma-ray satellite did not confirm the signal.

Normally, when a massive star reaches the end of its life, its core collapses into a single black hole. But if the star was spinning very rapidly, its core might stretch into a dumbbell shape and fragment into two clumps, each forming its own black hole. After the black hole pair formed, the star's outer envelope rushed inward toward them.

In order to power both the gravitational wave event and the gamma-ray burst, the twin black holes must have been born close together, with an initial separation of the size of the Earth, and merged within minutes. If more gamma-ray bursts are detected from gravitational wave events, they will offer a promising new method of measuring cosmic distances and the expansion of the universe.

By spotting the after glow of a gamma-ray burst and measuring its redshift, then comparing it to the independent distance measurement from LIGO, astronomers can precisely constrain the cosmological parameters.