LIGO black holes born from a single star?

The news of the first ever detection of gravitational waves was a major astronomical discovery. Now a further study of the event has revealed a gamma-ray burst that could point to the origin of the black holes behind the event.

Artist's simulation of two black holes merging. Was the black hole pair born from a single star?
Credit: SXS

The pair of black holes that merged to cause the first ever detection of a gravitational wave may have been born from a single, massive star.

On 14 September 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves caused by the merging of two massive black holes. This detection was confirmed on 11 February 2016.

A fraction of a second following the initial detection, the Fermi Space Telescope detected a gamma-ray burst. It is thought that this gamma ray was generated by the death of a star in which the two black holes resided.

"It's the cosmic equivalent of a pregnant woman carrying twins inside her belly," says astrophysicist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA).

When a star begins to die, its core often collapses to form a black hole. But if that star was spinning very fast, the core can stretch and break into two massive clumps, each forming its own black hole.

The type of star required for this to occur could be formed from two smaller stars in a binary pair. These stars orbit one another faster and faster as they get closer together, eventually creating a single, massive star that continues to spin incredibly fast.

In this case, once the star had died and the two black holes had formed, the star’s outer envelope would have been pushed outwards and then sucked inwards by the black holes. For such a powerful event that would cause a gravitational wave and a gamma-ray burst to occur, the black holes must have been born close together and merged in a very short space of time: possibly minutes.

This single black hole would then have fed on the stellar material being sucked inward, creating powerful jets of material blasting out from the black hole and causing the gamma-ray burst.

"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," says Loeb.

The team behind the study say that, if more gamma-ray bursts are detected during gravitational wave events, they could offer a new way of measuring cosmic distances and the expansion of the Universe. This would be done by observing the afterglow from the gamma-ray burst and measuring its redshift, which is a change in the wavelength of light that occurs when an object moves away from its observer at high speed. This redshift can be compared to the independent distance measurement from LIGO, enabling astronomers to accurately measure the distances involved.

"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.


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