Black hole heavyweights triggered gravity wave event

The recent detection of gravitational waves is a stunning confirmation of Albert Einstein’s theories and the start of a new way of observing the universe. And at the center of it all is a celebrity couple: the first known pairing of black holes and the most massive ones found outside of the cores of galaxies.

On September 14, the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, sensed a disturbance in spacetime caused by two massive black holes smashing together (SN Online: 2/11/16). “It’s quite an incredible discovery,” says Vikram Ravi, an astrophysicist at Caltech. “They’ve seen objects that I guess none of us outside the collaboration imagined they might see.” With masses of 29 and 36 suns, these black holes were roughly twice as massive as the previous record holders.

Those masses actually aren’t too shocking, says Jeffrey McClintock, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Very massive stars, though rare, should give rise to very massive black holes. What would have been more surprising, he says, is if LIGO failed to turn up any black holes this large. “If the nearest 1,000 stars had been investigated and we hadn’t found any planets, I would go back to church,” he says. “I feel the same way about two 30-solar-mass black holes.”

There are heavier black holes. Those monsters live in the centers of galaxies and can weigh billions of times as much as the sun. But they are different beasts entirely, probably built up as galaxies collide. Black holes such as those detected by LIGO are born when a massive star dies. And given their masses, “they likely formed in a fairly different environment than the Milky Way,” Ravi says.

How much mass a star ends up with at the end of its life depends partly on its store of elements heavier than helium. Atoms such as carbon, magnesium and iron present larger targets to the light that’s escaping a star. As light races outward, it bumps into these atoms, which in turn shove the surrounding gas along. The heavy elements behave like little snowplows attached to the photons, whittling away at the star’s mass as the light radiates into space. To make black holes as massive as LIGO’s, the original stars must have had fewer of these heavy elements than typical stars in our neighborhood, the LIGO team reports February 11 in the Astrophysical Journal Letters.  

One possibility is that the stars formed early in the universe before heavy elements had a chance to accumulate. At the other extreme, the stars could have formed more recently in a relatively nearby (or local) and pristine pocket such as a dwarf galaxy. “With one observation, it’s impossible to say if it’s on one side of the continuum or the other,” says Vicky Kalogera, a LIGO astrophysicist at Northwestern University in Evanston, Ill.

The best estimates put the collision in a galaxy about 1.3 billion light-years away (give or take a few hundred million light-years) in the southern sky, roughly in the direction of the Magellanic Clouds, two satellites of the Milky Way. A third LIGO facility, such as one proposed for India, will help narrow down precise positions of future detections. So would a simultaneous burst of electromagnetic radiation from the location of a collision. LIGO has agreements with telescopes around the world (and in space) to keep an eye out for any flashes of light that occur at the same time as a gravity wave detection. For LIGO’s debut, no observatories reported anything definitive. But the Fermi gamma-ray satellite did see something interesting, astrophysicist Valerie Connaughton and colleagues report online February 14 at arXiv.org.

“We found a little blip that’s weaker than anything we’d normally look at,” says Connaughton, of the Universities Space Research Association in Huntsville, Ala. At 0.4 seconds after LIGO’s detection, Fermi recorded a very faint flash of gamma rays. “We’d normally never pick it out of the data,” she says. Researchers can’t pinpoint precisely where the burst came from, but the direction is roughly consistent with LIGO’s.

If the black hole collision did blast out gamma rays, theorists are going to have some explaining to do. Merging black holes shouldn’t release any electromagnetic radiation. It’s only when neutron stars get involved that telescopes should see flashes of light. During a recent phone call with colleagues about the Fermi data, “the theorists were already arguing with each other,” Connaughton says.

But before the theorists get too worked up, researchers need to figure out if what Fermi saw had anything to do with LIGO’s black holes. “We’re definitely not saying we saw an [electromagnetic] counterpart,” says Connaughton. It could be just a coincidence. During nearly 67 hours of observing in September, Fermi saw 27 similar gamma ray bursts. The only way to be certain is to wait for more LIGO detections. “If it’s real, it’s not going to be a one-off,” she says.

LIGO’s debut detection appeared during a test run in September; researchers are currently analyzing LIGO data accumulated during the four months that followed, and another science run is planned for later this year. The team is optimistic about their chances of finding more events. LIGO could have sensed a collision between two 30-solar-mass black holes out to about 6 billion light-years away. Given that researchers found one (so far) in 16 days of data, and assuming that’s a typical couple of weeks in the universe, then researchers estimate that between two and 53 similar collisions occur per cubic gigaparsec per year. (One cubic gigaparsec is a volume of space roughly 4 billion light-years across.)

If those estimates are correct, scientists think LIGO could have detected up to about 10 more similar collisions in its first four months of operation, and possibly hundreds once the facility is running at full sensitivity. And that’s not including collisions of black holes with different masses, smashups of neutron stars or any other cosmic calamities that could rattle spacetime.

As more collisions are found, astronomers should get a better handle on where binary black holes form. “We may find they’re all in the local universe and none in the early universe,” Kalogera says. And that would tell researchers something about how massive star formation has changed throughout cosmic history. “We have high expectations now for a bigger sample in the near future.”