Elusive Birth of Black Holes Might be Visible After All

The creation of new black holes was thought to be unnoticable in most cases, at least with currently available technology on Earth. But a new theory predicts that most of these invisible events could actually be detectable when looking for the right signs.

While a small percentage of the (thought) overall cases of black hole births result in highly recognizable gamma-ray bursts of exploding stars, the old line of thinking goes that most black holes form uncharismatically when a star dies, with more of a disappearance, rather than a burst of energy.

But California Institute of Technology postdoctoral scholar Tony Piro thinks that the old notion that a dying star produces a black hole without much spectacle might not hold true.

"Maybe they're not as boring as we thought," Piro said in a Caltech news release.

According to well-established theory, when a massive star dies, its core collapses under its own weight. As it collapses, the protons and electrons that make up the core merge and produce neutrons. For a few seconds-before it ultimately collapses into a black hole-the core becomes an extremely dense object called a neutron star, which is as dense as the sun would be if squeezed into a sphere with a radius of about 10 kilometers (roughly 6 miles). This collapsing process also creates neutrinos, which are particles that zip through almost all matter at nearly the speed of light. As the neutrinos stream out from the core, they carry away a lot of energy-representing about a tenth of the sun's mass (since energy and mass are equivalent, per E = mc²).

In the 1980s a physicist suggested that as energy flows out from the core of the dying star a layer of hydrogen gas would be forced out, generating a shock wave that would rush from the star at 2 million miles per hour.

Using computer simulations, two astronomers at UC Santa Cruz, Elizabeth Lovegrove and Stan Woosley, recently found that when the shock wave strikes the outer surface of the gaseous layers, it would heat the gas at the surface, producing a glow that would shine for about a year-a potentially promising signal of a black-hole birth. Although about a million times brighter than the sun, this glow would be relatively dim compared to other stars. "It would be hard to see, even in galaxies that are relatively close to us," says Piro.

But now Piro says he has found a more promising signal. In his new study, he examines in more detail what might happen at the moment when the shock wave hits the star's surface, and he calculates that the impact itself would make a flash 10 to 100 times brighter than the glow predicted by Lovegrove and Woosley. "That flash is going to be very bright, and it gives us the best chance for actually observing that this event occurred," Piro explains. "This is what you really want to look for."

Such a flash would be dim compared to exploding stars called supernovae, for example, but it would be luminous enough to be detectable in nearby galaxies, he says. Piro reports that no black-hole flashes have been observed yet.

His research is published in The Astrophysical Journal.