May 5, 2013
Witnessing The Birth Of Black Holes
John P. Millis, Ph.D. for redOrbit.com — Your Universe Online
Stars sustain themselves by fusing elements in their core, producing even heavier atoms, releasing energy that keeps the immense force of gravity at bay. But eventually, the fusion process will no longer produce enough outward radiation pressure to sustain the star, and it will begin to collapse in on itself.
In the grandest of cases, the progenitor is a truly massive object, perhaps tens or hundreds of times the mass of our Sun. And the end of its life comes as a massive explosion called a supernova, or in the most extreme instances, a Gamma-ray Burst (GRB).
The object that is left behind is a black hole, an exotic object that is the focus of much study in astronomy. But supernovae and GRB´s massive enough to create black holes are somewhat rare, much too rare to be the sole mechanism for creating these objects. In fact, some suggest that many of the black holes we observe are not created in this manner.
Instead, a hypothetical event known as an 'Unnova' may be responsible for the birth of many of the black holes in our galaxy. This phenomenon would proceed much like a planetary nebula — the dying phase of lower mass stars such as our Sun — where the outer envelope of the star simply fades away into space, leaving behind the compressed core of the star. Only, the process would be more rapid and energetic.
In an Unnova the core would become compressed as the protons absorb the surrounding electrons, leading to a neutron dominated core. But instead of the outer layers of the star rebounding at the neutron core — the resulting shockwaves creating the well-known supernova event — the release of neutrinos from the neutronization process pushes matter away from the core.
As the inner layers of the star rush outward at nearly 2,000,000 miles per hour, they would interact with the outer envelope, creating a shockwave that drives the matter into outer space.
The energy released would leave a glow that could be visible for more than a year. But the brightness would be so low that it is unlikely that we´d be able to see it from Earth in all but the nearest systems.
On the surface, it seems that the Unnova would provide little insight into the formation of black holes, as they would be nearly impossible to detect. However, a new study by Cal-Tech physicist Tony Piro suggests that there may still be hope.
By carefully modeling the interactions of the matter being ejected from around the core of the star, he found that when the shock wave from the neutrino expression reaches the surface that a flash some 10 to 100 times greater than the afterglow would ignite. "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."
While the flash would still pale in comparison to supernovae, which are driven by much more intense shockwaves, the brightness would be sufficient to be seen in nearby galaxies.
Piro estimates that the flash would persist for about 3 to 10 days, peaking in the optical and ultraviolet wavelengths. With current technology, we should be able to find at least one of these events each year. But in the next decade the new Large Synoptic Survey Telescope (LSST) is expected to come online, which will provide a significant boost to our optical and ultraviolet survey capabilities.
According to Piro, "If LSST isn't regularly seeing these kinds of events, then that's going to tell us that maybe there's something wrong with this picture, or that black-hole formation is much rarer than we thought."
You can read more about this in the May 1 issues of the Astrophysical Journal Letters. The paper is titled "Taking the 'un' out of unnovae."