Monster x-ray machine blasts apart black hole theory
There is lightning being made in Albuquerque, New Mexico. But unlike the kind that shoots between sky and earth, this lightning takes place inside the Z machine at Sandia National Laboratories. The lightning made inside this miracle of engineering carries more than 1,000 times the electricity of a regular bolt, and is 20,000 times faster – so fast, in fact, that the pulse released would go as fast as traveling from Los Angeles to New York in slightly less than one second. The machine also produces intense X-rays, and researchers have just used this component of the its ability to shake up a long-held theory regarding black holes.
The Z machine is the world's most powerful source of laboratory radiation. It's used to study how materials act under extreme conditions (it can melt diamonds, for example) as well to conduct fusion experiments and run simulations of what happens during a nuclear explosion. In a recent study though, researchers used the machine to duplicate the X-rays that surround black holes. Such X-rays are emitted from the great dark voids when gas surrounding them in what's known as the accretion disc is heated to incredibly high temperatures before getting sucked in by the hole's gargantuan gravitational force.
Because black holes can't be directly studied due to their confounding habit of devouring everything – including energy – the best we can do is study what's known as X-ray spectra (the wavelengths of energy produced in the X-ray band of the electromagnetic spectrum) in their accretion discs created by the superheated gas known as plasma.
"There's lots of information in spectra. They can have many shapes," said NASA astrophysicist Tim Kallman, a co-author of the study. "Incandescent light bulb spectra are boring, they have peaks in the yellow part of their spectra. The black holes are more interesting, with bumps and wiggles in different parts of the spectra. If you can interpret those bumps and wiggles, you know how much gas, how hot, how ionized and to what extent, and how many different elements are present in the accretion disk."
In their study, the researchers upended a theory about ions in a black hole's accretion discs.
For about 20 years, it has been thought that iron ions were present in these accretion discs even if no spectral lines were visible. The thought was that these ions were sheared off from their atoms thanks to the massive gravitational force and radiation emitted from a black hole. This is different from what an ion would normally do – drop to a lower energy state by emitting photons. So even if astronomers couldn't see the photons from the ions, they believed they were still there.
But in the Sandia study, the researchers submitted a dime-sized piece of silicon to the powerful X-rays from the Z machine. Silicon was chosen for the study as it is one of the most abundant elements in the universe. However, the silicon study revealed that if there were no photons, then there were no ions, a finding that dramatically influences the way we have been measuring black holes for years.
"If we could go to the black hole and take a scoop of the accretion disk and analyze it in the lab, that would be the most useful way to know what the accretion disk is made of," said Sandia researcher and lead author Guillaume Loisel. "But since we cannot do that, we try to provide tested data for astrophysical models."
Those models will now have to change says Loisel, because what's in those theoretical "scoops" is different from what we've believed for about the last two decades — in short, the hypothesized iron ions are simply not there.
"Our research suggests it will be necessary to rework many scientific papers published over the last 20 years," he said. "Our results challenge models used to infer how fast black holes swallow matter from their companion star. We are optimistic that astrophysicists will implement whatever changes are found to be needed."
The team's work has been published in the journal Physical Review Letters.
Source: Sandia National Laboratories