2009-10-20

Researchers have figured out how to recreate the environment near a black hole in order to understand the processes that cause them to emit high energy X-rays.

Black holes are famous for having a gravitational field that is so potent that light cannot escape its pull. But that same gravitational pull causes nearby matter to reach energies that results in a prodigious amount of radiation, from regular light up to X-rays and beyond. Researchers have attempted to model the behavior of matter as it gets drawn into accretion disks near a black hole in order to understand this radiation, but the conditions in these areas are difficult to reproduce on Earth. Now, a consortium of researchers from China, Japan, and Korea have figured out how to use a 300 GigaWatt laser to reproduce conditions near the accretion disk, and have successfully reproduced the spectrum observed near both black holes and neutron stars.

In many cases, black holes and neutron stars form in binary systems, and their intense gravitational pull can be sufficient to strip matter off their companion star. That matter gets drawn into a disk of gas centered on the black hole, where some of it is slowly dragged into the black hole. This process can produce luminous jets of matter that move at close to the speed of light as it exits the system along magnetic field lines that extend from the poles of the black hole. Within the disk itself, the matter nearest the black hole is heated by energetic collisions as it's drawn towards the event horizon. The light from that matter also ionizes the material towards the outer edge of the disk, resulting in high energy radiation that we can detect using equipment like the space-based Chandra X-ray Observatory.

Astrophysicists have been able to model the behavior of this matter and get their models to produce the sort of spectrum we've observed from objects like Cygnus X-3. But it's been rather difficult to reproduce these conditions on Earth and perform a sanity check on the models.

The problem isn't, as you might expect, recreating the dense conditions where frictional heating prevails; we're actually relatively good at that. Instead, the challenge is that the matter nearer the edge of the disk isn't that hot, but it gets ionized by the intense radiation in the environment. So far, we've not been nearly so good at stripping electrons off a relatively cold gas.

The researchers used the GEKKO-XII laser at Osaka University to vaporize a plastic shell, producing the sort of hot, dense plasma we've typically studied. But they used a second laser to vaporize a nearby sample of silicon to an energy that was over an order of magnitude lower. Their timing was precise enough that the radiation from the hot plasma hit the silicon while it was still in a plasma, creating the sort of radiation-induced ionization that is thought to predominate in an accretion disk. The experiment was set up so that only the radiation emitted by the silicon was measured.

As an aside, it's worth spending a moment considering the technical achievement involved in getting two powerful lasers to fire into a confined space with such precise timing.

Although silicon is almost certainly not the most common element in an accretion disk, the radiation detected in the experiment is produced by electrons reoccupying the inner most orbitals, which should typify the materials that are common in the disk. The researchers refer to electron transitions occurring in "helium-like" silicon ions.

A comparison of the radiation produced by the experimental samples is shown directly above observational data obtained by Chandra from two binary systems, one with a neutron star, the other containing a black hole candidate, and it's clear that some of the features of the spectrum closely match those from natural sources. But the convenience of measuring this radiation in a controlled environment is also clear, given the apparent precision of the Chandra measurements.

As a result of the more detailed measurements, the authors have produced results that suggest astronomers may have been feeding their models a variable that's not entirely reflective of physical reality. The variable is supposed to reflect a combination of the radiation intensity and target density, and astronomers have assumed a range of typical values. Using this experimental system, however, the authors' have come up with a number that's outside the range used by astronomers.

It's important to note that this isn't a difference between experimental data and models. The authors of the new paper still had to use a model to understand what was happening in their sample to produce the spectrum they observed. The difference is that they were able to feed their model data with a much higher precision, and understood the conditions that produced the data lot better.

  Email Article

  Discuss in the Forum

Back