Accretion disks on demand

One of the best ways to study black holes is to observe the fireworks that occur when matter falls onto them. As material in the surrounding accretion disk (illustrated in the figure) gets heated and ionized into plasma, it emits x rays whose spectra embody the black hole’s properties, such as mass and spin. But so long as the vicinity of a stellar black hole cannot be directly resolved, the models that make sense of the spectra must rely on assumptions that can be directly tested only by re-creating an astrophysical plasma in the lab and measuring it.

Now Guillaume Loisel of Sandia National Laboratories and colleagues have done just that with Sandia’s Z machine. The powerhouse x-ray source blasted dime-sized samples of silicon, a suitable analogue for the iron that dominates the line emission from accretion disks. After isolating the silicon emission, which was 10^–7 as bright as the triggering pulse, Loisel and colleagues found that the spectrum resembled those measured from plasmas swirling around black holes and x-ray binaries.

What the researchers didn’t find was evidence of a process called resonant Auger destruction (RAD), in which certain excited ions emit an electron rather than a photon when dropping to the ground state. Theorists have invoked RAD to explain why some of the expected iron lines in accretion-disk spectra are absent. There were, however, no such stealth ions in the Z experiment—if there was no spectral line, then there were no ions in the plasma. The authors contend as incorrect the assumption that RAD totally wipes out the emission of some accretion-disk iron ions. If confirmed, they say that would necessitate revisions of previous research; other researchers say that’s too sweeping a conclusion. Nonetheless, the experimental results serve as a benchmark for testing accretion-disk emission models and will likely get scientists more focused on the implications of the presence or absence of various spectral lines. (G. P. Loisel et al., Phys. Rev. Lett. 119, 075001, 2017 .)