Ionized metals detected in the jet shot out by a black hole.
When a baby spits up on you, the content of that spit-up is somewhat irrelevant. The same might seem true for black holes. Black holes are messy eaters: a lot of their food winds up thrown away as winds or jets instead of going down their throats. You might think that, as long as the jettisoned material doesn’t hit you, you don’t need to care what it is.
But the particles that make up a black hole’s jet are important, and it’s surprisingly difficult to see what those particles are. Astronomers know that black holes’ twisted magnetic field lines launch the jets: material shoots out along these lines kind of like beads on a whirling wire. But they don’t know if the material comes from the accretion disk, containing electrons and protons, or if it originates from the current just outside the black hole, which would fill the jet with electrons and their antimatter counterparts, positrons.
That distinction could be important for stuff in the jet’s way. A proton and a positron are both positively charged, but a proton is about 1,800 times more massive. “From the perspective of the stuff being run into, it’s about the difference of a linebacker running into you [versus] a tennis ball,” explains Gregory Sivakoff (University of Alberta). “Of course, since you’re being run into at nearly the speed of light, you’re not going to be happy being run into by either.”
Protons would reveal themselves via emission lines from ionized atoms (because atoms have protons in their nuclei). Thus far, only one black hole has shown clear evidence of protons, but it’s an oddball system and its jets are probably atypical.
Now, María Díaz Trigo (ESO) and colleagues appear to have finally detected protons’ signature in the jet from a stellar-mass black hole gobbling material from its companion star. The system, 4U 1630-47, lies just south of Scorpius and occasionally shoots out a jet. The team observed it in radio with the Australia Telescope Compact Array and in X-ray with ESA’s XMM-Newton. When the jet was turned on (spotted in radio), the astronomers detected emission lines from ionized iron and nickel (spotted in X-ray).
What makes the signal from these highly ionized metals unique is that the material appears to be moving at about two-thirds the speed of light. That’s a couple hundred times the speed of winds that can blow off the tutu-like accretion disk, so it looks like the atoms — and therefore the protons — are in the jet itself.
This is a big deal, says Jeffrey McClintock (Harvard-Smithsonian Center for Astrophysics). Jets’ makeup has been a tough puzzle to unravel, and except for that other lone source, first spotted in the 1970s, no one’s definitively discovered jet protons before.
“I was blown away,” says Sivakoff. “More work needs to be done to confirm these results, but it looks like this is a great indication of proton-rich jets.”
The presence of protons implies that the accretion disk powers the jet. The disk is woven through with magnetic fields, and as it feeds the black hole, it shoves those magnetic field lines into the beast’s mouth until they thread the black hole like an olive threaded on strands of spaghetti. In this scenario, it’s the disk material that’s funneled out along the magnetic strands.
The accretion disk’s importance is unsurprising: jets usually show up when a sizeable corona of ionized gas grows around a black hole’s disk. It’s hard to understand why that would happen if the disk wasn’t involved in the jet’s creation, Sivakoff says.
But the final answer probably isn’t one mechanism or the other. Protons could also contaminate a jet thanks to the surrounding environment; even if that's not the case here, it could be elsewhere. “Often times in astronomy we eventually find out that when considering between option A and B, both contribute,” says Sivakoff. “I nickname this the Chinese menu effect.”
If jets really do contain protons, they could create gamma-rays and neutrinos when they smash into surrounding material, Díaz Trigo’s team suggests. Thus far neutrino observatories have had poor luck detecting individual sources, but that might be because they’re not yet sensitive enough. Gamma-ray observations with the Fermi space telescope and other instruments might have better luck.
Reference: M. Díaz Trigo et al. “Baryons in the relativistic jets of the stellar-mass black-hole candidate 4U 1630-47.” Nature, published online 13 November 2013.