Black Hole Spews Atoms

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.

Cygnus X-1
An artist's highly symbolic representation of the Cygnus X-1 black hole and its X-ray-hot inner accretion disk. Astronomers have now detected atoms in the jet from a black hole in a similar system, potentially revealing how the jet is powered.
NASA / CXC / M.Weiss
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.

8 thoughts on “Black Hole Spews Atoms

  1. Bruce

    So, if I’m understanding this correctly the X-rays being observed are coming from highly ionized iron and nickel after they experience a collision at two thirds c. But what is unclear to me is this: are these relatively heavy ions part of the jet, or are they being hit by the jet?

  2. Peter WilsonPeter Wilson

    The relatively heavy ions are part of the jet. The x-rays are not coming from collisions, but from electrons dropping down into the lowest orbitals around iron and nickel nuclei, which have been ionized 26 times, if I understand their Figure 1 correctly. Also, the jet has to be pointing almost straight at us in order to see radial velocity of 2/3 c.

  3. Bruce

    Thanks Peter. Then since iron and nickel are part of the jets all lighter elements would have to be part of the jets too, wouldn’t they? Fe and Ni are fused in the cores of massive stars just before and as they detonate as supernovas, which cause much of the Fe and Ni to be dispersed out into interstellar space. The star that formed the black hole must have injected these metals into its binary companion, and now this star is spilling some of this metal enriched material back into the BH’s accretion disk. But, is it also possible that fusion reactions are taking place inside the disk? (An earlier S&T newsblog reported a finding that Lithium can be produced at some disk/jet sites.) Could reactions like Si+Si=>Fe and Fe+He=>Ni be taking place in the disk, and not just in the cores of SN progenitors? If so then it is possible that we’re not just made of star stuff, but we could also have some BH accretion disk stuff inside us too.

  4. Peter WilsonPeter Wilson

    You’re welcome. Sounds right, except for part about fusion. Problem is, everything in the accretion disk is moving in the same direction. Intuitively, fusion requires a more-or-less head-on collision between nuclei, and it is hard to picture that happening in the accretion disk. Yet, as you point out, lithium is reported there, so intuition can be wrong. Either way, there’s certainly some material in us that rode the accretion disk; came “this close” to being swallowed forever by a BH but was shot out the poles instead.

  5. Bruce

    That sounds right too Peter, except that fusion inside the disk may be possible even with all the material moving in the same bulk direction. If plasma is hot enough and dense enough fusion can occur, whatever the overall movement might be. In ion collisions temperature supplies the energy to overcome the coulomb barrier, while the density controls the frequency of collisions. In reviewing a textbook on nuclear astrophysics I found that one of the reactions I mentioned in my last comment was incorrect. The direct fusion of two Silicon ions (if it occurs) would produce nickel, not iron. But the typical paths for the production of these two metals are Chromium + Helium => Iron and Iron + Helium => Nickel. Do reactions like these take place in BH accretion disks? It would depend upon on how hot and how dense the material in the disk is and how long the material stays in the disk. These reactions normally occur deep in the cores of massive stars just before they go supernova. I just thought it would be cool (or hot?) if these reactions also sometimes occur out were they can be more readily observed in BH accretion disks. The conditions probably aren’t extreme enough for much heavy ion fusion to occur, but then again you can’t get much more extreme than the vicinity of a BH. I also just find it fascinating how the laws of physics insure that all the elements we know and love and are made of are created and are then distributed out into the far reaches of interstellar space. BH’s aren’t the voracious feeders we’ve been led to believe. They’re much more efficient at redistributing material than at swallowing it. They might even prove to be essential for the widespread existence of life in the universe.

  6. Bruce Mayfield

    Yes. Didn’t even Einstein say something about God not playing dice with the universe? And, in addition to the movement of material in their jets, massive BH’s can also contribute via gravitational interactions to the dispersal of entire star systems out from stellar nurseries like the Orion nebula. See: This process of gravitational dispersal is an evidence of a principle Peter Wilson calls duality.

  7. Bruce

    For those seeking more info on this topic, it was also featured as today’s Astronomy Picture Of the Day (APOD).
    In that interesting discussion I learned that the max temperature of the accretion disks of low massed BH’s like this should be around 10^7 K, so only light nuclei fusion reactions should be possible. The Fe and Ni being shot out the jets must be coming from the companion star.