Much of the matter falling onto a black hole is channeled into powerful jets that blast it back out into space. Astronomers have used Doppler measurements to determine that particles in jets are moving close to the speed of light, making black holes some of the Universe's most powerful accelerators. How much energy involved in doing so depends on the jets' composition. It requires a lot more energy to accelerate a heavy particle like an atomic nucleus than it does an electron. So what types of particles are actually contained in these jets?

Astronomers have now found the signature of heavy particles moving at two-thirds the speed of light in the jet from a stellar-mass black hole candidate. María Díaz Trigo and colleagues monitored the X-ray and radio emissions from a binary system thought to contain a black hole and found strong emission most consistent with highly ionized iron atoms accelerated to high velocity. Since this system is typical of other black holes in the Milky Way, this new observation indicates that similar systems are likely to all blast protons and nuclei into the surrounding space.

Stellar-mass black holes are formed from the collapsed cores of stars significantly more massive than the Sun. As such, they have masses comparable to those of stars, as opposed to the supermassive black holes at the centers of galaxies, which may be millions or billions of times more massive. However, the physics of accretion—matter orbiting and falling onto a black hole—for both types of black hole is such that it produces jets and hot disks that emit ultraviolet or X-ray light.

The present system of interest is 4U 1630-47, a binary system consisting of a star and a companion object that's bright in X-ray emissions. Like the most famous of these systems, Cygnus X-1, the companion object is too massive and compact to be anything other than a black hole. The physical processes in these binaries is the same: the black hole strips gas from the star, which forms an accretion disk and jets.

Jets are the primary mechanism by which black holes interact with the surrounding medium, as they are high-energy and may contain a lot of mass. The process of pumping matter back into space is known as feedback. For supermassive black holes, feedback shapes the environment of the host galaxy far beyond the reach of the black hole's gravity. Even for stellar-mass black holes, feedback is an important process for the local environment.

However, astronomers had little conclusive evidence for the composition of jets, and they didn't know whether they contained baryons—heavy particles consisting of quarks rather than light particles like electrons. Baryons were identified in the jet from one black hole (known as SS 433), but that system is atypical.

A proton is about 2,000 times more massive than an electron, and atomic nuclei for elements other than hydrogen are much more massive still. The energy required to accelerate baryons is correspondingly greater, so finding heavy particles in jets is a measurement of the power of black holes and how much their feedback can influence the region nearby. Additionally, baryons in the jet and in interstellar gas act like a cosmic particle collider, producing cascades of neutrinos that could be detected by IceCube or other observatories.

In the case of 4U 1630-47, the researchers examined X-ray data collected by the space-based XMM-Newton observatory during a period when the system underwent a brief outburst in 2012. They compared these with concurrent radio observations from the Australian Telescope Compact Array (ACTA) and concluded the activity was consistent with a flare in the black hole jet.

However, the researchers also identified a source of strong emission in the spectrum that was most consistent with Fe XXV or Fe XXVI: iron ions missing 24 or 25 of their electrons, respectively. These baryons are on the order of 100,000 times the mass of an electron, requiring a lot of energy to accelerate; based on the emission, the iron was moving at as much as two-thirds the speed of light.

The physical picture of 4U 1630-47 is clear based on this data. Matter stripped from the companion star forms an accretion disk, which is hot enough to ionize any atom it harbors. The black hole's gravity accelerates the electrons, protons, and ions to a high fraction of the speed of light. Some of those particles are then blasted into interstellar space in jets. Earth happened to be nearly in line with the jet (the astronomers estimated roughly 15 to 30° aligned toward us) during the outburst. The energy involved pretty much rules out emission from the disk: only a jet is so energetic.

Since 4U 1630-47 is a fairly typical stellar-mass black hole, finding baryons in its jet is a good indication that other binary systems should spew them out as well. That has important implications for black hole feedback and its role in shaping interstellar environments. Since a number of similar binaries reside in our galaxy, they could also be sources for high-energy neutrinos and other cosmic rays.

Nature, 2013. DOI: 10.1038/nature12672 (About DOIs).