This gas cloud is pulsing with gamma-rays from a distant black hole, but how?

Around 15,000 light-years away, an active black hole seems to be periodically lighting up an otherwise unremarkable cloud of gas with gamma rays. But there's a catch. The gas cloud is around 100 light-years from the black hole - and exactly how the cloud is pulsing in time with it is a mystery.

It's unlikely, astrophysicists believe, that the pulsations are directly generated by the plasma jets that are emitted by active black holes. But it's possible the black hole is somehow blasting out cosmic rays in a manner that illuminates the gas cloud.

"This result challenges obvious interpretations and is unexpected from previously published theoretical models," said astrophysicist Jian Li of the Deutsches Elektronen-Synchrotron in Germany.

"It provides us with a chance to unveil the particle transport from SS 433 and to probe the structure of the magnetic field in its vicinity."

SS 433 is the black hole system, of a type known as a microquasar. It consists, obviously, of the black hole - a relatively titchy fella, weighing in at 10 to 20 times the mass of the Sun. Its binary companion is a main-sequence supergiant, at around 30 times the mass of the Sun.

The two are locked in very close orbit, just a hair over 13 days, so that the black hole is siphoning a steady stream of material off its stellar companion, spooling around it like water around a drain. As it does so, the material glows, just like a mini version of a quasar.

But not all the material gets slurped up. Some of it gets channelled away from the inner edge of the accretion disc around the outside of the black hole - we're not entirely sure how, but scientists think it's along magnetic field lines.

This material is then beamed away from - presumably - the black hole's poles in jets of plasma that can approach significant percentages of the speed of light.

In addition, the black hole in SS 433 is wobbling, like a spinning top, on its rotational axis. This wobble is called rotational precession, and it means the jets, rather than shooting in a straight line, spiral out into space, as seen in the video below of another precessing microquasar, V404 Cygni.

By observing SS 433 for a decade, Li and his team were able to determine that the black hole's precession has a periodicity of just over 162 days, with the jet losing its spiral shape and relaxing into a cone after a few cycles. And the gas cloud pulses with gamma radiation on the same time scales.

This might seem like an open-and-shut case, but there's a big problem. The cloud isn't actually within the extrapolated jet cone.

"Finding such an unambiguous connection via timing, about 100 light years away from the micro quasar, not even along the direction of the jets is as unexpected as [it is] amazing," Li said. "But how the black hole can power the gas cloud's heartbeat is unclear to us."

Unclear it may be, but that doesn't mean there are no potential explanations. It's possible that cosmic rays - high-energy protons that travel almost at the speed of light - are being produced by the black hole, either at the ends of the jets, or from its equator.

At the ends of the jets, or close to the black hole, the subatomic particles of cosmic rays could be ejected towards the cloud. When they hit it, the impact could then produce gamma rays, as described in a 2005 theoretical paper.

Or it's possible that the cosmic rays originate from the edge of the accretion disc.

"Energetically, the outflow from the disc could be as powerful as that of the jets and is believed to precess in solidarity with the rest of the system," explained astrophysicist Diego Torres of the  Institute of Space Sciences in Spain.

These cosmic rays would have to be emitted at a sufficient rate to produce the observed gamma ray emission. This, the researchers said, is difficult to reconcile with our current understanding of the source environment.

So, there's a lot more work to be done to understand the system - and just how it is lighting up a cloud of gas 100 light-years away.

"SS 433 continues to amaze observers at all frequencies and theoreticians alike," the researchers wrote, "and is certain to provide a testbed for our ideas on cosmic-ray production and propagation near microquasars for years to come."

The research has been published in Nature Astronomy.

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