Black Holes Caught in Tug-of-War
This optical and infrared image from the Digitized Sky Survey shows the crowded field around the micro-quasar GRS 1915+105 (GRS 1915 for short) located near the plane of our Galaxy. The inset shows a close-up of the Chandra image of GRS 1915, one of the brightest X-ray sources in the Milky Way galaxy.
Credit: NASA/CXC/Harvard/J.Neilsen/Palomar DSS2

Supermassive black holes that pack the heft of billions of suns have the capacity to regulate their energy during a tug-of-war with a hot radiation wind that blows in from their debris disks.

Now 10 years worth of observations from the Chandra X-ray Observatory have uncovered the first clear evidence of this mysterious phenomenon occurring in a small black hole just 14 times the mass of the sun.

"The supermassive black hole has thousands of stars nearby, a whole galaxy of matter to push around," said Joseph Nielson, an astrophysicist at the Harvard University. "So it's more like supermassive black holes have more opportunity to self-regulate."

The difficult patient

Black holes are so powerful that matter and even light cannot get out once trapped. But around black holes, a flurry of activity creates high-energy radiation — including radio waves and X-rays — that does flow out into space. Sometimes it all gets pretty messy, and some of the energy can be channeled into focused jets that shoot out in both direction along the axis of the black hole's rotation.

Chandra has kept an eye on a notoriously unpredictable black hole that has 14 states of varying brightness, including a "heartbeat" state where periodic spikes in brightness resemble an EKG reading on a heart monitor.

Those unknown fluttering states have complicated observations of the GRS 1915+105 system for more than two decades, even though every major ground and space observatory has taken a peek at one time or another.

"These light curves are so bizarre that the black hole has been described as 'pathological,'" Nielson told "So the fact that we can see past all that, to the fundamental physics of winds and jets, is very exciting."

Most black holes ordinarily have long periods of quiet followed by occasional outbursts of jets and other activity, and so GRS 1915 is not unusual in that respect. But the small and feisty black hole does stand out by having had active outbursts for 17 years.

On and off again

The black hole's energy jet continually fights a seesaw battle with the hot radiation wind, as revealed in Chandra's X-ray observations. That wind flows in from the hot inner regions of the dusty, gaseous accretion disk surrounding the black hole.

"The outer disk actually absorbs some of that energy, and essentially begins to evaporate," Nielson said. "As it flows away from the disk, it absorbs even more of that radiation and its momentum, until the wind attains speeds of 1000 km/s [621 mi/s] or more."

Evaporation of the outer disk deprives the black hole jet of the mass that serves as its fuel, and eventually chokes it off. But the jets start up again. How the jets start up again remains a mystery.

Scientists know more about possible mechanisms driving the hot wind, which can include factors beyond the thermal driving or X-ray heating in the case of GRS 1915. Magnetic fields can also drive such winds around small black holes, also known as micro-quasars.

"So our paper kind of raises a new mystery: why do some micro-quasars produce magnetically-driven winds and others produce thermally-driven winds?" Nielson noted. "Right now, we just don't know the answer."

An ongoing enigma

In any case, the jet appearances can also differ greatly in terms of how long they keep going.

"Maybe it's just for a few hours, but it could be for days or weeks," Nielson noted. "And some day, it will stop altogether -- no jets, no winds, just a quiet accretion disk slowly feeding the black hole."

But for now, GRS 1915 continues to provide useful information that researchers can apply to understanding its super-massive black hole cousins, which can defy observation for timescale reasons. An hour-long change in the small black hole would be equivalent to a timescale of 10,000 years in a super-massive black hole, if the latter weighed a billion times the mass of the sun.

Much more Chandra data also remains for just this one black hole, and researchers have yet to analyze it all.

"It's sort of like going back to see what's on each puzzle piece," Nielson said. "Chandra is great for this sort of thing, and we've got lots of data to pore over."