Erratic black hole

regulates itself

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 26 March, 2009

New analysis based on Chandra observations suggests that a special class of black hole may have a mechanism for shutting off their high speed jets and thus regulate the rate at which they grow.

Black holes vary in size from the supermassive - millions or billions times solar masses - down to the stellar mass black holes with typical masses of 7-25 Suns. Even the smaller black holes can pack quite a punch, launching powerful jets of particles and radiation similar to those characteristic of fully-fledged pulsars, and are therefore known as “micro-quasars”.

This optical and infrared image from the Digitized Sky Survey shows the crowded field around GRS 1915 located near the plane of our Galaxy. The inset shows a close-up of the Chandra image. Image: X-ray (NASA/CXC/Harvard/J.Neilsen); Optical and IR (Palomar DSS2).

In the new study, NASA’s Chandra X-ray Observatory has been used to study one of our own Galaxy’s micro-quasars, known as GRS 1915+105, for the last ten years. GRS 1915 contains a black hole about 14 times the mass of the Sun and is feeding off material from a nearby companion star. This stolen material creates an accretion disc around the black hole.

The results of eleven separate Chandra observations show erratic and complicated behaviour, including 14 different patterns of variation that range from timescales of seconds to months. Scientists suspect that these variations are caused by a connection between the disc and the radio jet seen in GRS 1915, but this is not yet fully understood. One idea is that the jets may be periodically choked off when a hot wind is driven off the accretion disc around the black hole, depriving it of matter that would have otherwise fueled it. Conversely, once the wind dies down, the jet can re-emerge.

“We think the jet and wind around this black hole are in a sort of tug of war,” says Joseph Neilsen, Harvard graduate student and lead author of the paper appearing in the journal Nature. “Sometimes one is winning and then, for reasons we don’t entirely understand, the other one gets the upper hand.”

Chandra X-ray Image of GRS 1915. Image: NASA/CXC/Harvard/J.Neilsen.

The observations also show that the wind and the jet carry a similar quantity of matter away from the black hole, evidence that the black hole has a method for regulating its accretion rate. This may be related to the toggling between mass expulsion via either a jet or a wind from the accretion disc. Self-regulation is a common topic when discussing supermassive black holes, but this is the first clear evidence for it in stellar-mass black holes.

“It is exciting that we may be on the track of explaining two
mysteries at the same time: how black hole jets can be shut down and also how black holes regulate their growth,” says co-author Julia Lee, assistant professor in the Astronomy department at the Harvard-Smithsonian Center for Astrophysics.

Despite their variation in mass of several factors of millions, micro-quasars and quasars should show a similarity in behavior when their very different physical scales are taken into account.
“If quasars and micro-quasars behave very differently, then we have a big problem to figure out why, because gravity treats them the same,” says Neilsen. “So, our result is actually very reassuring, because it’s one more link between these different types of black holes.”

The main difference between these two extremes, however, is the timescale for changes in the black holes’ variations. To illustrate, an hour-long cycle for changes seen in GRS 1915 would correspond to about 10,000 years for a supermassive black hole that weighs a billion times the mass of the Sun. “We cannot hope to explore at this level of detail in any single supermassive black hole system,” says Lee. “So, we can learn a tremendous amount about black holes by just studying stellar-mass black holes like this one.”

Although there are still many mysteries concerning this self-regulating behaviour, especially in pinpointing what exactly triggers the jets to re-ignite, Neilsen concludes: “We think our work is a step in the right direction.”

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