Closest Black Hole to Earth Discovered

photo: These three radio images, made from observations at the Very Large Array in Soccorro, New Mexico, show the activity of V 4641's jets on Sept. 16 and 17, 1999. The first image shows a jet shooting outward about 125 astronomical units -- three times the distance from the sun to Pluto. It moves at about 90 percent the speed of light on a path that is slightly inclined towards Earth. Just 30 minutes later, the jet had faded noticeably (center image). By the following day the intensity had dropped 40 times (right), but it remained for about three weeks until its signal faded into the background. Credit: Robert Hjellming, National Radio Astronomy Observatory

The dramatic tantrum last fall from an often-overlooked star has betrayed the existence of the nearest black hole yet discovered in the Milky Way -- one that should be put in a class all its own, a team of astronomers announced Friday.

The black hole, which is associated with a visible star called V 4641, is being called a micro-quasar because it exhibited – for a few days in September – the brilliant behavior associated with quasars. It sent out tremendous bursts of X-ray radiation and shot out jets of plasma at some 90 percent the speed of light, said Robert Hjellming, of the National Radio Astronomy Observatory.

Hjellming and colleagues from the Massachusetts Institute of Technology made the announcement here at the meeting of the American Astronomical Society.

Quasars are thought to be black holes billions of times more massive than the sun that lie at the heart of active galactic nuclei. They are extremely energetic X-ray emitters that shoot out tremendous fountains of plasma at velocities approaching the speed of light. These jets stretch for thousands of light-years, puzzling astrophysicists who must struggle to answer how black holes provide the oomph and direction for such powerful columns. (A light-year is 5.88 trillion miles.)

Although V 4641 is billions of times smaller than any quasar -- weighing somewhere between three and 10 solar masses -- astronomers who were looking at the object recognized the behavior. Only three other black holes have earned the micro-quasar distinction, but this is the closest one ever seen. It lies just 1,600 light-years from Earth on the way to the center of the Milky Way in the direction of the constellation Sagittarius.

The object drew tremendous attention from astronomers last September after it erupted in a giant X-ray burst, dimmed, and then fired to life again the following day. (Click here to read space.com's coverage of the burst.)

Within several hours of its initial detection, X-ray-observing satellites and radio telescopes around the world were targeting the object. Most were just in time to catch a few hiccups of activity and watch as the emissions faded out of sight.

Using the Very Large Array, a collection of 27 radio antennas in the New Mexico desert, Hjellming obtained a set of radio images that revealed the object’s near-light-speed jets. (To read more about that discovery and what it tells physicists about black holes, see space.com's October 25 article.)

Now, Hjellming has released a set of images that show just how fast the object’s jets faded.

These three radio images, made from observations at the Very Large Array in Soccorro, New Mexico, show the activity of V 4641's jets on Sept. 16 and 17, 1999. The first image shows a jet shooting outward about 125 astronomical units -- three times the distance from the sun to Pluto. It moves at about 90 percent the speed of light on a path that is slightly inclined towards Earth. Just 30 minutes later, the jet had faded noticeably (center image). By the following day the intensity had dropped 40 times (right), but it remained for about three weeks until its signal faded into the background. Credit: Robert Hjellming, National Radio Astronomy Observatory.

The September theatrics of V 4641 caused astronomers to reclassify the object from a variable star to an X-ray binary system. Historically, only a star has been visible, but the energetic X-ray activity was a dead giveaway that the star was secretly involved with some invisible neighbor – almost certainly a black hole – said Ron Remillard, an astrophysicist at the Massachusetts Institute of Technology.

The startling thing about this object, though, is not its X-ray bursts or its jets, Remillard said. It is the fact that after it burst to prominence, the object dimmed almost immediately. This makes it unique, even among micro-quasars, he said.

Generally the X-ray light curve of such objects flares up and then remains very bright for many months. Finally, it begins to dim very gradually. In any typical X-ray binary system, any outburst of visible light associated with an X-ray burst would last even longer than the X-ray emission.

The general understanding about such systems is that the black hole has an accretion disk made up of in-falling material swirling around its perimeter. This material becomes superheated as it approaches the inner ring of the disk and emits X-rays. The material further out in the disk would be heated by the X-rays streaming from the inner ring, and would give off a hot glow of visible light.

But V 4661 doesn’t do this. Bucking the formula again, the object’s visible light became intensely bright, then dimmed almost immediately even before the main X-ray event began.

What might be happening, Remillard suggested, is that the black hole does not have an accretion disk, and its companion star is shedding material into it rather directly.

It might be "sending matter over in some rapid fashion without necessarily going through a large disk. It’s exciting this jet mechanism in a rather pure form," he said. In this scenario, the effects of a sudden tumbling of stellar material into the black hole might cause the immediate and short-lived effect seen in September.

"We think we have quite a novel system here to pay great attention to," he said.

V 4641 is now obscured by the sun, but when it emerges in a few months, Remillard said he expects a lot more observing work to be done. It is likely, he said, that the star will throw a repeat performance.