Astronomers Discover Unusual Star System With Possible Black Hole In Our Galaxy

An Ohio State University astronomer is part of a team that has identified a new andunusual binary star system in our galaxy. The system includes a normal star paired with a dark and massiveobject such as a neutron star or black hole that is ejecting two high-speed jets of material.

The massive object’s companion, the star CI Camelopardalis (CI Cam for short), isn’t new to astronomers-- it was first spotted in 1933 when observers noted the peculiar wavelengths of light it emitted. But when aNASA satellite picked up an intense burst of X-rays from the star early last week, CI Cam thrust itself intothe center of an astronomical debate about how binary star systems evolve.

R. Mark Wagner, an Ohio State astronomy research scientist inresidence at Lowell Observatory in Flagstaff, Az., knewsomething extraordinary was happening when he received thecoordinates of the X-ray outburst from NASA Goddard SpaceFlight Center, and then, very soon afterward, receivedcoordinates for a radio outburst in the same area. The radiosignals were recorded by the Very Large Array (VLA) radiotelescope at the National Radio Astronomy Observatory inSocorro, New Mexico.

“Only a handful of stars are known radio sources,” saidWagner, “and they usually turn out to be part of exotic binarysystems that contain neutron star or black hole companions.”

Wagner found that the coordinates of the radio signals matched the location of CI Cam. He and Sumner G.Starrfield, a professor of physics and astronomy at Arizona State University, quickly recorded its spectrumwith the Perkins 72-inch telescope at Lowell.

Each chemical element has its own unique spectrum, and Wagner noticed the presence of large quantities ofiron -- a characteristic that made CI Cam unusual 65 years ago. But this time the spectrum also indicated thepresence of ionized helium -- a sure sign that CI Cam was part of an unusual binary system that had justundergone a cataclysmic event. The star also appeared about 10 times brighter than normal.

“That was it,” said Wagner. “We had an X-ray source, a radio source, and a bright object with the opticalsignatures of an X-ray source all tied together, so there was no doubt that CI Cam was the optical and radiocounterpart of the original X-ray source.”

Wagner said that CI Cam fits the description of an X-ray nova, a binary star system consisting of a normalstar and a massive companion object such as a neutron star or black hole which undergoes large X-rayeruptions. Decades can elapse between such events.

“What is obvious now is that CI Cam is a binary system,” said Wagner. “There’s not just one star, but twoobjects -- one of which is either a neutron star or a black hole. We don’t know yet which one. But CI Camis very different from many other binaries we know that have neutron stars or black holes.”

That is in part because CI Cam’s X-ray burst peaked 10 hours after it began, and then faded away almostcompletely over the next two days.

“None of us expected that, because typical X-ray novae have outbursts that last for about a year, so the briefoutburst was an indication that something is very different about this one. Optically, the star also appears verydifferent from what we’ve seen up until this point,” said Wagner.

He theorized that CI Cam must be an old, giant star that blasts its companion with streams of atomic materialin a dense wind. The material coalesces into a disk that orbits the massive companion and slowly spiralsinward under its strong gravitational pull.

The disk may have become unstable at some point, so that all the material began to fall onto the companionat once. In such a case, whether the companion is a small, dense neutron star or a black hole, the matter willbe compressed into a very small space -- perhaps less than 50 miles across, where it is heated to about 10million degrees and expelled away from the companion in a burst of X-rays, like those that were observed bythe NASA satellite.

Therein lies someof the currentcontroversybecauseastronomers don’tagree on whatcauses theoutbursts. Recentobservations ofother X-raynovae suggest thatan unstable disk isthe most likelyexplanation, but abrief period ofsustained heavymass loss fromthe giant star ontothe massivecompanion couldhave producedthe same result. In either case, Wagner suggests that the accretion of material from the normal star onto theneutron star or black hole may have occurred at a supercritical level, which could have sent material shootingoutward from the north and south poles in the form of two aligned high-speed jets.

The most recent results from astronomers at the VLA corroborate this view, since they have discovered thepresence of high-speed jets of material emanating from CI Cam. The VLA is also estimating the distance tothe star. Preliminary estimates suggest CI Cam is roughly 3,000 light years from Earth, well within our galaxy.

Other collaborators in this effort include researchers at the Massachusetts Institute of Technology, whodeveloped the device on board the NASA satellite that first received a signal from CI Cam. The device,called the All Sky Monitor, scans the sky looking for new X-ray sources.

Editor's Note: The original news release, with links to more images and further information, can be found at http://www.acs.ohio-state.edu/units/research/archive/cicam.htm.