Written by Pam Frost
April 15, 1998
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 a NASA satellite picked up an intense burst of X-rays from the star early last week, CI Cam thrust itself into the center of an astronomical debate about how binary star systems evolve.
R. Mark Wagner, an Ohio State astronomy research scientist in residence at Lowell Observatory in Flagstaff, Az., knew something extraordinary was happening when he received the coordinates of the X-ray outburst from NASA Goddard Space Flight Center, and then, very soon afterward, received coordinates for a radio outburst in the same area. The radio signals were recorded by the Very Large Array (VLA) radio telescope at the National Radio Astronomy Observatory in Socorro, New Mexico.
"Only a handful of stars are known radio sources," said Wagner, "and they usually turn out to be part of exotic binary systems 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 spectrum with the Perkins 72-inch telescope at Lowell.
Each chemical element has its own unique spectrum, and Wagner noticed the presence of large quantities of iron -- a characteristic that made CI Cam unusual 65 years ago. But this time the spectrum also indicated the presence of ionized helium -- a sure sign that CI Cam was part of an unusual binary system that had just undergone 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 optical signatures of an X-ray source all tied together, so there was no doubt that CI Cam was the optical and radio counterpart 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 normal star and a massive companion object such as a neutron star or black hole which undergoes large X-ray eruptions. 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 two objects -- one of which is either a neutron star or a black hole. We don't know yet which one. But CI Cam is 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 almost completely 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 brief outburst was an indication that something is very different about this one. Optically, the star also appears very different 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 material in a dense wind. The material coalesces into a disk that orbits the massive companion and slowly spirals inward under its strong gravitational pull.
The disk may have become unstable at some point, so that all the material began to fall onto the companion at once. In such a case, whether the companion is a small, dense neutron star or a black hole, the matter will be compressed into a very small space -- perhaps less than 50 miles across, where it is heated to about 10 million degrees and expelled away from the companion in a burst of X-rays, like those that were observed by the NASA satellite.
Therein lies some of the current controversy because astronomers don't agree on what causes the outbursts. Recent observations of other X-ray novae suggest that an unstable disk is the most likely explanation, but a brief period of sustained heavy mass loss from the giant star onto the massive companion could have produced the same result. In either case, Wagner suggests that the accretion of material from the normal star onto the neutron star or black hole may have occurred at a supercritical level, which could have sent material shooting outward 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 the presence of high-speed jets of material emanating from CI Cam. The VLA is also estimating the distance to the 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, who developed 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.
Top -- The location of CI Cam (indicated by the arrow) and the X-ray error circle as measured by the Proportional Counter Array (PCA) on board the NASA Rossi X-ray Timing Explorer spacecraft is shown in this visible light picture of the surrounding region extracted from the Space Telescope Science Institute Digital Sky Survey. The region shown in the figure covers an area of the sky comparable to one-quarter the diameter of the full moon.
Bottom -- The location of CI Cam on the sky with respect to prominent constellations and bright stars is shown. The location of CI Cam is indicated by an X (just above center and slightly to the left). The chart is suitable for an observer at 40 degrees north latitude facing northwest at 9 pm EDT on April 15. CI Cam is not visible to the unaided eye but can be seen using a small telescope.
Photos are available via FTP from ftp.lowell.edu/pub/rmw/cicam
Additional photos from the VLA available from ftp.aoc.nrao.edu/pub/press