The University of Chicago News Office
Feb. 26, 1996 Press Contact: Steve Koppes
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University of Chicago astrophysicists help unravel a cosmic mystery

University of Chicago astronomers are chasing a cosmic mystery–a bizarre, pulsing object near the center of our galaxy–that may be a never-before-seen signature of the death throes of a star. University of Chicago astrophysicists Don Lamb and Coleman Miller, and Northwestern University physicist Ronald Taam, describe it as the last gasps of a dying, low-mass X-ray binary star system, in a paper submitted Feb. 9 to Astrophysical Journal Letters.

A news story describing the astronomical anomaly will appear in the Friday, Feb. 23, issue of Science.

The object, GRO J1744-28, was first observed in early December, 1995, by an Earth-orbiting satellite, the Burst and Transient Source Experiment on board the Compton Gamma Ray Observatory. BATSE detected a source near the center of the Milky Way that was emitting X-ray and gamma-ray radiation, in both regular pulses and erratic bursts–at a rate of up to 18 per hour. Astronomers nicknamed the object a bursting pulsar.

“This repeated bursting behavior from a pulsar is unlike anything we’ve ever seen before,” said Lamb, Professor of Astronomy & Astrophysics at Chicago.

Because of the pulsing behavior, astronomers think the object consists of two stars locked in orbit around each other, called a binary. The regular pulsations are like a lighthouse beacon, as the two stars rotate around each other. One of the stars is a small but very dense neutron star–the remnant of a star that exploded in a supernova and then collapsed to an incredibly dense, dark core about the size of the city of Chicago. Because of the timing of the pulses scientists have already determined that the two stars orbit each other once every 12 days.

Lamb and his colleagues say the neutron star is being orbited by a smaller star that’s dying, puffing up so that its outer hydrogen shell is being pulled to the neutron star. Because of the neutron star’s incredible density and high magnetic field, the matter is crashing to the surface, igniting thermonuclear explosions that release bursts of high-energy radiation. “We’re looking at the death throes of a star,” said Lamb. “We know a lot about how that happens when a star dies alone, but we don’t know what happens when it’s in a binary system.”

Observers are scrambling to find counterparts to the X-ray source in other regions of the electromagnetic spectrum. The more information gleaned from the sky, the greater constraints on the models that theorists come up with. BATSE cannot pinpoint a source but information from other satellites and telescopes pointed at that region of the sky have begun pouring in.

In early January, a second satellite, the recently launched X-ray Timing Explorer, detected X-ray bursts coincident with the BATSE observations. XTE narrowed the BATSE error box–the area on the sky from which the source emanates–and gave ground-based observers an opportunity to search for counterparts in other regions of the electromagnetic spectrum.

On Feb. 6 and 8 (reported Feb. 9), the Very Large Array radio telescope in Socorro, New Mexico, observed a radio counterpart in the same region of the sky, but it has yet to be confirmed as coming from the same source.

On Feb. 10, Lamb and his colleagues at the University of Chicago, announced in a telegram to the astronomy community (IAU circular #6310) that they had found what appears to be an optical and infrared counterpart to the radio source, using the remote-controlled Astrophysical Research Consortium 3.5-meter telescope at Apache Point, New Mexico. “The radio source lies in the error box with the X- ray source,” said Lamb. “But there still needs to be something –like a periodic brightening– to link the X-ray with the radio source, which would confirm that we’ve found it in the optical.”

The bursting pulsar has brought together an unusual collaboration of theoretical and observational astronomers at Chicago. In addition to Miller and Lamb, the “Chicago Collective” includes Research Associates Jean Quashnock (a theorist) and Robert Nichol (an observer) and graduate students David Cole, Daniel Vanden Berk and Scott Severson (observers). All are in Astronomy & Astrophysics. They are joined by observing specialists Ed Bergeron, Karen Gloria, and Dan Long.at Apache Point Observatory.

The work is funded by the National Aeronautics and Space Administration. Miller and Quashnock are supported by Compton Gamma-Ray Observatory Fellowships.

 

http://www-news.uchicago.edu/releases/96/960226.cosmic.mystery.shtml
Last modified at 03:50 PM CST on Wednesday, June 14, 2000.

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