Weekly Science Update

Wednesday, March 26, 2008
An Accreting Black Hole
Black holes are often the sources of powerful emission despite their reputation for being implacable sinks for matter and energy. A black hole can be ringed by a disk of matter, and when infalling material interacts with that accretion disk, radiation and matter can be ejected -- sometimes in powerful jets. Astronomers are working to better understand the nature of black holes to probe the fundamental nature of gravity in extreme conditions, and also to understand small black holes, which are the ashes of massive stars that burned all of their nuclear fuel and completed their productive lives.

A black hole is so simple that it can be completely described by only three parameters: its mass, its spin, and its electric charge. A big challenge facing astronomers is determining a black hole's mass. This task is made much easier when the black hole is a member of a binary star pair -- two objects that orbit each other. The periodic orbital motions are precisely determined by the masses, and measurement of the motions determines the masses. Happily, the companion star of a black hole in binary can provide infalling material to interact with the disk and provide enough radiation to measure. In most cases some of the radiation is this scenario is X-ray emission.

When the companion star losing mass to the black hole is less massive than the black hole, and when the X-ray emission has relatively modest energy and is emitted in rare bursts, the pair is called a "soft X-ray transient." There are about ten known soft X-ray transients, but they are particularly interesting because during the long intervals between bursts the orbital motion of the companion stars can be precisely determined, and hence the mass of the black hole can be ascertained.

CfA astronomers Joe Neilsen, Saeqa Vrtilek, and Danny Steeghs used the 6.5-m Clay Telescope to monitor the light from a soft X-ray transient, A0620-00. This close binary pair was known to have an orbital period of only 7.75 hours, and in a few nights the team was able to measure many cycles. Writing in the latest issue of the journal Monthly Notices of the Royal Astronomical Society, the team reports determining the most likely mass of the black hole to be 11.1 solar-masses, with some uncertainty depending on the precise angle of the orbit with respect to the view from earth. The scientists were also able to determine that the disc itself is bright and variable, is not radially symmetric, and has crescent-shaped zones whose brightness contributes significantly (44%) to the optical light, and is modulated at the orbital period. The results help to confirm the theory of small black holes and to improve the characterization of the accretion disks around them.

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