Quasiperiodic Oscillations Probe Galactic X‐Ray Sources

“It’s very frustrating,” says William Priedhorsky, an x‐ray astronomer at Los Alamos, “that the brightest x‐ray sources in the sky are the ones we have had to be most speculative about.” The sources of Priedhorsky’s lament are the so‐called galactic bulge sources, energetic enigmas found primarily in the galaxy’s central bulge. Theorists had long speculated that the x‐ray emissions of a bulge source originate from hot gas falling from a low‐mass star onto the surface of its rapidly rotating neutron star companion. Unfortunately, evidence for this model had been indirect; the purported neutron star lies hidden beneath a plasma shroud. But recently the situation changed dramatically when a collaboration of researchers from the Max Planck Institute for Extraterrestrial Physics, the University of Amsterdam and MIT reported observations of rapid, quasiperiodic x‐ray oscillations from the bright bulge source GX5—1. Subsequent observations of other bulge sources by Günter Hasinger (Max Planck Institute, Garching) and collaborators, and by Priedhorsky and John Middleditch (Los Alamos) have uncovered five more quasiperiodic oscillators. The existence of QPOs may provide astronomers with their first handle on the bulge source’s neutron star. As a bonus they may also offer solutions to a few long‐standing puzzles of galactic astronomy.

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