Do Globular Clusters Spit Out X-ray Stars?.
THE DENSE STELLAR BEEHIVES KNOWN AS globular clusters are fertile breeding grounds for all kinds of astronomical exotica: millisecond pulsars, blue stragglers, maybe even midsize black holes. Now astronomers can add to the list microquasars --binary-star systems spouting gas jets at nearly the speed of light.
Living in an environment orders of magnitude denser than the Sun's neighborhood, each star in a globular cluster is likely to experience a close encounter with at least one other star during a cluster's multibillion-year lifetime. Binary-star collisions are particularly dramatic: the orbital energy in a binary can be swapped among other stars or binaries. Occasionally a binary can even be flung out of its parent cluster.
That may be the source of Scorpius X-1, according to I. Felix Mirabel and Irapuan Rodrigues (French Atomic Energy Commission). Sco X-1 is the brightest star in the night sky to X-ray eyes. It was discovered in 1962 by Geiger counters aboard an Aerobee rocket. Astronomers now know that Sco X-1 is a tightly bound binary system with a swollen gaseous star orbiting a neutron star--a superdense ball of nuclear matter. Gas spills from the evolved, 12th-magnitude star into the neutron star's strong gravitational grip. The process heats the gas to temperatures measured in millions of degrees, which causes the system to shine copiously in X-rays. The binary also sporadically spurts out jets of relativistic particles, making it a microquasar, albeit an unusual one: most microquasars contain black holes, not neutron stars (S&T: May 2002, page 32).
By combining recent measurements of Sco X-1's space motion with a model for the Milky Way's gravitational field, Mirabel and Rodrigues have made an educated guess about its past travels. Sco X-1, they say, careens tightly around our Milky Way's core, then loops outward on a highly eccentric orbit that takes it nearly 14,000 light-years above the Milky Way's plane. This orbit resembles those of two globular clusters thought to originate in the Milky Way's halo, the scientists report in the January 30th issue of the European journal Astronomy & Astrophysics. "Probably this neutron star picked up its companion and was thrown out of its globular cluster by a close encounter with other stars at the cluster's core," Mirabel says.
Mirabel and Rodrigues admit that Sco X-1 may instead have gotten its eccentric orbit from an impulsive kick when the neutron star's progenitor went supernova. In fact, along with four other astronomers they came to just this conclusion in the case of a different microquasar, GRO J1655-40, whose motion on the sky they measured with the Hubble Space Telescope (February issue, page 24). A supernova kick could reconcile Sco X-1 with an origin in the Milky Way's midplane. But it's unlikely, say Mirabel and Rodrigues, that Sco X-1's closely paired stars would have remained in orbit around one another after enduring a supernova explosion strong enough to propel the binary into its loopy, high-energy Milky Way orbit.