Method to madness of black hole, neutron star eruptions
Posted: June 6, 2001

In the fiery machinery of the night sky, where neutron stars and black holes wrapped in binary systems can flare and burst randomly, astronomers have uncovered a predictable mathematical pattern in the X- ray light emitted over time.

Drs. Patricia Boyd and Alan Smale of NASA's Goddard Space Flight Center in Greenbelt, MD, have followed the history of X-ray emission from three binary star systems over the last several years and uncovered a unifying concept: The number of days between the low points of emission in each binary system is random yet always based on multiples of a single constant number.

Artist's concept of an X-ray binary system which could produce variations characterized by random integer multiples of a single constant. Top: A normal star and an X-ray star orbit about their common center of mass. A spiral wave of denser material is present in the accretion disk of hot gas spiraling into the X-ray star. Center: A quarter of an orbit later, the spiral wave lines up with the maximum pull of gravity between the two stars. Bottom: If the resulting tidal forces on the disk have a fixed probability of setting off a disruption, then the X-ray radiation will fade at intervals whose lengths are a constant times some random small integer. Photos: NASA-GSFC
The scientists say this never-before-seen pattern reflects the physics of how matter swirls about and finally pours onto a neutron star, a star composed of nuclear matter that has collapsed under its own gravity, or into a black hole. They presented their findings Tuesday at the 198th Meeting of the American Astronomical Society in Pasadena, CA.

"Neutron stars and black holes can be simultaneously predictable and random, like a dice roll," said Boyd. "After many rolls, statistics tell us something about the dice, that they each have six unique sides. Likewise, in binary star systems, we see that lengths of the long variations (the dice rolls) can be characterized over time by the dynamics of the two stars (the shape and numbers on the dice)."

To obtain an uninterrupted history of a binary star system, the scientists used an instrument aboard NASA's Rossi X-ray Timing Explorer called the All-Sky Monitor (ASM). The ASM has assembled a continuous, five-year digital record of nearly all local star systems known to flicker in X-ray radiation.

Black holes and neutron stars often reside in binary star systems, sharing an orbit with a healthy, hydrogen-burning star. Sometimes, when the orbits bring the two companions close together or when the healthy star flares, gravity pulls gas from the healthy star toward the black hole or the neutron star. The journey, arduous enough for the gas to glow hot in X-ray radiation, follows a path called an accretion disk. Because a black hole is invisible and a neutron star is so tiny (only 10-20 kilometers across), astronomers best learn about these objects from the dynamics of the very visible accretion disk.

Boyd and Smale have uncovered a new tool to probe the physics of the accretion disk, one that combines the predictability of geometry and the randomness of disk disturbances. Their subjects are two probable black holes, Cygnus X-3 and LMC X-3, and one neutron star, Cygnus X-2.

Cygnus X-2 has an orbital period, or length, of 9.8 days. Boyd and Smale found that the time between minimum X-ray brightness is always a whole- number multiple of 9.8 -- for example 77.7 days, 58.8 days or 49 days, which are 8, 6 and 5 times 9.8. One cannot predict what multiple will come next; this is random. The orbital period and the presence of whole- number multiples, though, are constant.

Long-term variations in LMC X-3 and Cygnus X-3 follow the same general rule: The lengths of the variations are always a whole number multiplied by a constant. Finding similar behavior in such different systems implies that the mechanism for disk disturbances must be tied to something as predictable as a clock.

What could cause such clockwork in a chaotic, flaring system? The clumpiness and angle of the accretion disk may be one factor. Scientists believe that accretion disks can be warped and tilted from the plane where the two stars orbit. Gravity makes a tilted disk wobble like a spinning top. If a clump in the accretion disk passed between the two stars as the disk wobbled, the increased gravitational forces might set off the mechanism that disrupts the accretion disk.

The theoretical details of weaving together both random and predictable behavior have yet to be worked out. "The interplay between periodic and random components in these systems is a puzzle," said Smale. "Future ASM data will either show the pattern to continue or reveal an even more complex behavior."

Boyd and Smale work within Goddard's Laboratory for High Energy Astrophysics through their appointments by the University of Maryland, Baltimore County, and the Universities Space Research Association, respectively. The ASM was built by the Massachusetts Institute of Technology, Cambridge.