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.

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 present their findings today 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.

Additional information, illustrations and animation are available on the
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