In the neutron-star system 4U 1636-536, Miller finds evidence for X-ray bursts
that begin with two hot spots, on opposite sides of the neutron star.
While some flares do indeed exhibit an extended cascade of soft X-rays on the heels of the intense, hard X-ray burst
, X-ray component.
Astronomers have long hoped to use the x-ray bursts
to determine the radius of neutron stars more precisely.
These X-ray bursts
seemed suspiciously reminiscent of cosmic gamma-ray bursts--except for an absence of gamma rays.
He and his colleagues knew that according to established theory, the X-ray bursts
could indicate a black hole: Such radiation is generated when surface material from the visible star succumbs to an unusually strong gravitational tug and falls rapidly onto a disk of matter surrounding its invisible, compact companion.
For example, brief X-ray bursts
(each only a few seconds long) occur on the Sun at the onset of a flare.
He adds that any gas that comes closer to the pulsar must penetrate that star's magnetic field, creating X-ray bursts
in the process.
During the second outburst it displayed fast X-ray bursts
, typical of thermonuclear flashes, which showed it must have a neutron star companion.
X-ray bursts apparently occur when sufficient material collects on a neutron star's surface to initiate a runaway thermonuclear reaction.
If an accretion disk partially blocks the view, the intensity of any detected X-ray burst represents only a fraction of its true intensity.
In this model, the energy required to drive the thermonuclear explosion necessary to produce an X-ray burst
would come from a special sequence of nuclear fusion reactions in which nuclei capture protons to create new isotopes.
Another, more subtle, way that recycled pulsars reveal their fast spin rates is during an X-ray burst
, a magnificent flare that low-mass X-ray binaries sometimes emit.