Twinkle, Twinkle (really fast!), Little Star
The Rossi X-ray Timing Explorer (RXTE) has discovered
stars that emit streams of
X-rays pulsing over
1,000 times a
The pulses are not strictly periodic, but vary slightly from cycle to
cycle. Astronomers call them "quasi-periodic oscillations (QPOs)".
This just means that the pulses are almost, but not quite, periodic.
A neutron star is the superdense remains of an exploded star that
gravitationally collapsed back in on itself to form a small, compressed core
of neutrons. It is not unusual for a neutron star to emit X-rays. When a
neutron star is in a
with a sun-like star,
gravitationally pulled off this stellar
companion. As the matter falls toward the neutron star, it emits X-rays. We
can think of it this way: "It's the sound of matter going splat,"
says UC Berkeley astrophysicist Jonathan Arons.
Sometimes the emitted X-rays are pulsed, or modulated, by the spinning of
the neutron star. The pulse period seen in the X-ray emission is exactly
the same as the spin period of the star. However, another type of pulsation,
the QPO, was found in the mid-1980s by the European X-ray Observatory
Satellite (EXOSAT). The cycle times of these QPOs
were between 6 and 20 times a second for most of the sources in which such
behavior was observed. It was also noticed that the average period of the
oscillations varied as the overall X-ray brightness of the source varied.
The brighter the source was in X-rays, the shorter the QPO period.
Scientists would say this as "the central
as source intensity."
It was theorized that this modulation of X-rays was due to the difference in
frequency between the matter's orbital period around the neutron star and the
spin period of the neutron star. This difference, called the beat
frequency, would explain the 6 - 20 Hz QPOs that EXOSAT observed. It was
hard to confirm this theory, however, because most of the systems in which
the QPOs were found did not allow for a direct measurement of the neutron
star spin period.
Because of its much higher
time resolution, RXTE has not only
discovered QPOs at much higher frequencies than 6-20 Hz, it
may also have verified the "beat frequency" model. RXTE has directly
observed the neutron star spin frequency, the frequency of the orbiting material,
and the beat frequency for a particular X-ray source. Named 4U 1728-34, the
frequency of the orbiting material was found to be around 1100 Hz, the
neutron star rotation frequency to be 363 Hz, and the
beat frequency (or difference between the two) to be around 700 Hz. And,
yet, these numbers are a large step up from 6 and 20 Hz. X-rays pulses
which happen as often as 1,100 times a second are indeed unusual. As an
alternate explanation to the beat frequency theory, some scientists
now speculate instead that these rapid QPOs are caused by hot bubbles of
bursting on the neutron star's surface and colliding with the infalling
As RXTE continues to observe the high-energy Universe over the years to
come, astronomers hope to get the data which will not only allow them to
understand what is happening, but why, and how.