The fastest periodic signals seen to date in astronomy have been observed by the Rossi X-ray Timing Explorer(RXTE) in binary stars systems in which one star is an ultra-dense neutron star accreting material from its companion. In February 1996, two groups analyzing data at Goddard Space Flight Center began to see evidence for these ultrafast variations in the X-ray light curves of 4U 1728-34 (led by Tod Strohmayer of Goddard) and Sco X-1 (led by Michiel van der Klis of the University of Amsterdam). The oscillations they observed are nearly (but not quite) periodic and have frequencies of about 1000 cycles per second (hertz). Hence these signals are called "kilohertz (kHz) quasi-periodic oscillations (QPO)".
The figure above shows the "power spectrum" of the X-ray signal detected from 4U 1728-34. The "power spectrum" measures the strength of the time signal frequency as a function of time. A "good" clock (one that would be strictly periodic) would show a bright red horizontal streak in the image above. The 2 bright red streaks that are visible tell astrophysicists that there are at least 2 signals from this star, one near 1000 kHz and one at 600 kHz, and that the frequencies of these signals increase with time (the "streaks" have a positive slope, indicating that whatever's happening, it's happening more frequently as time goes on).
To date these signals have been seen in about 15 neutron stars in the galaxy. In many of the sources one sees two frequencies which vary with source brightness, though the exact relationship is different for different objects. For some of the sources the difference between the frequencies remains constant, and is thought to be telling us the rotation rate of the neutron star. The rotation rates seen so far are all between about 250 and 350 hertz (i.e., rotations per second).
These kHz QPO are among the most important scientific results to date of
RXTE, since the material giving rise to the kHz QPO is thought to be
produced by gas spiraling towards the neutron stars supersonically and
colliding with the neutron star's surface. These observations allow
scientists to measure the physical nature of the ultracondensed matter that
makes up the neutron star's interior under conditions impossible to
reproduce on earth.
