stars were unknown as a result. These neutron stars are thought to have relatively low magnetic fields so the gas does not fall mostly onto their magnetic poles, as in accreting pulsars. Because their magnetic fields are so low, the accretion disk can get very close to the neutron star before being disrupted by the magnetic field. The spectral variability of these neutron stars was seen to correspond to changes in the QPOs. Typical QPO frequencies were found to be between about 1 and 60 Hz. The fastest oscillations were found in a spectral state called the Horizontal Branch, and were thought to be a result of the combined rotation of the matter in the disk and the rotation of the collapsed star (the "beat frequency model"). During the Normal Branch and Flaring Branch, the star was thought to approach its Eddington luminosity at which the force of the radiation could repel the accreting gas. This could give rise to a completely different kind of oscillation. Observations starting in 1996 with the Rossi X-ray Timing Explorer could detect faster variability, and it was found that neutron stars and black holes emit X-rays that have QPOs with frequencies up to 1000 Hz or so. Often "twin peak" QPOs were found in which two oscillations of roughly the same power appeared at high amplitudes. These higher frequency QPOs may show behavior related to that of the lower frequency QPOs. Measuring black holes QPOs can be used to determine the mass of black holes. The technique uses a relationship between black holes and the inner part of their surrounding disks, where gas spirals inward before reaching the event horizon. The hot gas piles up near the black hole and radiates a torrent of X-rays, with an intensity that varies in a pattern that repeats itself over a nearly regular interval. This signal is the QPO. Astronomers have long suspected that a QPO's frequency depends on the black hole's mass. The congestion zone lies close in for small black holes, so the QPO clock ticks quickly. As black holes increase in mass, the congestion zone is pushed farther out, so the QPO clock ticks slower and slower. See also Broad iron K line Quasiperiodicity Neutron-star oscillation References X-rays Observational astronomy