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When a compact object (e.g. a neutron star or a black hole) is in a binary system, matter is gravitationally pulled off the stellar companion. As the matter falls toward the compact object, it emits X-rays. Sometimes the emitted X-rays are pulsed, or modulated and the pulse period is found to be exactly the same as the spin period of the neutron star.
However, another type of pulsation: Quasi-periodic Oscillation (sources QPOs), was found in the mid-1980s by the European X-ray Observatory Satellite (EXOSAT). It was 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. It was theorized that this modulation of X-rays was due to the difference in frequency between the matter's orbital period around the compact object and the spin period of the compact object. This difference, called the beat frequency, would explain the 6 - 20 Hz QPOs that EXOSAT observed. X-ray pulses which happen much more often (several hundred times a second) have been observed in the meantime as well and are indeed unusual. As an alternative explanation to the beat frequency theory, some scientists now speculate instead that these rapid QPOs are caused by hot bubbles of radiation bursting close to the compact object and colliding with the infalling matter.
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