The discovery of 0.2 Hz quasi-periodic oscillations in the X-ray flux of the transient 42 second pulsar EXO 2030 + 375

The discovery of about 0.2 Hz quasi-periodic oscillations (QPOs) in the 1-15 keV X-ray flux of the transient 42 s pulsar EXO 2030 + 375 is reported. The detection of QPOs in the power spectra of this pulsar was made possible by the application of a newly developed technique to decouple the power spectrum peaks. QPOs were detected during an outburst, when the source was close to its maximum observed luminosity. Upper limits to the QPO strength during the rest of the outburst are consistent with the mean value observed of about 3.5 percent rms. During the outburst the QPOs centroid frequency decreased by about 12 percent, as the luminosity decreased by about 25 percent. The observed QPOs strength, frequency, and frequency-luminosity dependence are all in remarkable agreement with the predictions of the beat frequency model for QPOs and provide the first quantitative confirmation of it. Simple models in which the QPOs signal is produced close to the magnetospheric boundary are ruled out as only about 0.2 percent of the total accretion luminosity can be liberated at this distance from the neutron star surface. 31 refs