Discovery of fast X-ray oscillations during the 1998 giant flare from SGR 1900+14

We report the discovery of complex high-frequency variability during the 1998 August 27 giant flare from SGR 1900 + 14 using the Rossi X-Ray Timing Explorer (RXTE). We detect an approximate to 84 Hz quasi-periodic oscillation (QPO) during a 1 s interval beginning approximately 1 minute after the initial hard spike. The amplitude is energy-dependent, reaching a maximum of 26% (rms) for photons above 30 keV and is not detected below 11 keV, with a 90% confidence upper limit of 14% (rms). Remarkably, additional QPOs are detected in the average power spectrum of data segments centered on the rotational phase at which the 84 Hz signal was detected. Two signals, at 53.5 and 155.1 Hz, are strongly detected, while a third feature at 28 Hz is found with lower significance. These QPOs are not detected at other rotational phases. The phenomenology seen in the SGR 1900 + 14 flare is similar to that of QPOs recently reported by Israel et al. from the 2004 December 27 flare from SGR 1806-20, suggesting that they may have a common origin, perhaps torsional vibrations of the neutron star crust. Indeed, an association of the four frequencies (in increasing order) found in SGR 1900 + 14 with l = 2, 4, 7, and 13 toroidal modes appears plausible. We discuss our findings in the context of this model and show that if the stars have similar masses, then the magnetic field in SGR 1806 - 20 must be about twice as large as in SGR 1900 + 14, broadly consistent with estimates from pulse timing. We briefly discuss how mode identifications could lead to constraints on the nuclear equation of state.