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 theAugust 27, 1998 giant flare from SGR 1900+14 using the Rossi X-ray TimingExplorer (RXTE). We detect an 84 Hz oscillation (QPO) during a 1 s intervalbeginning approximately 1 min after the initial hard spike. The modulationamplitude is energy dependent, reaching a maximum of 26% (rms) for photonsabove 30 keV, and is not detected below 11 keV, with a 90% confidence upperlimit of 14% (rms). Remarkably, additional QPOs are detected in the averagepower spectrum of data segments centered on the rotational phase at which the84 Hz signal was detected. Two signals, at 53.5 and 155.1 Hz, are stronglydetected, while a third feature at 28 Hz is found with lower significance.These QPOs are not detected at other rotational phases. The phenomenology seenin the SGR 1900+14 flare is similar to that of QPOs recently reported by Israelet al. from the December 27, 2004 flare from SGR 1806-20, suggesting they mayhave a common origin, perhaps torsional vibrations of the neutron star crust.Indeed, an association of the four frequencies (in increasing order) found inSGR 1900+14 with l = 2, 4, 7, and 13 toroidal modes appears plausible. Wediscuss our findings in the context of this model and show that if the starshave similar masses then the magnetic field in SGR 1806-20 must be about twiceas large as in SGR 1900+14, broadly consistent with magnetic field estimatesfrom pulse timing.Comment: 13 Pages, 5 figures, AASTeX, accepted for publication in the Astrophysical Journal Letter