TITLE: GCN CIRCULAR NUMBER: 9645 SUBJECT: GRB 090709A: Quasiperiodic variations in the BAT light curve DATE: 09/07/10 01:00:23 GMT FROM: Craig Markwardt at NASA/GSFC/UMD C. B. Markwardt (GSFC/UMD), F. P. Gavriil (GSFC/UMBC), D. M. Palmer (LANL), W. H. Baumgartner (GSFC/UMBC), S. D. Barthelmy (GSFC) We performed a timing analysis of GRB 090709A. Visually, there appears to be a quasiperiodic signal at period near 8 seconds. More formally, a power spectrum of the full burst shows an apparent strong excess at 8.06 seconds. This analysis is based on a de-trended 64 msec BAT light curve (15-350 keV), T-50 s to T+150 s, where the trend was based on a smoothed version of the light curve (smoothing scale 10 sec). After normalizing the power spectrum by the local noise level (0.2 to 0.6 Hz), we find a peak power of ~52, which has a probability of occurring by random chance of ~1e-6 after adjusting for the number of trials. The results are relatively insensitive to trend smoothing timescale, to within a factor of ~2. The number of trials is based on the number of independent Fourier frequencies examined (~2000) and an estimated 100 BAT bursts long enough in duration to perform such an analysis. We consider this to be a conservative estimate: we are likely to have overestimated the number bursts for which we could do this analysis; and the 0.2 to 0.6 Hz range used to compute the noise level has the most noise, which decreases the overall peak significance. The amplitude of the modulations is as high as ~70% of the smoothed light curve, and as small as 10%. The signal is not strictly coherent. The "Q factor", the ratio of peak centroid frequency to FWHM, is ~11 (whereas a coherent signal with a duration of ~200 seconds could be as sharp as Q=24). We believe such a detection is the first time strong near-periodic oscillations have been seen at this significance for classical gamma-ray bursts. Periodic variations at 8 seconds would be typical for galactic magnetars. If this source is at a redshift of 8.5-10 (Aoki et al. GCN 9634, Morgan et al. GCN 9635), then the intrinsic period would be ~800 milliseconds, which falls outside of the magnetar range. If on the other hand the source is at low redshift (Butler, GCN 9639), the observed period would be typical of magnetars. Figures GRB 090709A BAT light curve http://gcn.gsfc.nasa.gov/gcn/other/grb090709a_bat_lc.gif GRB 090709A BAT light curve after removing trend http://gcn.gsfc.nasa.gov/gcn/other/grb090709a_bat_detrended.gif GRB 090709A BAT light curve (black) and trend (red) http://gcn.gsfc.nasa.gov/gcn/other/grb090709a_bat_trend.png GRB 090709A power spectrum of detrended BAT light curve, normalized by local noise level http://gcn.gsfc.nasa.gov/gcn/other/grb090709a_powspec.png