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GCN Circular 9645

Subject
GRB 090709A: Quasiperiodic variations in the BAT light curve
Date
2009-07-10T01:00:23Z (15 years ago)
From
Craig Markwardt at NASA/GSFC/UMD <craigm@milkyway.gsfc.nasa.gov>
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
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