GCN Circular 9696

N. Mirabal (U. Complutense de Madrid) and E. V. Gotthelf (Columbia U.)
report:

In light of recent reports of quasi-periodic variations in the
gamma-ray light curve of GRB 090709A (Markwardt et al., GCN 9645
Not found
 
 
;
Golenetskii et al., GCN 9647
Not found
 
 
, Gotz et al., GCN 9649
Not found
 
 
; Ohno et al.,
GCN 9653
Not found
 
 
), we have analyzed Swift XRT observations of GRB 090709A
obtained between 2009 July 9.319 UT and 2009 July 11.862 UT
(79s - 61 hr after the trigger assuming T at 2009 July 9, 07:38:34 UT).
This period overlaps the BAT light curve from T+79 s to T+150 s
(Markward et al., GCN 9645
Not found
 
 
). The first set of observations (390 s) was
taken in WT mode (1.77 ms timing resolution),  followed by three
pointings in PC mode (2.5 s timing resolution). Fast Fourier
Transform (FFT) analysis applied to the light curves
in each interval shows no significant power down to the Nyquist
limit of f < 283 Hz and f < 2 Hz, for the WT and PC mode data,
respectively. Furthermore, the power spectrum of the first 120 s
of WT mode data (79-469 s after the trigger) does not reveal
any significant signal. The FFTs show typical characteristics of both
white and red noise during the interval. The absence of oscillations in
the X-ray band appears to rule out coherent quasi-periodic variations
extending beyond T+150 s. We note that the 8-second cycle derived from
the gamma-ray light curve seems at odds with the spin period P ~ 1 ms
required to produce an energy release Egamma ~ 10^50 erg from a magnetar
engine (Thompson et al. 2004, ApJ, 611, 380), unless the neutron star 
was born
a millisecond pulsar and spun down to a 1-8 s period (depending on the
actual GRB redshift) on a ~10 s timescale.

The power spectrum of the GRB 090709A light curve (WT mode, 79-469 s
after the trigger) can be found at:

http://www.gae.ucm.es/~mirabal/grbs/grb090709a/grb090709a_fft.gif