GCN Circular 9696
Subject
GRB 090709A: Swift XRT Timing Analysis
Event
Date
2009-07-21T12:18:24Z (17 years ago)
From
Nestor Mirabal at U.Complutense de Madrid <mirabal@gae.ucm.es>
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;
Golenetskii et al., GCN 9647, Gotz et al., GCN 9649; Ohno et al.,
GCN 9653), 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). 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