We have searched for Gravitational Waves (GWs) associated with the SGR
1806-20 hyperflare of 27 December 2004. This event, originating from a Galactic
neutron star, displayed exceptional energetics. Recent investigations of the
X-ray light curve's pulsating tail revealed the presence of Quasi-Periodic
Oscillations (QPOs) in the 30 - 2000 Hz frequency range, most of which
coincides with the bandwidth of the LIGO detectors. These QPOs, with
well-characterized frequencies, can plausibly be attributed to seismic modes of
the neutron star which could emit GWs. Our search targeted potential
quasi-monochromatic GWs lasting for tens of seconds and emitted at the QPO
frequencies. We have observed no candidate signals above a pre-determined
threshold and our lowest upper limit was set by the 92.5 Hz QPO observed in the
interval from 150 s to 260 s after the start of the flare. This bound
corresponds to a (90% confidence) root-sum-squared amplitude h_rssdet^90% =
4.5e-22 strain Hz^-1/2 on the GW waveform strength in the detectable
polarization state reaching our Hanford (WA) 4 km detector. We illustrate the
astrophysical significance of the result via an estimated characteristic energy
in GW emission that we would expect to be able to detect. The above result
corresponds to 7.7e46 erg (= 4.3e-8 M_sun c^2), which is of the same order as
the total (isotropic) energy emitted in the electromagnetic spectrum. This
result provides a means to probe the energy reservoir of the source with the
best upper limit on the GW waveform strength published and represents the first
broadband asteroseismology measurement using a GW detector.Comment: 13 pages, 2 tables, 3 figures, submitted to Phys. Rev.