Search for gravitational wave radiation associated with the pulsating tail of the SGR $1806-20$ hyperflare of 27 December 2004 using LIGO

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 quasiperiodic 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 quasimonochromatic GWs lasting for tens of seconds and emitted at the QPO frequencies. We have observed no candidate signals above a predetermined 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 ${\mathrm{h}}_{\mathrm{rss}\mathrm{\text{−}}\det }^{90\%}=4.5\times {10}^{-22}\mathrm{\text{strain}}{\mathrm{Hz}}^{-1/2}$ 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 $7.7\times {10}^{46}\mathrm{erg}$ ($=4.3\times {10}^{-8}{\mathrm{M}}_{\odot }{\mathrm{c}}^2$), 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.

Figure 1

The strain-equivalent sensitivity of the H1 detector at the time of the hyperflare, the fourth and fifth science runs (S4, S5), and its design sensitivity.

Figure 2

A block diagram of the analysis sketching the signal flow.

Figure 3

Search sensitivity to different waveform families and for different data-quality cuts. The cuts are relative to the off-source RMS distribution calculated in segments 125 ms long and for $2\sigma$ cuts (dark gray crosses) and $4\sigma$ cuts (light gray crosses). Top left panel: SG waveform injections as a function of quality factor $Q$ varied from $Q=600$ to $Q={10}^6$. The dashed line represents the average sensitivity ($5.1\times {10}^{-22}\mathrm{\text{strain}}{\mathrm{Hz}}^{-1/2}$) for injections with $Q>5\times {10}^3$ (where the sensitivity is essentially flat) and a $2\sigma$ cut. Top right panel: 40 s long WNB waveform injections as a function of burst bandwidth ranging from 1 Hz to 11 Hz. Within the parameter space explored, the sensitivity is essentially constant. Bottom left and right panels: PM and AM waveform injections as a function of modulation depth for a modulation frequency of 100 mHz.