Search for Gravitational Wave Bursts from Six Magnetars

Soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are thought to be magnetars: neutron stars powered by extreme magnetic fields. These rare objects sporadically produce gamma-ray bursts which could be accompanied by gravitational waves (GWs). We present the results of a search for GW bursts from six galactic magnetars that is sensitive to neutron star $f$-modes, thought to be the most efficient GW emitting oscillatory modes in compact stars. One of them, SGR 0501+4516, is likely $\sim1$\,kpc from Earth, an order of magnitude closer than magnetars targeted in previous GW searches. A second, AXP 1E 1547.0$-$5408, gave a burst with an estimated isotropic energy $>10^{44}$ \,erg which is comparable to the giant flares. We find no evidence of GWs associated with a sample of 1279 electromagnetic triggers from six magnetars occurring between November 2006 and June 2009, in GW data from the LIGO, Virgo, and GEO600 detectors. Our lowest model-dependent GW emission energy upper limits for band- and time-limited white noise bursts in the detector sensitive band, and for $f $-mode ringdowns (at 1090\,Hz), are $3.0\times10^{44} d_{1}^2 $\,erg and $1.4\times10^{47} d_{1}^2 $\,erg respectively, where $d_{1} = \frac{d_{\mathrm{0501}}}{1 \mathrm{kpc}}$ and $d_{\mathrm{0501}}$ is the distance to SGR 0501+4516. These limits are an order of magnitude lower than any previous, and approach the range of electromagnetic energies seen in SGR giant flares for the first time.