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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE,General Relativity and Quantum Cosmology, gr-qc
Abstract:
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 are characterized by repeated and sometimes spectacular gamma-ray
bursts. The burst mechanism might involve crustal fractures and excitation of
non-radial modes which would emit 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 ~1
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 3.0x10^{44} d_1^2 erg and 1.4x10^{47} d_1^2 erg respectively,
where d_1 = d_{0501} / 1 kpc and d_{0501} is the distance to SGR 0501+4516.
These limits on GW emission from f-modes 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.