Gravitational Waves from Long-Duration Simulations of the Dynamical Bar Instability

The direct detection of gravitational radiation presents one of the greatest scientific challenges of our day. With interferometers such as LIGO, VIRGO, GEO, and TAMA [1] expected to be operating in the next few years, and a new generation of spherical resonant mass detectors under study [2,3], the calculation of the signals expected from various astrophysical sources has a high priority. Accurate calculations of the waveforms are needed to enable both the detection and identification of sources [4]. In particular, short duration bursts are expected to be more difficult to detect than longer-lived signals. One interesting class of sources includes rapidly rotating compact objects that develop the rotationallyinduced “bar instability”. This instability derives its name from the bar-like deformation it induces. The resultant object is potentially a strong source of gravitational radiation because of its highly nonaxisymmetric structure. Examples of compact astrophysical objects that may rotate rapidly enough to encounter this instability include stellar cores that have expended their nuclear fuel and are prevented from undergoing further collapse by centrifugal forces [5–10]; a neutron star spun up by accretion from a binary companion [11,12]; and the remnant of a compact binary merger [13,14]. Such global rotational instabilities in fluids arise from nonaxisymmetric modes e±imφ, where m = 2 is known as the “bar mode”. It is convenient to parametrize them by β = Trot/|W |, (1)