Gravitational Radiation from Newborn Magnetars in the Virgo Cluster

There is growing evidence that two classes of high-energy sources, the SoftGamma Repeaters and the Anomalous X-ray Pulsars contain slowly spinning``magnetars'', i.e. neutron stars whose emission is powered by the release ofenergy from their extremely strong magnetic fields (>10^15 G. We show here thatthe enormous energy liberated in the 2004 December 27 giant flare fromSGR1806-20 (~5 10^46 erg), together with the likely recurrence time of suchevents, requires an internal field strength of > 10^16 G. Toroidal magneticfields of this strength are within an order of magnitude of the maximum fieldsthat can be generated in the core of differentially-rotating neutron starsimmediately after their formation, if their initial spin period is of a fewmilliseconds. A substantial deformation of the neutron star is induced by thesemagnetic fields and, provided the deformation axis is offset from the spinaxis, a newborn fast-spinning magnetar would radiate for a few weeks a stronggravitational wave signal the frequency of which (0.5-2 kHz range) decreases intime. The signal from a newborn magnetar with internal field > 10^16.5 G couldbe detected with Advanced LIGO-class detectors up to the distance of the Virgocluster (characteristic amplitude h_c about 10^-21). Magnetars are expected toform in Virgo at a rate approx. 1/yr. If a fraction of these have sufficientlyhigh internal magnetic field, then newborn magnetars constitute a promising newclass of gravitational wave emitters.Comment: Accepted for publication on ApJ Letter