Gravitational Radiation from Newborn Magnetars in the Virgo Cluster

There is growing evidence that two classes of high-energy sources, the Soft Gamma Repeaters and the Anomalous X-ray Pulsars contain slowly spinning “magnetars”, i.e. neutron stars whose emission is powered by the release of energy from their extremely strong magnetic fields (> 1015 G). We show here that the enormous energy liberated in the 2004 December 27 giant flare from SGR 1806-20 (∼ 5×1046 erg), together with the likely recurrence time of such events, requires an internal field strength of > ∼ 10 16 G. Toroidal magnetic fields of this strength are within an order of magnitude of the maximum fields that can be generated in the core of differentially-rotating neutron stars immediately after their formation, if their initial spin period is of a few milliseconds. A substantial deformation of the neutron star is induced by these magnetic fields and, provided the deformation axis is offset from the spin axis, a newborn fast-spinning magnetar would radiate for a few weeks a strong gravitational wave signal the frequency of which (∼ 0.5 − 2 kHz range) decreases in time. The signal from a newborn magnetar with internal field > 1016.5 G could be detected with Advanced LIGO-class detectors up to the distance of the Virgo cluster (characteristic amplitude hc ∼ 10−21). Magnetars are expected to form in Virgo at a rate > ∼ 1 yr −1. If a fraction of these have sufficiently high internal magnetic field, then newborn magnetars constitute a promising new class of gravitational wave emitters. Subject headings: gravitational waves — stars: magnetic fields — stars: neutron — stars: individual(SGR 1806-20)