Gravitational Radiation from Newborn Magnetars in the Virgo Cluster

Abstract

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 ergs), together with the likely recurrence time of such events, requires an internal field strength of 1016 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 on the order 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 of ≥1 yr-1. If a fraction of these have sufficiently high internal magnetic fields, then newborn magnetars constitute a promising new class of gravitational wave emitters.