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 ( > 10 ^ { 15 } G ) .
We show here that the enormous energy liberated in the 2004 December 27 giant flare from SGR 1806-20 ( \sim 5 \times 10 ^ { 46 } erg ) , together with the likely recurrence time of such events , requires an internal field strength of \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 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 ( \sim 0.5 - 2 kHz range ) decreases in time .
The signal from a newborn magnetar with internal field > 10 ^ { 16.5 } G could be detected with Advanced LIGO-class detectors up to the distance of the Virgo cluster ( characteristic amplitude h _ { c } \sim 10 ^ { -21 } ) .
Magnetars are expected to form in Virgo at a rate \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 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 .