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
SGR1806-20 (~5 10^46 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 > 10^16.5 G could
be detected with Advanced LIGO-class detectors up to the distance of the Virgo
cluster (characteristic amplitude h_c about 10^-21). Magnetars are expected to
form in Virgo at a rate approx. 1/yr. If a fraction of these have sufficiently
high internal magnetic field, then newborn magnetars constitute a promising new
class of gravitational wave emitters.Comment: Accepted for publication on ApJ Letter
