  In this paper we study the spin-evolution and gravitational-wave luminosity
of a newly born millisecond magnetar, formed either after the collapse of a
massive star or after the merger of two neutron stars. In both cases we
consider the effect of fallback accretion, and consider the evolution of the
system due to the different torques acting on the star, namely the spin up
torque due to accretion and spin-down torques due to magnetic dipole radiation,
neutrino emission, and gravitational wave emission linked to the formation of a
`mountain' on the accretion poles. Initially the spin period is mostly affected
by the dipole radiation, but at later times accretion spin the star up rapidly.
We find that a magnetar formed after the collapse of a massive star can accrete
up to 1 M_{\odot} , and survive on the order of 50 s before collapsing to a
black hole. The gravitational wave strain, for an object located at 1 Mpc, is
h_c \sim 10^{-23} at kHz frequencies, making this a potential target for next
generation ground based detectors. A magnetar formed after a binary neutron
star merger, on the other hand, accretes at the most 0.2 M_{\odot}, and emits
gravitational waves with a lower maximum strain of the order of h_c \sim
10^{-24} , but also survives for much longer times, and may possibly be
associated with the X-ray plateau observed in the light curve of a number of
short gamma-ray burst.
