  Newly born magnetars are good candidate sources of ultrahigh energy cosmic
rays. These objects can in principle easily accelerate particles to the highest
energies required to satisfy the ultrahigh energy cosmic ray scenario
(E~10^{20-21} eV), thanks to their important rotational and magnetic energy
reservoirs. Their acceleration mechanism, based on unipolar induction, predicts
however a hard particle injection that does not fit the observed ultrahigh
energy cosmic ray spectrum. Here we show that an adequate distribution of
initial voltages among magnetar winds can be found to soften the spectrum. We
discuss the effect of these distributions for the stochastic gravitational wave
background signature produced by magnetars. The magnetar population
characteristics needed to fit the ultrahigh energy cosmic ray spectrum could
lead in most optimistic cases to gravitational wave background signals enhanced
of up to four orders of magnitudes in the range of frequency 1-100 Hz, compared
to the standard predictions. These signals could reach the sensitivities of
future detectors such as DECIGO or BBO.
