'Empirical prior, parametrized by the false alarm probability ``p0``
See  eq. 21 in Scargle (2012)
Note that there was an error in this equation in the original Scargle
paper (the "log" was missing). The following corrected form is taken
from http://arxiv.org/abs/1304.2818'

def p0_prior(self, N):

    return (4 - np.log(((73.53 * self.p0) * (N ** (-0.478)))))
