  We report on a ROSAT-HRI observation of the soft X-ray transient Cen X-4
during quiescence. We discover a variation in the flux by a factor of 3 in less
than four days. This relatively fast variation, the first observed from a
quiescent soft X-ray transient, rules out some of the emission mechanisms that
have been proposed for the quiescent flux. Accretion either onto the neutron
star surface or onto the magnetospheric boundary is clearly favored.
