  A moderate level of variability has been detected in the quiescent luminosity
of several neutron star soft X-ray transients. Spectral variability was first
revealed by Chandra observations of Aql X-1 in the four months that followed
the 2000 X-ray outburst. By adopting the canonical model for quiescent spectrum
of soft X-ray transients, i.e. an absorbed neutron star atmosphere model plus a
power law tail, Rutledge et al. (2002a) concluded that the observed spectral
variations can be ascribed to temperature variations of the neutron star
atmosphere. These results can hardly be reconciled with the neutron star
cooling that is expected to take place in between outbursts (after deep crustal
heating in the accretion phase). Here we reanalyse the Chandra spectra of Aql
X-1, together with a long BeppoSAX observation in the same period, and propose
a different interpretation of the spectral variability: that this is due to
correlated variations of the power law component and the column density (>5, a
part of which might be intrinsic to the source), while the temperature and flux
of the neutron star atmospheric component remained unchanged. This lends
support to the idea that the power law component arises from emission at the
shock between a radio pulsar wind and inflowing matter from the companion star.
