The evolution of the high-energy tail in the quiescent spectrum of the soft X-ray transient Aquila X-1

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 4 months that followed the 2000 X-ray outburst. By adopting the canonical model for the quiescent spectrum of soft X-ray transients, i.e., an absorbed neutron star atmosphere model plus a power-law tail, in 2002 Rutledge et al. concluded that the observed spectral variations could 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 reanalyze 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 it 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 remain unchanged. This lends support to the idea that the power-law component arises from emission at the shock between the radio pulsar wind and in. owing matter from the companion star.