The Evolution of the High‐Energy Tail in the Quiescent Spectrum of the Soft X‐Ray Transient Aquila X‐1

Abstract

A moderate level of variabilityhas been detected in the quiescent luminosity of several neutronstar soft X-ray transients. Spectral variability was first revealedby Chandra observations of Aql X-1 in the 4 months that followedthe 2000 X-ray outburst. By adopting the canonical model for thequiescent spectrum of soft X-ray transients, i.e., an absorbed neutronstar atmosphere model plus a power-law tail, in 2002 Rutledge et al.concluded that the observed spectral variations could be ascribed totemperature variations of the neutron star atmosphere. These results canhardly be reconciled with the neutron star cooling that is expected totake place in between outbursts (after deep crustal heating in theaccretion phase). Here we reanalyze the Chandra spectra of Aql X-1,together with a long BeppoSAX observation in the same period, andpropose a different interpretation of the spectral variability: thatit is due to correlated variations of the power-law component andthe column density (>5, a part of which might be intrinsic to thesource), while the temperature and flux of the neutron star atmosphericcomponent remain unchanged. This lends support to the idea that thepower-law component arises from emission at the shock between theradio pulsar wind and inflowing matter from the companion star.