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 luminosityof several neutron star soft X-ray transients. Spectral variability was firstrevealed by Chandra observations of Aql X-1 in the four months that followedthe 2000 X-ray outburst. By adopting the canonical model for quiescent spectrumof soft X-ray transients, i.e. an absorbed neutron star atmosphere model plus apower law tail, Rutledge et al. (2002a) concluded that the observed spectralvariations can be ascribed to temperature variations of the neutron staratmosphere. These results can hardly be reconciled with the neutron starcooling that is expected to take place in between outbursts (after deep crustalheating in the accretion phase). Here we reanalyse the Chandra spectra of AqlX-1, together with a long BeppoSAX observation in the same period, and proposea different interpretation of the spectral variability: that this is due tocorrelated variations of the power law component and the column density (>5, apart of which might be intrinsic to the source), while the temperature and fluxof the neutron star atmospheric component remained unchanged. This lendssupport to the idea that the power law component arises from emission at theshock between a radio pulsar wind and inflowing matter from the companion star.Comment: 6 pages, 2 figures. Accepted for publication on Ap