A year in the life of the low-mass X-ray transient Aql X-1

Abstract

The Swift satellite monitored the quiescence of the low-mass X-ray binary transient Aql X-1 on a weekly basis during the 2012 March-November interval. A total of 42 observations were carried out in the soft X-ray (0.3-10 keV) band with the X-ray telescope on board Swift. We investigated the X-ray variability properties of Aql X-1 during quiescence by tracking luminosity variations and characterizing them with a detailed spectral analysis. The source is highly variable in this phase and two bright flares were detected, with peak luminosities of ~4 × 1034 erg s-1 (0.3-10 keV). Quiescent X-ray spectra require both a soft thermal component below ~2 keV and a hard component (a power-law tail) above ~2 keV. Changes in the power-law normalization alone can account for the overall observed variability. Therefore, based on our data set, the quiescent X-ray emission of Aql X-1 is consistent with the cooling of the neutron star core and with mechanisms involving the accretion of matter on to the neutron star surface or magnetosphere. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.