Mining the Aql X-1 long-term X-ray light curve

Abstract

Aql X-1 is the prototypical low-mass X-ray binary transient. The Rossi X-ray Timing Explorer All-Sky Monitor provided ~16 yr coverage, revealing 20 outbursts. This is by far the most extensive legacy of outbursts from the same source.We investigated the outburst characteristics in terms of energetics, peak luminosities, durations, decays and recurrence times.We found that bright outbursts (peak luminosity ≳1037 erg s-1) equal dimmer outbursts (≲1036.6 erg s-1) in number. The peak luminosity does not correlate with outburst energetics, durations or quiescent times. We analysed the latest stages of the outbursts, searching for exponential and/or linear decays. Light-curve modelling led to constraints on the outer disc radius and enabled us to estimate the viscosity and the irradiation parameters. The former is larger than has been obtained for other, shorter orbital period transients, while the latter is somewhat smaller. This might be related to the longer orbital period of Aql X-1 with respect to other transient X-ray binaries. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.