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

TL;DR

This study analyzes 20 years of X-ray data from Aql X-1, revealing insights into outburst properties, energetics, and accretion disk parameters, highlighting differences from shorter-period transients.

Contribution

It provides the most extensive analysis of Aql X-1 outbursts, including modeling of decay phases to estimate disk and irradiation parameters, and compares these with other transients.

Findings

Bright and dim outbursts are equally numerous.

Peak luminosity does not correlate with outburst energetics or duration.

Outer disk radius and irradiation parameters differ from shorter-period transients.

Abstract

Aql X-1 is the prototypical low mass X-ray binary transient. The Rossi X-ray Timing Explorer All Sky Monitor provided a ~16 yr coverage revealing 20 outbursts. This is by far the most extensive legacy of outbursts from the same source. We investigated the outbursts characteristics in terms of energetics, peak luminosities, durations, decays and recurrence times. We found that bright outbursts (peak luminosity >10^{37} erg s^-1) equal in number dimmer outbursts (<10^{36.6} erg s^-1). 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 modeling led to constraints on the outer disk radius and enabled us to estimate the viscosity and the irradiation parameters. The former is larger than what 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.