Optical and X-ray variations during five outbursts of Aql X-1 in 3.6 yr from 2016

Abstract

ABSTRACT We analysed optical/X-ray quasi-simultaneous light curves of Aql X-1, obtained by Monitor of All-sky X-ray Image, Zwicky Transient Facility, and Las Cumbres Observatory Global Telescope network in about 3.6 yr from 2016, for understanding electromagnetic radiation mechanisms during its outbursts. As a result, we confirmed that five outbursts had detected in the epoch, and that three outbursts underwent the X-ray state transition across Low-Hard, In-Transition, and High-Soft state while remaining two outbursts stayed in the Low-Hard state. We found that the optical spectral energy distribution in the High-Soft state is consistent with a simplified irradiated disc model, and that the optical colour/magnitude variation can be explained by variations in the X-ray luminosity and the disc geometrical thickness.