Optical and X-ray variations during 5 outbursts of Aql X-1 in 3.6 years from 2016

TL;DR

This study analyzed optical and X-ray light curves of Aql X-1 over 3.6 years, revealing five outbursts with different state transitions and linking optical spectral features to X-ray luminosity and disk geometry.

Contribution

It provides new insights into the electromagnetic radiation mechanisms during outbursts of Aql X-1, especially the optical spectral behavior in different X-ray states.

Findings

Confirmed 5 outbursts with 3 undergoing state transitions

Optical spectrum in High-Soft state matches irradiated disk model

Optical variations explained by X-ray luminosity and disk geometry

Abstract

We analyzed optical/X-ray quasi-simultaneous light curves of Aql X-1, obtained by MAXI (Monitor of All-sky X-ray Image), ZTF (Zwicky Transient Facility) and LCO (Las Cumbres Observatory) in about 3.6 years from 2016, for understanding electromagnetic radiation mechanisms during its outbursts. As a result, we confirmed that 5 outbursts had detected in the epoch, and that 3 outbursts underwent the X-ray state transition across Low-Hard, In-Transition, and High-Soft state while remaining 2 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 disk model, and that the optical color/magnitude variation can be explained by variations in the X-ray luminosity and the disk geometrical thickness.