Optical nebular emission following the most luminous outburst of Aquila X-1

TL;DR

This study presents optical spectroscopy of Aquila X-1 during its 2016 outburst, revealing a nebular phase possibly caused by ejecta or an extended disc atmosphere, and discusses implications for outflow detection in X-ray binaries.

Contribution

First detailed optical spectroscopic analysis of Aquila X-1's outburst, identifying nebular phases and exploring outflow signatures in low mass X-ray binaries.

Findings

No definitive wind signatures detected.

Detection of strong, broad Halpha emission indicating nebular phase.

Optical nebular phases may be common in LMXB outbursts.

Abstract

Aquila X-1 is a prototypical neutron star low mass X-ray binary and one of the most studied X-ray transients. We present optical spectroscopy obtained with the Gran Telescopio Canarias (10.4 m) during the 2016 outburst, the brightest in recent times, which showed a standard evolution with hard and soft accretion states. Our data set includes a dense coverage of the brightest phases of the event, as well as the decay towards quiescence. We searched for optical winds by studying the profiles and evolution of the main emission lines and found no indisputable wind signatures, such as P-Cyg profiles. Nonetheless, our detailed analysis of the particularly strong and broad Halpha emission line, detected at the end of the outburst, is consistent with the presence of a nebular phase produced by optically thin ejecta at ~800 km/s or, alternatively, an extended disc atmosphere. We discuss these possibilities as well as the similarities with the phenomenology observed in other black hole and neutron star systems. Our study suggests that optical nebular phases might be a relatively common observational feature during the late stages of low mass X-ray binaries outbursts, enabling to probe the presence of outflows at low-to-intermediate orbital inclinations.