Discovery of optical and infrared accretion disc wind signatures in the black hole candidate MAXI J1348-630

TL;DR

This study reports the detection of optical and infrared signatures of accretion disc winds in the black hole candidate MAXI J1348-630, revealing wind features during different outburst phases and suggesting winds are common in low mass X-ray binaries.

Contribution

First detection of optical/infrared accretion disc wind signatures in MAXI J1348-630, demonstrating winds are observable in low inclination low mass X-ray binaries.

Findings

Detection of broad emission line wings in H-alpha during state transition.

Blue-shifted absorption features indicating wind velocities around -500 km/s.

Presence of flat-top profiles throughout the outburst.

Abstract

MAXI J1348-630 is a low mass X-ray binary discovered in 2019 during a bright outburst. During this event, the system sampled both hard and soft states following the standard evolution. We present multi-epoch optical and near-infrared spectroscopy obtained with X-shooter at the Very Large Telescope. Our dataset includes spectra taken during the brightest phases of the outburst as well as the decay towards quiescence. We study the evolution of the main emission lines, paying special attention to the presence of features commonly associated with accretion disc winds, such as blue-shifted absorptions, broad emission line wings and flat-top profiles. We find broad emission line wings in H-alpha during the hard-to-soft transition and blue-shifted absorption troughs at ~-500 km/s in H-beta, HeI-5876, H-alpha and Pa-beta during the bright soft-intermediate state. In addition, flat-top profiles are seen throughout the outburst. We interpret these observables as signatures of a cold (i.e. optical to infrared) accretion disc wind present in the system. We discuss the properties of the wind and compare them with those seen in other X-ray transients. In particular, the wind velocity that we observe is low when compared to those of other systems, which might be a direct consequence of the relatively low binary inclination, as suggested by several observables. This study strengthen the hypothesis that cold winds are a common feature in low mass X-ray binaries and that they can also be detected in low inclination objects via high-quality optical and infrared spectroscopy.