Mass ratio determination from Halpha lines in Black-Hole X-ray transients

TL;DR

This paper establishes a new method to estimate black hole mass ratios in X-ray transients using Halpha emission line profiles, enabling studies of fainter systems and expanding our understanding of black hole properties.

Contribution

The paper introduces a novel correlation between the mass ratio and Halpha line profile ratios, allowing mass ratio estimation from faint targets and deepening black hole characterization methods.

Findings

Mass ratio q correlates with DP/FWHM ratio in Halpha lines.

Mass ratios can be estimated with ~32% uncertainty.

Application to faint transients predicts specific mass ratios.

Abstract

We find that the mass ratio q in quiescent black hole (BH) X-ray transients is tightly correlated with the ratio of the double peak separation (DP) to the full-width-half maximum (FWHM) of the Halpha emission line, log q = -6.88 -23.2 log (DP/FWHM). The correlation is explained through the efficient truncation of the outer disc radius by the 3:1 resonance with the companion star. This is the dominant tidal interaction for extreme mass ratios q=M2/M1<~0.25, the realm of BH (and some neutron star) X-ray transients. Mass ratios can thus be estimated with a typical uncertainty of ~32%, provided that the Halpha profile used to measure DP/FWHM is an orbital phase average. We apply the DP/FWHM-q relation to the three faint BH transients XTE J1650-500, XTE J1859+226 and Swift J1357-0933 and predict q=0.026 (+0.038-0.007), 0.049 (+0.023-0.012) and 0.040 (+0.003-0.005), respectively. This new relation, together with the FWHM-K2 correlation presented in Paper I (casares 2015) allows the extraction of fundamental parameters from very faint targets and, therefore, the extension of dynamical BH studies to much deeper limits than was previously possible. As an example, we combine our mass ratio determination for Swift J1357-0933 with previous results to yield a BH mass of 12.4+-3.6 Msun. This confirms Swift J1357-0933 as one of the most massive BH low-mass X-ray binaries in the Galaxy.