A FWHM-K2 correlation in Black-Hole transients

TL;DR

This study identifies a tight correlation between Halpha emission line width and donor star velocity in black-hole transients, enabling mass function estimation from low-resolution spectra, and distinguishes them from neutron stars and cataclysmic variables.

Contribution

It introduces a new FWHM-K2 correlation for black-hole transients and demonstrates its potential for efficient black hole detection using low-resolution spectroscopy.

Findings

FWHM of Halpha correlates with donor star velocity K2 in black-hole transients.

CVs above the period gap follow a flatter FWHM-K2 correlation.

Distinct FWHM-EW regions differentiate black holes from cataclysmic variables.

Abstract

We compare Halpha emission profiles of 12 dynamically confirmed black holes (BHs) and 2 neutron star X-ray transients (SXTs) in quiescence with those of a sample of 43 Cataclysmic Variables (CVs), also quiescent. The full-width-half maximum (FWHM) of the Halpha line in SXTs is tightly correlated with the velocity semi-amplitude of the donor star K2=0.233(13) FWHM. This new correlation, when combined with orbital periods (i.e. through photometric light curves), opens the possibility to estimate compact object mass functions from single integration, low-resolution spectroscopy. On the other hand, CVs above the period gap are found to follow a flatter correlation, a likely consequence of their larger mass ratios. We also find that the FWHM traces the disc velocity at ~42% R_L1, independently of binary mass ratio. In addition, for a given FWHM, BHs tend to have lower EWs than CVs. This might be explained by the fact that CVs must be seen at higher inclinations to mimic the same projected disc velocities as BH SXTs. For the same reason CVs with FWHM>~1500 km/s are mostly eclipsing while none of our sample BHs are. Further, we show that there is a vacant/unoccupied region for CVs in the FWHM-EW plane defined by FWHM > 2568 ( 1 - (9/EW)**2 )**0.5 (km/s). Both the FWHM-K2 correlation and the FWHM-EW plane can be exploited, together with photometric light curves, to efficiently discover quiescent BHs in deep Halpha surveys of the Galactic Plane.