A Halpha metric for identifying dormant black holes in X-ray transients

TL;DR

This paper introduces a new Ha line-based metric to distinguish dormant black holes from cataclysmic variables in X-ray transients, improving detection accuracy in large surveys.

Contribution

The study develops an empirical Ha FWHM and EW metric that effectively separates black hole transients from CVs, aiding in identifying dormant black holes.

Findings

FWHM and EW decrease with orbital period in both BHs and CVs.

BH transients exhibit larger Ha EWs than CVs at similar periods.

The proposed metric can select ~80% of BH transients while rejecting ~78% of CVs.

Abstract

Dormant black holes (BHs) in X-ray transients can be identified by the presence of broad Ha emission lines from quiescent accretion discs. Unfortunately, short-period cataclysmic variables (CVs) can also produce broad Ha lines, especially when viewed at high inclinations, and are thus a major source of contamination. Here we compare the full width at half maximum (FWHM) and equivalent width (EW) of the Ha line in a sample of 20 quiescent BH transients and 354 CVs (305 from SDSS I to IV) with secure orbital periods (Porb) and find that: (1) FWHM and EW values decrease with Porb, and (2) for a given Porb both parameters are typically larger in BH transients than in CVs. We also compile spectral types for 17 low-mass companions in BH transients from the literature and derive an empirical Porb-Teff calibration. Using this, we conclude that the decrease in EW with Porb is mostly driven by the dilution of the Ha flux by the donor star continuum, which dominates the r-band spectrum for Porb>~0.2 d. At shorter periods, the larger contribution of the disc to the total r-band flux introduces significant scatter in the EWs due to the changing visibility of the disc projected area with binary inclination. On the other hand, the larger EWs observed in BHs can be explained by their extreme mass ratios (which limit the fractional contribution of the companion to the total flux) and the absence of a white dwarf component (important at Porb<~0.085 d). Finally, we present a tentative metric, based on Ha FWHM and EW information, and provide optimal cuts to select ~80 % of the BH X-ray transients, while rejecting ~78 % of the CVs in our sample. Such a metric, combined with other multi-frequency diagnostics, can help detect new dormant BH X-ray transients in blind large-scale surveys such as HaWKs and its pathfinder, Mini-HaWKs.