A H<i>α</i> metric for identifying dormant black holes in X-ray transients

Dormant black holes in X-ray transients can be identified by the presence of broad H α emission lines from quiescent accretion discs. Unfortunately, short-period cataclysmic variables can also produce broad H α 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 H α line in a sample of 20 quiescent black hole transients and 354 cataclysmic variables (305 from SDSS I to IV) with secure orbital periods ( P orb ) and find that: (1) FWHM and EW values decrease with P orb , and (2) for a given P orb both parameters are typically larger in black hole transients than in cataclysmic variables. We also compile spectral types for 17 low-mass companions in black hole transients from the literature and derive an empirical P orb − T eff calibration. Using this, we conclude that the decrease in EW with P orb is mostly driven by the dilution of the H α flux by the donor star continuum, which dominates the r -band spectrum for P orb ≳ 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 black holes 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 P orb ≲ 0.085 d). Finally, we present a tentative metric, based on H α FWHM and EW information, and provide optimal cuts to select ∼80% of the black hole X-ray transients, while rejecting ∼78% of the cataclysmic variables in our sample. Such a metric, combined with other multi-frequency diagnostics, can help detect new dormant black hole X-ray transients in blind large-scale surveys such as H α WKs and its pathfinder, Mini-H α WKs.