Hibernating black holes revealed by photometric mass functions

TL;DR

This paper introduces the photometric mass function (PMF) as a new method to efficiently identify quiescent black holes in the Galaxy by analyzing Halpha emission line widths, significantly improving discovery rates.

Contribution

The paper presents the concept of the photometric mass function and demonstrates its effectiveness in detecting quiescent black holes using Halpha line widths and a dedicated survey strategy.

Findings

Halpha widths can be measured with customized filters.

A width cut-off at 2200 km/s effectively removes most contaminants.

The proposed survey could find ~50 new black holes in 800 sq. deg.

Abstract

We present a novel strategy to uncover the Galactic population of quiescent black holes (BHs). This is based on a new concept, the photometric mass function (PMF), which opens up the possibility of an efficient identification of dynamical BHs in large fields-of-view. This exploits the width of the disc Halpha emission line, combined with orbital period information. We here show that Halpha widths can be recovered using a combination of customized Halpha filters. By setting a width cut-off at 2200 km/s we are able to cleanly remove other Galactic populations of Halpha emitters, including ~99.9% of cataclysmic variables (CVs). Only short period (Porb<2.1 h) eclipsing CVs and AGNs will contaminate the sample but these can be easily flagged through photometric variability and, in the latter case, also mid-IR colours. We also describe the strategy of a deep (r=22) Galactic plane survey based on the concept of PMFs: HAWKs, the HAlpha-Width Kilo-deg Survey. We estimate that ~800 sqr deg are required to unveil ~50 new dynamical BHs, a three-fold improvement over the known population. For comparison, a century would be needed to produce an enlarged sample of 50 dynamical BHs from X-ray transients at the current discovery rate.