Is the eclipsing binary RR Dra dancing with a hidden tertiary black hole candidate?

Abstract

ABSTRACT Analysis of timing variation of the eclipsing binary RR Dra implies the existence of an additional object with mass of no less than 3.5(±0.2) M⊙. This third object is orbiting around the central eclipsing pair once every 98(±1) yr, making the whole a hierarchical triple system. However, reliable photometric solutions based on light curves from Transiting Exoplanet Survey Satellite and WASP show that contribution of the third-light takes only about 2 per cent of the total luminosity. It could photometrically be attributed to other unrelated star located within the aperture. The tertiary component is thus a massive object but invisible in optical wavelengths. Besides, evidence of misalignment between the barycenter and the centre of light of the system is also found. This strengthen the existence of a hidden black hole candidate in the form of the third body. The potential black hole may play an essential role in extracting angular momentum from the central binary pair, forming the current state. As a good laboratory, RR Dra is an interesting system that can help to study stellar-mass black hole under the circumbinary case.