Search for Dormant Black Holes in Ellipsoidal Variables III. The OGLE BULGE Short-Period Sample

Abstract

This is the third of a series of papers that presents an algorithm to search for close binaries with massive, possibly compact, unseen secondaries. The detection of such a binary is based on identifying a star that displays a large ellipsoidal periodic modulation, induced by tidal interaction with its companion. In the second paper of the series we presented a simple approach to derive a robust modified minimum mass ratio (mMMR), based on the observed ellipsoidal amplitude, without knowing the primary mass and radius, assuming the primary fills its Roche lobe. The newly defined mMMR is always smaller than the actual mass ratio. Therefore, a binary with an mMMR larger than unity is a good candidate for having a massive secondary, which might be a black hole or a neutron star. This paper considers 10, 956 OGLE short-period ellipsoidals observed towards the Galactic Bulge. We re-analyse their modulation and identify 136 main-sequence systems with mMMR significantly larger than unity as candidates for having compact-object secondaries, assuming their observed periodic modulations reflect indeed the ellipsoidal effect. Obviously, one needs follow-up observations to find out the true nature of these companions.

High Tide: A Systematic Search for Ellipsoidal Variables in ASAS-SN

“…While the sample before visual inspection has a single peak at 𝑃 ∼ 0.6 days, the final ELL candidate distribution shows three peaks. This distribution is consistent with the sample of OGLE ellipsoidal variables in the Galactic bulge (Soszyński et al 2016;Gomel et al 2021). The three peaks likely correspond to the conditions for observing ellipsoidal variability at different evolutionary stages.…”