NGC 2004 #115: a black hole imposter containing three luminous stars

TL;DR

This study reveals that NGC 2004 #115, previously thought to host a black hole, actually contains a luminous star, challenging the identification of dormant black hole candidates based solely on light curves and radial velocities.

Contribution

The paper demonstrates that the unseen companion in NGC 2004 #115 is a luminous star, not a black hole, using detailed light curve analysis and modeling, highlighting potential misinterpretations in dormant black hole candidate systems.

Findings

The companion is a 2-3 solar mass luminous star, not a black hole.

Ellipsoidal variability indicates a lower inclination than previously assumed.

A triple system model explains the observed light curve and radial velocities.

Abstract

NGC 2004 #115 is a recently identified black hole (BH) candidate in the Large Magellanic Cloud (LMC) containing a B star orbiting an unseen companion in a 2.9 day orbit and Be star tertiary. We show that the unseen companion is not a $25\,M_{\odot}$ BH, but a $(2-3)\,M_{\odot}$ luminous star. Analyzing the OGLE and MACHO light curves of the system, we detect ellipsoidal variability with amplitude 10 times larger than would be expected if the companion were a $25\,M_{\odot}$ BH, ruling out the low inclination required for a massive companion. The light curve also shows a clear reflection effect that is well-modeled with a $2.5\,M_{\odot}$ main-sequence secondary, ruling out a lower-mass BH or neutron star companion. We consider and reject models in which the system is a binary containing a stripped star orbiting the Be star: only a triple model with an outer Be star can explain both the observed light curve and radial velocities. Our results imply that the B star, whose slow projected rotation velocity and presumed tidal synchronization were interpreted as evidence for a low inclination (and thus a high companion mass), is far from being tidally synchronized: despite being in a 2.9 day orbit that is fully or nearly circularized ($e < 0.04$), its surface rotation period appears to be at least 20 days. We offer cautionary notes on the interpretation of dormant BH candidates in binaries.