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

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 d orbit and a Be star tertiary. We show that the unseen companion is not a 25 M_⊙ BH, but a (2−3) M_⊙ luminous star. Analysing the Optical Gravitational Lensing Experiment (OGLE) and MAssive Compact Halo Object (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_⊙ BH, ruling out the low inclination required for a massive companion. The light curve also shows a clear reflection effect that is well modelled with a 2.5 M_⊙ 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 d orbit that is fully or nearly circularized (e < 0.04), its surface rotation period appears to be at least 20 d. We offer cautionary notes on the interpretation of dormant BH candidates in binaries.

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