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 day orbit and Be star tertiary. We show that the unseen companion is not a $25M_{\odot }$ 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 $25M_{\odot }$ 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 $2.5M_{\odot }$ 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.

Accepted to MNRAS

References in corpus (16)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars
• Tertiary companions to close spectroscopic binaries
• The VLT-FLAMES Survey of Massive Stars: Observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
• A wide star-black-hole binary system from radial-velocity measurements
• The 'hidden' companion in LB-1 unveiled by spectral disentangling
• I Spy Transits and Pulsations: Empirical Variability in White Dwarfs Using Gaia and the Zwicky Transient Facility
• Is HR 6819 a triple system containing a black hole? -- An alternative explanation
• A naked-eye triple system with a nonaccreting black hole in the inner binary
• A stripped-companion origin for Be stars: clues from the putative black holes HR 6819 and LB-1
• The effect of tides on near-core rotation: analysis of 35 Kepler $\gamma$ Doradus stars in eclipsing and spectroscopic binaries
• Birth of the ELMs: a ZTF survey for evolved cataclysmic variables turning into extremely low-mass white dwarfs
• A comparison of the dynamical and model-derived parameters of the pulsating eclipsing binary KIC9850387
• Inverse Tides in Pulsating Binary Stars
• $\nu$ Gem: a hierarchical triple system with an outer Be star