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