To be or not to be a black hole: detailed binary population models as a
  sanity check
  We use the self-consistent stellar populations in the Binary Population A
Spectral Synthesis (BPASS) models to assess whether NGC1850-BH1 is a black
hole. Using search criteria based on reported physical properties in the
literature we purposefully search for suitable systems with a black hole (or
compact object) companion: we do not find any. Good matches to the observations
are found in models where the bright component is a stripped star and the
companion is natively (meaning we did not impose this in our search) 1 to 2.3
magnitudes fainter than the primary in the optical bands. This alone can
explain the lack of detection of the companion in the MUSE spectra without the
need to invoke rapid rotation, although the conservative mass transfer
exhibited by these particular models is likely to lead to rapidly rotating
companions which could further smear their spectroscopic signatures. We advise
that future claims of unseen black holes in binary systems would benefit from
exploring detailed binary evolution models of stellar populations as a sanity
check
