Self - bound CFL stars in binary systems : are they “ hidden ” among the black hole candidates ?

The identification of black holes is one of the most important tasks of modern astrophysics. Candidates have been selected among binary stars based on a high mass function, and seriously considered when the lower mass limit exceeds ∼ 3M⊙. More recently the absence of (Type I) thermonuclear bursts has been advanced as an additional criterion in favor of the black hole interpretation, since the absence of a solid surface naturally precludes the accumulation and ignition of accreting material. We discuss in this Letter the possibility that selfbound stars made of CFL-paired quarks mimic the behavior of at least the low-mass end black holes as a result of a) higher maximum masses than ordinary neutron stars b) low steady luminosities due to the bare surface properties c) impossibility of generating Type I bursts because of the complete absence of normal matter crusts at their surfaces. These features caution about a positive identification of event horizons based on the lack of bursts.