2016MNRAS.455L..91A

We identified two optical counterparts of brightest ultraluminous X-ray sources (ULXs) in galaxies NGC 5474 and NGC 3627 (M66). The counterparts in Hubble Space Telescope images are very faint, their V magnitudes are 24.7 (MV ~ -4.5) and 25.9 (MV ~ -4.2), respectively. NGC 5474 X-1 changes the X-ray flux more than two orders of magnitude, in its bright state it has LX ~ 1.6x10⁴⁰ erg/s, the spectrum is best fitted by an absorbed power law model with a photon index Γ ~ 0.94. M66 X-1 varies in X-rays with a factor of ∼ 2.5, its maximal luminosity being 2.0x10⁴⁰ erg/s with Γ ~ 1.7. Optical spectroscopy of the NGC 5474 X-1 has shown a blue spectrum, which however was contaminated by a nearby star of 23 mag, but the counterpart has a redder spectrum. Among other objects captured by the slit are a background emission-line galaxy (z = 0.359) and a new young cluster of NGC 5474. We find that these two ULXs have largest X-ray-to-optical ratios of LX/Lopt ∼ 7000 for NGC 5474 X-1 (in its bright state) and 8000 for M66 X-1 both with the faintest optical counterparts ever measured. Probably their optical emission originates from the donor star. If they have super-Eddington accretion discs with stellar-mass black holes, they may also have the lowest mass accretion rates among ULXs such as in M81 X-6 and NGC 1313 X-1.