2018ApJ...854..176V

We study the optical counterparts of ultraluminous X-ray sources (ULXs) NGC 4559 X-10 and NGC 4395 ULX-1. Their absolute magnitudes, after taking reddening into account, are M_V ≃ -5.3 and M_V ≃ -6.2, respectively. The spectral energy distribution (SED) of the NGC 4559 X-10 counterpart is well fitted by a spectrum of an A-type star, whereas NGC 4395 ULX-1 has a blue power-law spectrum. Optical spectroscopy of NGC 4395 ULX-1 has shown a broad He II λ4686 emission, which puts this object in line with all of the other spectrally studied ULXs. Analyzing the absolute magnitudes of 16 well-studied ULX counterparts, we find that the intrinsically fainter counterparts with M_V >= -5.5 have, on average, blackbody-like or power-law SEDs. In other ULX counterparts with M_V <= -5.5, we may observe a power-law-like SED; the only one donor is in NGC 7793 P13 (the ULX-pulsar). One may suggest that as the original accretion rate decreases (but nevertheless remains supercritical), the optical luminosity of the wind becomes lower and the donor star might dominate. Using the Swift archival X-ray data for NGC 4395 ULX-1, we have found a period of 62.8 ± 2.3 days. The X-ray phase curve of the source is very similar to the precession curve of SS 433.