A multiband look at ultraluminous X-ray sources in NGC 7424

We studied the multiband properties of two ultraluminous X-ray sources (2CXO J225728.9-410211 = X-1 and 2CXO J225724.7-410343 = X-2) and their surroundings, in the spiral galaxy NGC 7424. Both sources have approached X-ray luminosities L-X similar to 10(40) erg s(-1) at some epochs. Thanks to a more accurate astrometric solution (based on Australia Telescope Compact Array and Gaia data), we identified the point-like optical counterpart of X-1, which looks like an isolated B8 supergiant (M approximate to 9M(circle dot), age approximate to 30 Myr). Instead, X-2 is in a star-forming region (size of about 100 pc x 150 pc), near young clusters and ionized gas. Very large telescope long-slit spectra show a spatially extended region of He ii lambda 4686 emission around the X-ray position, displaced by about 50 pc from the brightest star cluster, which corresponds to the peak of lower ionization line emission. We interpret the He ii lambda 4686 emission as a signature of X-ray photoionization from the ULX, while the other optical lines are consistent with UV ionization in an ordinary He ii region. The luminosity of this He++ nebula puts it in the same class as other classical photoionized ULX nebulae such as those around Holmberg II X-1 and NGC 5408 X-1. We locate a strong (5.5 GHz luminosity $\nu \, L_{\nu } \approx 10<^>{35}$ erg s(-1)), steep-spectrum, unresolved radio source at the peak of the low-ionization lines, and discuss alternative physical scenarios for the radio emission. Finally, we use WISE data to obtain an independent estimate of the reddening of the star-forming clump around X-2.