Two eclipsing ultraluminous X-ray sources in M 51

We present the discovery, from archival Chandra and XMM-Newton data, of X-ray eclipses in two ultraluminous X-ray sources (ULXs), located in the same region of the galaxy M 51: CXOM51 J132940.0$+$471237 (ULX-1, for simplicity) and CXOM51 J132939.5$+$471244 (ULX-2). Three eclipses were detected for ULX-1, two for ULX-2. The presence of eclipses puts strong constraints on the viewing angle, suggesting that both ULXs are seen almost edge-on and are certainly not beamed towards us. Despite the similar viewing angles and luminosities ($L_{\rm X} \approx 2 \times 10^{39}$ erg s$^{-1}$ in the $0.3$-$8$ keV band for both sources), their X-ray properties are different. ULX-1 has a soft spectrum, well fitted by Comptonization emission from a medium with electron temperature $kT_e \approx 1$ keV. ULX-2 is harder, well fitted by a slim disk with $kT_{\rm in} \approx 1.5$-$1.8$ keV and normalization consistent with a $\sim 10 M_{\odot}$ black hole. ULX-1 has a significant contribution from multi-temperature thermal plasma emission ($L_{\rm X,mekal} \approx 2 \times 10^{38}$ erg s$^{-1}$); about $10$% of this emission remains visible during the eclipses, proving that the emitting gas comes from a region slightly more extended than the size of the donor star. From the sequence and duration of the Chandra observations in and out of eclipse, we constrain the binary period of ULX-1 to be either $\approx 6.3$ days, or $\approx 12.5-13$ days. If the donor star fills its Roche lobe (a plausible assumption for ULXs), both cases require an evolved donor; most likely a blue supergiant, given the young age of the stellar population in that galactic environment.