A 33 hour period for the Wolf-Rayet/black hole X-ray binary candidate NGC 300 X-1

Context. NGC 300 X-1 is the second extragalactic candidate, after IC 10 X-1, in the rare class of Wolf-Rayet/compact object X-ray binary systems exemplified in the Galaxy by Cyg X-3. From a the oretical point of view, accretion onto a black hole in a detac hed system is possible for large orbital periods only if the mass of the relativistic object is high or the velocity of the accr eted wind is low. Aims. We analysed a 2 week SWIFT XRT light curve of NGC 300 X-1 and searched for periodicities. Methods. Period searches were made using Lomb-Scargle periodogram analysis. We evaluated the confidence level using Monte Carlo simulations. Results. A period of 32.8±0.4 h (3� error) was found for NGC 300 X-1 with a confidence level >99%. Furthermore, we confirm the high irregular variability during the high flux level, as alr eady observed in the XMM-Newton observations of the source. A folded XMM-Newton light curve is shown, with a profile that is in agreement with SWIFT. The mean absorbed X-ray luminosity in the SWIFT observations was 1.5× 10 38 erg s −1 , close to the value derived from the XMM-Newton data. Conclusions. While Cyg X-3 has a short period of 4.8 h, the period of NGC 300 X-1 is very close to that of IC 10 X-1 (34.8±0.9 h). These are likely orbital periods. Possibility of formation of accretion disk for such high orbital periods strongly dep ends on the terminal velocity of the Wolf-Rayet star wind and black-hole mass. While low masses are possible for wind velocities. 1000 km s −1 , these increase to several tens of solar masses for velociti es > 1600 km s −1 and no accretion disk may form for terminal velocities larger than 1900 km s −1 .