The Type Ib supernova 2010O: an explosion in a Wolf-Rayet X-ray binary?

The Type Ib supernova 2010O was recently discovered in the interacting starburst galaxy Arp 299. We present an analysis of two archival Chandra X-ray observations of Arp 299 taken before the explosion and show that there is a transient X-ray source at a position consistent with the supernova. Due to the diffuse emission, the background is difficult to estimate. We estimate the flux of the transient from the difference of the two X-ray images and conclude that the transient can be described by a 0.225-keV blackbody with a luminosity of 2.5 +/- 0.7 x 10(39) erg s(-1) for a distance of 41 Mpc. These properties put the transient in between the Galactic black hole binary XTE J1550-564 and the ultraluminous X-ray binaries NGC 1313 X-1 and X-2. The high level of X-ray variability associated with the active starburst makes it impossible to rule out a chance alignment. If the source is associated with the supernova, it suggests SN 2010O is the explosion of the second star in a Wolf-Rayet X-ray binary, such as Cyg X-3, IC 10 X-1 and NGC 300 X-1.