Abstract
We propose to obtain the first Chandra image of the nearest very luminous (Lx=1e38-1e39 erg/s) recurrent supersoft X-ray source (SSS) currently in outburst. The source, located in NGC300, has a 5.4-hr period and is ultraluminous in the high state. In AO-7, we obtained a Chandra HRC-I observation to locate the position in order to search for the optical counterpart in archival HST and VLT images. However, the source was off with a 99% upper limit of 3.5e37 erg/s (ATel#1560). NGC300 was observed with Swift on 2008 5/20, 6/4, and 6/19. The SSS was clearly detected with XRT in all observations and all the source photons are from below 0.7 keV, consistent with a SSS. The 0.2-2 keV unabsorbed luminosity is about 1.5-2e38 erg/s (ATel#1560). If the companion can be identified, we can set a strong constraint on the donor and conclude whether the compact object is an IMBH. It would also provide important information about the formation of SSS in a black hole binary system.