2008ApJS..177..181L

Luminous supersoft X-ray sources found in the Milky Way and Magellanic Clouds are likely white dwarfs that steadily or cyclically burn accreted matter on their surface, which are promising Type Ia supernova progenitors. Observations of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources that are generally hotter and more luminous, including some ultraluminous supersoft sources (ULSs) that are possibly intermediate-mass black holes of a few thousand solar masses. In this paper we report our X-ray spectral and timing analysis for M81-ULS 1, a ULS in the nearby spiral galaxy M81. M81-ULS 1 has been persistently supersoft in 17 Chandra ACIS observations spanning 6 years, and its spectrum can be described by either a kT_bb~70 eV blackbody for a ∼1.2 M_☉ white dwarf, or a kT_in~80 eV multicolor accretion disk for a ≳10³ M_☉intermediate-mass black hole. In two observations, the light curves exhibited dramatic flux drop/rise on timescales of 10³ s, reminiscent of eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive search for periodicity in the reasonable range of 50 ks to 50 days failed to reveal an orbital period. The failure to reveal any periodicity is consistent with the long period (≥30 yr) predicted for this system given the optical identification of the secondary with an asymptotic giant star. In addition, the eclipse-like dramatic flux changes in hours are hard to explain under the white dwarf model, but can in principle be explained by disk temperature changes induced by accretion rate variations under the intermediate-mass black hole model.