  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-ULS1, an ultraluminous supersoft source in the nearby
spiral galaxy M81. M81-ULS1 has been persistently supersoft in 17 Chandra ACIS
observations spanning six years, and its spectrum can be described by either a
$kT_{bb}\approx70$ eV blackbody for a $\sim1.2M_\odot$ white dwarf, or a
$kT_{in} \approx 80$ eV multicolor accretion disk for a $\gtrsim10^3M_\odot$
intermediate mass black hole. In two observations, the light curves exhibited
dramatic flux drop/rise on time scales of $10^3$ seconds, reminiscent of
eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive
search for periodicity in the reasonable range of 50 ksec to 50 days failed to
reveal an orbital period. The failure to reveal any periodicity is consistent
with the long period ($\ge30$ yrs) predicted for this system given the optical
identification of the secondary with an asymptotic giant star. Also, 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.
