A variable ultraluminous supersoft X-ray source in ''the antennae'': Stellar-mass black hole or white dwarf?

The Chandra monitoring observations of The Antennae'' (NGC 4038/4039) have led to the discovery of a variable, luminous, supersoft source (SSS). This source is detected only at energies below 2 keV and, in 2002 May, reached count rates comparable to those of the nine ultraluminous X-ray sources (ULXs) detected in these galaxies. Spectral fits of the SSS data give acceptable results only for a similar to90 - 100 eV blackbody spectrum with an intrinsic absorption column of N-H similar to (2-3) x 10(21) cm(-2). For a distance of 19 Mpc, the best-fit observed luminosity increases from 1.7 x 10(38) ergs s(-1) in 1999 December to 8.0 x 10(38) ergs s(-1) in 2002 May. The intrinsic, absorption-corrected, best-fit luminosity reaches 1.4 x 10(40) ergs s(-1) in 2002 May. The assumption of unbeamed emission would suggest a black hole of greater than or similar to 100M(.). However, if the emission is blackbody at all times, as suggested by the steep soft spectrum, the radiating area would have to vary by a factor of similar to10(3), inconsistent with gravitational energy release from within a few Schwarzschild radii of a black hole. Viable explanations for the observed properties of the SSS are provided by anisotropic emission from either an accreting nuclear-burning white dwarf or an accreting stellar-mass black hole.