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 ~90-100 eV blackbody spectrum with an intrinsic absorption column of NH ~ (2-3) × 1021 cm-2. For a distance of 19 Mpc, the best-fit observed luminosity increases from 1.7 × 1038 ergs s-1 in 1999 December to 8.0 × 1038 ergs s-1 in 2002 May. The intrinsic, absorption-corrected, best-fit luminosity reaches 1.4 × 1040 ergs s-1 in 2002 May. The assumption of unbeamed emission would suggest a black hole of gsim100 M☉. 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 ~103, 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.