2003ApJ...591..843F

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 N_H~(2-3)x10²¹ cm^–2. For a distance of 19 Mpc, the best-fit observed luminosity increases from 1.7x10³⁸ ergs/s in 1999 December to 8.0x10³⁸ ergs/s in 2002 May. The intrinsic, absorption-corrected, best-fit luminosity reaches 1.4x10⁴⁰ ergs/s in 2002 May. The assumption of unbeamed emission would suggest a black hole of ≳100 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.