The Chandra monitoring observations of The Antennae ( NGC 4038/39 ) have led to the discovery of a variable , luminous , supersoft source ( SSS ) .
This source is only detected 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 \sim 100–90 eV blackbody spectrum with an intrinsic absorption column of N _ { H } \sim 2 - 3 \times 10 ^ { 21 } cm ^ { -2 } .
For a distance of 19 Mpc , the best-fit observed luminosity increases from 1.7 \times 10 ^ { 38 } erg s ^ { -1 } in 1999 December to 8.0 \times 10 ^ { 38 } erg s ^ { -1 } in 2002 May .
The intrinsic , absorption-corrected best-fit luminosity reaches 1.4 \times 10 ^ { 40 } erg s ^ { -1 } in 2002 May .
The assumption of unbeamed emission would suggest a black hole of \gtrsim 100 M _ { \odot } .
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 \sim 10 ^ { 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 .