1WGA J1216.9+3743: Chandra Finds an Extremely Steep Ultraluminous X-Ray Source

We report the discovery of a new ultraluminous X-ray source (ULX) in the nearby galaxy NGC 4244 from Chandra archival data. The source, 1WGA J1216.9+3743, is one of the least luminous and softest ULXs discovered so far. Its X-ray spectrum is the best available for a representative of the soft ULXs, a class of sources recently discovered by Chandra and XMM-Newton. 1WGA J1216.9+3743 appears pointlike in the Chandra image and has a complex spectral shape; a multicolor disk model, suitable for brighter ULXs, is inadequate for this source. The 1WGA J1216.9+3743 spectrum is heavily absorbed (NH ~ 1-4 × 1021 cm-2) and very soft. The best-fit power-law model gives Γ ~ 5 and implies a luminosity L(0.5-10 keV) ~ 1039 ergs s-1. A comparison with previous detections shows that, despite the variability displayed by the source during the Chandra observation, the 1WGA J1216.9+3743 count rate, spectral shape, and absorption are practically unchanged over a 9 yr period. We also performed deep optical imaging of the field containing the X-ray source and found a possible R ~ 23.7 counterpart.