1WGAJ1216.9+3743: Chandra Finds an Extremely Steep Ultraluminous X-ray Source

Abstract

We report the discovery of a new ultraluminous X-ray source (ULX) in the nearby galaxy NGC 4244 from Chandra archival data. The source, 1WGAJ1216.9+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. 1WGAJ1216.9+3742 appears point-like in the Chandra image and has a complex spectral shape: a multicolor disk model, suitable for brighter ULXs, is inadequate for this source. 1WGAJ1216.9+3743 spectrum is heavily absorbed (NH ~ (1-4)E21 cm^-2),and very soft. The best-fit power-law model gives a photon index ~ 5 and implies a luminosity L(0.5-10 keV) ~ 1E39 erg/s. A comparison with previous detections shows that, despite the variability displayed by the source during the Chandra observation, 1WGAJ1216.9+3743 count rate, spectral shape and absorption are practically unchange over a 9-year period. We performed also deep optical imaging of the field containing the X-ray source, and found a possible R ~ 23.7 counterpart.

7 pages, 3 figures, emulateapj5 style, accepted version, scheduled for ApJ v582 n2 January 10, 2003 issue

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• Chandra Multi-wavelength Project (ChaMP). I. First X-ray Source Catalog
• The Ultraluminous X-ray Source NGC 1313 X-2 (MS 0317.7-6647) and its Environment
• Blank Field Sources in the ROSAT HRI Brera Multiscale Wavelet catalog
• The Diffuse and Compact X-ray Components of the Starburst Galaxy Henize~2-10
• Candidate isolated neutron stars in the 4XMM-DR10 catalog of X-ray sources
• Optical Counterpart of The Ultraluminous X-ray Source NGC 1313 X-2