The Birth of an Ultraluminous X-Ray Source in M83

A previously undetected (L_X < 10³⁶ erg s^-1) source in the strongly star-forming galaxy M83 entered an ultraluminous state between 2009 August and 2010 December. It was first seen with Chandra on 2010 December 23 at L_X ≈ 4 × 10³⁹ erg s^-1 and has remained ultraluminous through our most recent observations in 2011 December, with typical flux variation of a factor of two. The spectrum is well fitted by a combination of absorbed power-law and disk blackbody models. While the relative contributions of the models vary with time, we have seen no evidence for a canonical state transition. The luminosity and spectral properties are consistent with accretion powered by a black hole with M _BH ≈ 40-100 M _⊙. In 2011 July we found a luminous, blue optical counterpart that had not been seen in deep Hubble Space Telescope observations obtained in 2009 August. These optical observations suggest that the donor star is a low-mass star undergoing Roche lobe overflow, and that the blue optical emission seen during the outburst is coming from an irradiated accretion disk. This source shows that ultraluminous X-ray sources (ULXs) with low-mass companions are an important component of the ULX population in star-forming galaxies and provides further evidence that the blue optical counterparts of some ULXs need not indicate a young, high-mass companion, but rather that they may indicate X-ray reprocessing.

Based on observations made with NASA's Chandra X-Ray Observatory, the NASA/ESA Hubble Space Telescope (HST), Swift, the 6.5 m Magellan Telescopes located at Las Campanas Observatory, and the Gemini Observatory. NASA's Chandra Observatory is operated by Smithsonian Astrophysical Observatory under contract NAS83060, and the data were obtained through program GO1-12115. The HST observations were obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555. The new HST observations were obtained through programs GO-12513 and GO-12683. Data in the HST archive from program GO-11360 were also used. The ground-based observations were obtained from the Magellan I Telescope at the Las Campanas Observatory and from the Gemini South Telescope of the Gemini Observatory, both awarded through NOAO, which is operated by Association of Universities for Research in Astronomy, Inc., for the National Science Foundation.