Discovery of a transient ultraluminous X-ray source in an old field population

Ultraluminous X-ray sources (ULXs) are often associated with spiral galaxies and/or star forming regions, suggesting many are high-mass X-ray binaries. ULXs in old populations also exist, but usually have X-ray luminosities of at most a few times 10^39 erg/s. Here we present the discovery of a ULX in an elliptical galaxy with an X-ray luminosity of about 5x10^39 erg/s. Based on cross-matching with optical data, this ULX is not located in a globular cluster, although it could have been ejected from one. The source faded by a factor of at least 30 in three years. We discuss the possible origin and properties of this ULX, also in light of its unusually hard X-ray spectrum.