Discovery of a transient ultraluminous X-ray source in the elliptical galaxy M86

We report the discovery of the transient ultraluminous X-ray source (ULX) CXOU J122602.3+125951 (hereafter M86 tULX-1), located 3'52″ (19 kpc) north-west of the centre of the giant elliptical galaxy M86 (NGC 4406) in the Virgo Cluster. The spectrum of M86 tULX-1 can be fit by a power law plus multicolour disc model with a 1.0^{+0.8}_{-2.6} index and a 0.66^{+0.17}_{-0.11} keV inner disc temperature, or by a power law with a 1.86 ± 0.10 index. For an isotropically emitting source at the distance of M86, the luminosity based on the superposition of spectral models is (5 ± 1) × 10³⁹ erg s^-1. Its relatively hard spectrum places M86 tULX-1 in a hitherto unpopulated region in the luminosity-disc temperature diagram, between other ULXs and the (sub-Eddington) black-hole X-ray binaries. We discovered M86 tULX-1 in an archival 148-ks 2013 July Chandra observation, and it was not detected in a 20-ks 2016 May Chandra observation, meaning it faded by a factor of at least 30 in three years. Based on our analysis of deep optical imaging of M86, it is probably not located in a globular cluster. It is the brightest ULX found in an old field environment unaffected by recent galaxy interaction. We conclude that M86 tULX-1 may be a stellar-mass black hole of ∼30-100 M_⊙ with a low-mass giant companion, or a transitional object in a state between the normal stellar-mass black holes and the ultraluminous state.