We report the discovery of the transient ultraluminous X-ray source ( ULX ) CXOU J122602.3+125951 ( hereafter M86 tULX-1 ) , located 3 \arcmin 52 \arcsec ( 19 kpc ) northwest 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 \pm 0.10 index .
For an isotropically emitting source at the distance of M86 , the luminosity based on the superposition of spectral models is { ( 5 \pm 1 ) \times 10 ^ { 39 } } 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 { \sim } 30 - 100 \mathrm { M } _ { \odot } with a low-mass giant companion , or a transitional object in a state between the normal stellar-mass black holes and the ultraluminous state .