We report the discovery of a new ultraluminous X-ray source ( ULX ) ( catalog 2XMM J125048.6+410743 ) within the spiral galaxy M94 .
The source has been observed by ROSAT , Chandra , and XMM-Newton on several occasions , exhibiting as a highly variable persistent source or a recurrent transient with a flux variation factor of \gtrsim 100 , a high duty cycle ( at least \sim 70 % ) , and a peak luminosity of L _ { X } \sim 2 \times 10 ^ { 39 } erg s ^ { -1 } ( 0.2–10 keV , absorbed ) .
In the brightest observation , the source is similar to typical low-luminosity ULXs , with the spectrum showing a high-energy cutoff but harder than that from a standard accretion disk .
There are also sporadical short dips , accompanied by spectral softening .
In a fainter observation with L _ { X } \sim 3.6 \times 10 ^ { 38 } erg s ^ { -1 } , the source appears softer and is probably in the thermal state seen in Galactic black-hole X-ray binaries ( BHBs ) .
In an even fainter observation ( L _ { X } \sim 9 \times 10 ^ { 37 } erg s ^ { -1 } ) , the spectrum is harder again , and the source might be in the steep-powerlaw state or the hard state of BHBs .
In this observation , the light curve might exhibit \sim 7 hr ( quasi- ) periodic large modulations over two cycles .
The source also has a possible point-like optical counterpart from HST images .
In terms of the colors and the luminosity , the counterpart is probably a G8 supergiant or a compact red globular cluster containing \sim 2 \times 10 ^ { 5 } K dwarfs , with some possible weak UV excess that might be ascribed to accretion activity .
Thus our source is a candidate stellar-mass BHB with a supergiant companion or with a dwarf companion residing in a globular cluster .
Our study supports that some low-luminosity ULXs are supercritically accreting stellar-mass BHBs .