DISCOVERY OF A HIGHLY VARIABLE DIPPING ULTRALUMINOUS X-RAY SOURCE IN M94

Abstract

We report the discovery of a new ultraluminous X-ray source (ULX) 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 gsim100, a high duty cycle (at least ~70%), and a peak luminosity of L X ~ 2 × 10[superscript 39] erg s[superscript –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 ~ 3.6 × 10[superscript 38] erg s[superscript –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 ~ 9 × 10[superscript 37] erg s[superscript –1]), the spectrum is harder again, and the source might be in the steep-power-law state or the hard state of BHBs. In this observation, the light curve might exhibit ~7 hr (quasi-)periodic large modulations over two cycles. The source also has a possible point-like optical counterpart from Hubble Space Telescope images. In terms of the colors and the luminosity, the counterpart is probably a G8 supergiant or a compact red globular cluster containing ~2 × 10[superscript 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.