An accurate Chandra localization of NGC 925 J0227+33: identifying the donor star of an ultraluminous X-ray source

There are many open questions regarding the nature of ultraluminous X-ray sources, especially concerning the masses of their compact objects. The only way to obtain reliable masses for black holes in ULXs is through dynamical mass measurements of their donor stars. Our near-infrared survey of nearby ULXs has yielded 3 new spectroscopically identified donor stars. A high resolution image resolved one of these into 3 separate sources. We plan to measure the radial velocity curve of the donor star and determine a lower limit for the mass of the compact object in this ULX, but for this we need to identify which of the 3 NIR sources is the real counterpart. We propose a new, 10 ks Chandra observation to obtain an accurate position for the ULX and uniquely identify its donor star.