Revealing the Nature of the Ultraluminous X-ray sources in NGC 4861

ULXs are defined as off-nuclear X-ray sources with isotropic luminosities much higher than the Eddington limit for a solar mass black hole (Lx~1.3e38 erg/s). Typical X-ray luminosities of ULXs are in between 10^39 and 10^41 erg/s. The physical nature of ULXs has been an enigma because of their high energy output. Some ULXs show strong variability suggesting that they are accreting compact objects. Assuming the emission is isotropic, some of the very bright ULXs may harbor intermediate-mass black holes. Some ULXs may just be stellar mass black holes with radiation pressure-dominated or slim accretion disks that cause super-Eddington luminosities. Alternatively, some ULXs may be young X-ray luminous supernova remnants in a high density medium, or hypernova remnants. Optical counterparts of some ULXs are found to be consistent with high mass stars with sometimes evidence for variability and blue spectra possibly indicative of an accretion disc. In addition, a large fraction of ULXs appear to be associated with extended nebulae; supershells hundreds of parsecs in diameter. These large supershells are found to be powered by photoionization and/ or shock-excitation from the ULXs. In this talk, I will present our new results of ULXs in the metal-deficit blue compact dwarf galaxy NGC 4861. Previous studies reveal two ULXs (both ~10^40 erg/s) in this galaxy. From our INT H-alpha and [SII] images, we find that both sources are in the proximity of regions of diffuse nebulosity, which may be powered by the ULXs. Moreover, ULX-1 is found to be associated with a bright H-alpha point source with a [SII] counterpart. Our new Chandra observation together with several archival HST images confirm the association, which will shed light on understanding the nature of these objects.