Optical Counterparts of ULXs in Two Dwarf Galaxies: NGC 4861 and NGC 4449

Abstract

We present the results of a search for optical candidates of Ultraluminous X-ray sources (ULXs) in two dwarf galaxies: NGC 4861 and NGC 4449 using Hubble Space Telescope HST archival data. With a precise astrometry, we confirm that NGC 4861 X1 associated with an HII complex as reported by Thuan et al. ( 2014) and we conclude that NGC 4861 X2 resides in a young star group with 400±80 M . We also find that NGC 4449 X7 is associated with three optical candidates within an error radius of 0. 2 at the 90% confidence level. Absolute magnitudes (M 𝑣 ) of these candidates are determined as −5.0 and −4.1. The age and mass values for the three candidates by using stellar evolutionary tracks are estimated as 40-50 Myr and ∼8 M , respectively. The locations of optical candidates suggest possible association with a nearby group of stars. In addition, we analyzed the previously unused archival data of XMM-Newton, Chandra and Swift where the sources detected. Although the X-ray spectral data do not allow us to discriminate between physical models, long-term data at hand are consistent with the sources being in luminous hard states.