Monthly Notices of the Royal Astronomical Society | 2021
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 {\\it HST} archival data. With a precise astrometry, we confirm that NGC 4861 X1 associated with an HII complex as reported by \\cite{2014MNRAS.441.1841T} and we conclude that NGC 4861 X2 resides in a young star group with 400$\\pm$80 M$\\odot$. We also find that NGC 4449 X7 is associated with three optical candidates within an error radius of 0$\\farcs$2 at the 90$\\%$ 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 $\\sim$8 M$\\odot$, 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 {\\it XMM-Newton}, {\\it Chandra} and {\\it 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.