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arXiv · 2105.02302

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

M. Ozdogan Ela·A. Akyuz·N. Aksaker·S. Avdan·İ. Akkaya Oralhan·A. Vinokurov·S. Allak·Y. Solovyeva·K. Atapin·D. Bizyaev
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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.

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M. Ozdogan Ela, A. Akyuz, N. Aksaker, S. Avdan, İ. Akkaya Oralhan, A. Vinokurov, S. Allak, Y. Solovyeva, K. Atapin, D. Bizyaev. 2021-05-05. Optical Counterparts of ULXs in Two Dwarf Galaxies: NGC 4861 and NGC 4449. https://doi.org/10.1093/mnras%2Fstab1321

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