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.

14 pages, 8 tables, 6 figures, Accepted for publication in MNRAS

References in corpus (22)

• X-ray Properties of Black-Hole Binaries
• An Ultraluminous X-ray Source Powered by An Accreting Neutron Star
• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907
• Discovery of a 0.42-s pulsar in the ultraluminous X-ray source NGC 7793 P13
• Discovery of coherent pulsations from the Ultraluminous X-ray Source NGC 7793 P13
• A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source
• Keck HIRES Spectroscopy of Extragalactic HII Regions: C and O Abundances from Recombination Lines
• The discovery of weak coherent pulsations in the ultraluminous X-ray source NGC 1313 X-2
• The powerful jet of an off-nuclear intermediate-mass black hole in the spiral galaxy NGC 2276
• Spectral State Transitions of the Ultraluminous X-ray Sources X-1 and X-2 in NGC 1313
• Optical Properties of the Ultraluminous X-ray Source Holmberg IX X-1 and its Stellar Environment
• LEGUS and Halpha-LEGUS Observations of Star Clusters in NGC 4449: Improved Ages and the Fraction of Light in Clusters as a Function of Age
• New Hubble Space Telescope imaging of the counterparts to six ultraluminous X-ray sources
• Multiband counterparts of two eclipsing ultraluminous X-ray sources in M 51
• CG X-1: an eclipsing Wolf-Rayet ULX in the Circinus galaxy
• The slim-disk state of the ultraluminous X-ray source in M83
• The X-ray properties of the dwarf Magellanic-type galaxy NGC 55
• The ultraluminous X-ray source HoII X-1: kinematic evidence of its escape from the cluster
• Optical Counterparts of ULXs and Their Host Environments in NGC 4490/4485
• X-ray spectral and optical properties of a ULX in NGC 4258 (M106)
• Observations of ultraluminous X-ray sources with the 6-m telescope of the SAO RAS

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• The Transient Ultra-luminous X-ray Source, ULX-4, in M51
• New Transient ULX Candidate in NGC 4254: Evidence of Circumbinary Disk?