Abstract
Ultraluminous X-ray sources (ULXs) are extra-galactic, non-nuclear sources with X-ray luminosity in excess of 1039 erg s–1. It has been thought that the majority of ULX populations are stellar-mass objects accreting matter at a super-Eddington rate. Although ULX studies are often focused in the X-ray regime, this work studied the ULXs in the optical regime, identified as the ULX counterparts (CTPs). The optical variability of nine CTPs were observed using the 2.4-m Thai National Telescope. Out of the nine ULXs, we detected three ULXs exhibiting strong variability up to ∼1 magnitude, suggesting that the CTP light does not come from the donor star’s emission. The paper discusses the physical origins of the variability which potentially explain the observed light curves.
Highlights
Ultraluminous X-ray source (ULXs) are extragalactic, non-nuclear point sources with X-ray luminosity (LX) in excess of the Eddington limit for a 10 solar mass (M⊙) black hole (LX ≥ 1039 erg s−1; see [1] for a recent review)
Given that the counterparts at optical wavelengths (CTPs) are associated with the ULXs, theoretically, the optical light could arise from the donor star emission, outer disc emission, or reprocessing emission of X-rays photons on the outer disc
Since the FWHM of the Thai National Telescope (TNT) ULTRASPEC observed under good weather conditions could be ∼1′′ [7], the ULX counts extracted from the defined region could be contaminated by nearby object emissions, distorting the ULX light curves
Summary
Ultraluminous X-ray source (ULXs) are extragalactic, non-nuclear point sources with X-ray luminosity (LX) in excess of the Eddington limit for a 10 solar mass (M⊙) black hole (LX ≥ 1039 erg s−1; see [1] for a recent review). Out of the nine ULXs, we detected three ULXs exhibiting strong variability up to ∼1 magnitude, suggesting that the CTP light does not come from the donor star’s emission. ULXs might have OB supergiant companions if the optical lights observed are dominated by the donor star emission.
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