Optical variability studies of Ultraluminous X-ray sources

Abstract

Ultraluminous X-ray source (ULXs) are an extra-galactic, non-nuclear point source with X-ray luminosity (LX) in excess of the Eddington limit for a 10 solar mass (M⨀) black hole (LX ≳ 1039 erg s-1 ). It has been suggested that the ULX population is heterogeneous while the majority of them might be a stelar-mass compact object - e.g. black hole - which is accreting matter at a super-Eddington rate. Indeed, the recent observational studies have revealed that some ULXs might be powered by a neutron star, i.e. a neutron star ULX, implying that they are really a super-Eddington accretor. Actually, most ULX studies focus in the X-ray energy band since the sources are very bright in this regime. However, for some ULXs, their optical counterparts (CTPs) could be identified which means that the ULXs can be also studied optically. In this thesis, we study the optical variability of nine ULX CTPs using the 2.4-m Thai National Telescope during the observational season 2017 - 2019. The results suggest that all ULX sample shows a significant optical variability of ≳ 0.3 magnitude, indicating that the optical light of ULX CTPs is unlikely to originate from their donor star emission. In addition, we have also argued that the flux dips found in the light curves of some ULXs could suggest that they are the neutron star ULX entering the propeller phase, in which the accretion is suppressed by the magnetic field. For the ULX Holmberg IX X-1, NGC 4395 ULX1 and M101 ULX-1, they are studied further during the 2018 season.