Simultaneous X-ray/UV Observations of Ultraluminous X-ray Source Holmberg II X-1 with Indian Space Mission Astrosat

Abstract

We present the results of eight epochs of simultaneous UV and X-ray observations of the highly variable ultraluminous X-ray source (ULX) Holmberg II X-1 with AstroSat—Indian multiwavelength space satellite. During the entire observation period from late 2016 to early 2020, Holmberg II X-1 showed a moderate X-ray luminosity about of 8×10398×1039 erg s−1−1 and a hard power-law spectrum with Γ≲1.9Γ≲1.9. Due to low variability of the object in X-rays (by a factor 1.5) and insignificant variability in the UV range (upper limit approximately 25%) we could not find reliable correlation between flux changes in these ranges. Inside each particular observation, the X-ray variability amplitude is higher, reaching a factor of 2–3 respect to the mean level, however, it is observed in the form of relatively short stochastic bursts. We discussed our results in terms of three models of a heated donor star, a heated disk and a heated wind, and estimated the lower limit to the variability which would allow to reject at least part of them.