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

• Published in: Astrophysical bulletin 2022-09, Vol.77 (3), p.231-245
• Main Authors: Vinokurov, A., Atapin, K., Bordoloi, O. P., Sarkisyan, A., Kashyap, U., Chakraborty, M., Rahna, P. T., Kostenkov, A., Solovyeva, Y., Fabrika, S., Safonova, M., Gogoi, R., Sutaria, F., Murthy, J.
• Format: Article
• Language: English
• Subjects: Astronomy; Astrophysics and Cosmology; Indian spacecraft; Luminosity; Physics; Physics and Astronomy; Satellite observation; Space missions; Variability; X ray sources; X-ray astronomy; X-rays
• ISSN: 1990-3413; 1990-3421
• DOI: 10.1134/S1990341322030129

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 erg s and a hard power-law spectrum with . 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.