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 $8 × {{10}^{{39}}}$ erg s$^{{ - 1}}$ and a hard power-law spectrum with $Γ \lesssim 1.9$. 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.
- Publication:
-
Astrophysical Bulletin
- Pub Date:
- September 2022
- DOI:
- 10.1134/S1990341322030129
- arXiv:
- arXiv:2205.05204
- Bibcode:
- 2022AstBu..77..231V
- Keywords:
-
- accretion;
- accretion disks;
- X-rays: binaries;
- X-rays: individual: Holmberg II X-1;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 16 pages, 5 figures, Submitted to Astrophysical Bulletin