Simultaneous X-ray/UV observations of ultraluminous X-ray source Holmberg II X-1 with Indian space mission AstroSat

TL;DR

This study presents simultaneous UV and X-ray observations of the ultraluminous X-ray source Holmberg II X-1 with AstroSat, revealing moderate luminosity, low variability, and discussing implications for different heating models.

Contribution

First simultaneous UV and X-ray observational analysis of Holmberg II X-1 with AstroSat, providing insights into its variability and emission mechanisms.

Findings

Holmberg II X-1 showed moderate X-ray luminosity of 8×10^39 erg/s.

X-ray variability was modest, with a factor of 1.5, and UV variability was insignificant.

No reliable correlation between UV and X-ray flux changes was found.

Abstract

We present the results of 8 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 of $8\times10^{39}$ erg/s and a hard power-law spectrum with $\Gamma \lesssim 1.9$. Due to low variability of the object in X-rays (by a factor 1.5) and insignificant variability in the UV range (upper limit $\approx25$%) we could not find reliable correlation between flux changes in these ranges. Inside each particular observation, the X-ray variability amplitude is higher, it reaches a factor of 2-3 respect to the mean level at the time scales of $\sim10$ ks or even shorter. 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.