Long-term X-ray/UV observations of the ultraluminous X-ray source HolmbergII X-1.

Abstract

Ultraluminous X-ray sources (ULXs) are point-like extra-galactic sources with luminosities ex-ceeding the Eddington limit for stellar-mass black holes. The results of our recent observations of one of these sources – Holmberg II X-1 – with the Astrosat observatory prompted us to look at the long-term brightness and spectral variability of this source using the Swift/XRT observations. We found that after being in the state of low variability ( ≲ 2 times) and hard spectrum ( Γ ∼ 1 . 8) for about ten years, the source returned to the state with high variability ( ≳ 10 times ) and a soft spectrum ( Γ ∼ 2 . 5). Thus, during the time of the Swift monitoring of more than 15 years, the ULX experienced state transitions at least 3 times. Also we found that the period of 27 days observed in the variable state and vanished in the steady state appeared again as the source returned to the variable state. Further analysis and modeling are necessary to shed light on the possible mechanisms of the observed state transitions of Holmberg II X-1. The Multifaceted Universe: Theory and Observations 2022