Variability of the Ultraluminous X-ray Source M 74 X-1 in X-ray and Optical Ranges

Abstract

Using observations of M 74 X-1 obtained with the Chandra X-ray Observatory and the Hubble Space Telescope, we identified this source in the optical range. Analysis of other archival data allowed us to track the evolution of M 74 X-1 over two decades of observations, as well as to search for a correlation between the object's variability in the X-ray and optical ranges based on several available quasi-simultaneous observations. As a result, we revealed an approximately linear decrease in the object's brightness of about $$1\overset{\textrm{m}}{.}9$$ in the $$V$$ band in the 2005–2021 time range, which is atypical for most ultra-luminous X-ray sources, and, presumably, is not caused by a change in the X-ray luminosity of the object. At the same time, we found changes in the nature of the short-term X-ray variability of M 74 X-1: by 2021, the stochastic variability of the object decreased and the quasi-periodic oscillations on a timescale of about 1.5 hours seen in observations of 2001–2002 disappeared.