Spectral and temporal properties of ultra-luminous X-ray source NGC 55 ULX1

Abstract

ABSTRACT We investigate the spectral and temporal properties of ultra-luminous X-ray source (ULX) NGC 55 ULX1 using Swift, XMM-Newton and NuSTAR observations conducted during 2013–2021. In these observations, the source flux varies by a factor of ∼5–6, and we identify the source mainly in the soft ultraluminous (SUL) state of ULXs. We fit the X-ray spectra with a two thermal component model consisting of a blackbody (for the soft component) and a disc (for the hard component), and the soft component dominates in these observations. The soft component in the SUL state shows properties similar to that of ultraluminous supersoft sources, for example, an anticorrelation between the characteristic radius and temperature of the blackbody component. In addition, we observe a positive correlation between the blackbody and inner disc temperatures when the X-ray spectra are fitted with the two-thermal component model. The source exhibits marginal evidence of X-ray flux dips in the Swift and XMM-Newton observations at different intensity levels. We explain the observed spectral and temporal properties of the source by invoking the supercritical radiatively driven outflow mechanism.