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We investigate the spectral and temporal properties of ultra-luminous X-raysource (ULX) NGC~55~ULX1 using {\it Swift}, {\it XMM-Newton} and {\it NuSTAR}observations conducted during 2013--2021. In these observations, the sourceflux varies by a factor of $\sim 5-6$, and we identify the source mainly in the$soft~ultraluminous$ (SUL) state of ULXs. We fit the X-ray spectra with a twothermal component model consisting of a blackbody (for the soft component) anda disc (for the hard component), and the soft component dominates in theseobservations. The soft component in the SUL state shows properties similar tothat of ultraluminous supersoft sources, for example, an anti-correlationbetween the characteristic radius and temperature of the blackbody component.In addition, we observe a positive correlation between the blackbody and innerdisc temperatures when the X-ray spectra are fitted with the two-thermalcomponent model. The source exhibits marginal evidence of X-ray flux dips inthe {\it Swift} and {\it XMM-Newton} observations at different intensitylevels. We explain the observed spectral and temporal properties of the sourceby invoking the supercritical radiatively driven outflow mechanism.

Spectral and Temporal Properties of Ultra-luminous X-ray Source NGC 55 ULX1