Broadband X­ray Spectral Properties of NGC 55 ULX1

We investigate the broadband X-ray 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
anti-correlation 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. The observed
spectral and temporal properties of the source can be explained by the
supercritical radiatively driven outflow mechanism.