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

TL;DR

This study analyzes the spectral and temporal behavior of ULX NGC 55 ULX1 over several years, revealing a dominant soft ultraluminous state, correlations in thermal components, and evidence of flux dips, explained by supercritical outflows.

Contribution

It provides detailed spectral and temporal analysis of ULX NGC 55 ULX1, highlighting its soft ultraluminous state and correlations in thermal components, advancing understanding of ULX emission mechanisms.

Findings

Source mainly in soft ultraluminous state

Anti-correlation between radius and temperature of soft component

Evidence of flux dips at different intensity levels

Abstract

We investigate the spectral and temporal properties of ultra-luminous X-ray source (ULX) NGC~55~ULX1 using {\it Swift}, {\it XMM-Newton} and {\it NuSTAR} observations conducted during 2013--2021. In these observations, the source flux 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 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 {\it Swift} and {\it 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.