Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1

TL;DR

This study analyzes spectral variability in the ultraluminous X-ray source NGC 55 ULX1 using Swift and Chandra data, revealing changes in spectral components possibly linked to disc outflows.

Contribution

It provides the first detailed spectral variability analysis of NGC 55 ULX1 across multiple observations, suggesting outflows influence spectral changes.

Findings

Spectral variability consistent with changes in normalization and column density.

Spectral changes resemble those in other ULXs.

Possible link between outflows and spectral variability.

Abstract

NGC 55 ULX1 is a bright Ultraluminous X-ray source located 1.78 Mpc away. We analysed a sample of 20 Swift observations, taken between 2013 April and August, and two Chandra observations taken in 2001 September and 2004 June. We found only marginal hints of a limited number of dips in the light curve, previously reported to occur in this source, although the uncertainties due to the low counting statistics of the data are large. The Chandra and Swift spectra showed clearly spectral variability which resembles those observed in other ULXs. We can account for this spectral variability in terms of changes in both the normalization and intrinsic column density of a two-components model consisting of a blackbody (for the soft component) and a multicolour accretion disc (for the hard component). We discuss the possibility that strong outflows ejected by the disc are in part responsible for such spectral changes.