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

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.

9 pages, 6 figure; accepted to be published on MNRAS

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• Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1
• On the nature of the ultraluminous X-ray source Holmberg II X-1
• A transient ultraluminous X-ray source in NGC 55
• XMM-Newton multi-year campaign on NGC 55 ULX-1: Resolving the wind and its variability with RGS
• Ultraluminous X-ray sources in the group-centric elliptical galaxy NGC 5813
• Spectral and Temporal Properties of Ultra-luminous X-ray Source NGC 55 ULX1