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

Journal
Date Issued
2015
Author(s)
•
•
•
Motta, Sara
•
DOI
Description
We thank the anonymous referee for the useful comments and suggestions. FP, PE, LZ, and AW acknowledge financial support from the INAF research grant PRIN-2011-1 (‘Challenging Ultraluminous X-ray sources: chasing their BHs and formation pathways’) and the ASI/INAF contract I/004/11/0 and ASI/INAF n. I/037/12/0. PE acknowledges a Fulbright Research Scholar grant administered by the U.S.–Italy Fulbright Commission and is grateful to the Harvard–Smithsonian Center for Astrophysics for hosting him during his Fulbright exchange. This research is based on observations made by the Chandra X-ray Observatory and has made use of software provided by the Chandra X-ray Center (CXC, operated for NASA by SAO) in the application packages ciao and ChIPS . This research is based on observations with the NASA/UKSA/ASI mission Swift .
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-component 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.
Volume
448
Issue
2
Start page
1153
Uri
Url
Issn Identifier
0035-8711
Ads BibCode
2015MNRAS.448.1153P
Rights
open.access
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