tokens: The authors thank the anonymous referee for useful and constructive comments which improved the paper. We want to thank A. Tielens and J. Slavin for providing the hot gas spectrum shown in Fig. 5 . The research leading to these results has received funding from the European Union Seventh Framework Programme FP7-2011 under grant agreement No. 284405. C.R. and M.G. acknowledge funding by the Austrian Science Fund (FWF): project number P24790. The computational results presented have been achieved using the Vienna Scientific Cluster (VSC). This publication was supported by the Austrian Science Fund (FWF). This research has made use of NASA’s Astrophysics Data System. All figures were made with the free Python module Matplotlib ( Hunter 2007 ). This research made use of Astropy, a community-developed core Python package for Astronomy ( Astropy Collaboration et al. 2013 ).
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