  The merger of two white dwarfs (WD) is a natural outcome from the evolution
of many binary stars. Recently, a WD merger product, IRAS 00500+6713, was
identified. IRAS 00500+6713 consists of a central star embedded in a circular
nebula. The analysis of the optical spectrum of the central star revealed that
it is hot, hydrogen and helium free, and drives an extremely fast wind with a
record breaking speed. The nebula is visible in infrared and in the [O III]
line images. No nebula spectroscopy was obtained prior to our observations.
Here we report the first deep X-ray imaging spectroscopic observations of IRAS
00500+6713. Both the central star and the nebula are detected in X-rays,
heralding the WD merger products as a new distinct type of strong X-ray
sources. Low-resolution X-ray spectra reveal large neon, magnesium, silicon,
and sulfur enrichment of the central star and the nebula. We conclude that IRAS
00500+6713 resulted from a merger of an ONe and a CO WD, which supports earlier
suggestion for a super-Chandrasekhar mass of this object. X-ray analysis
indicates that the merger was associated with an episode of carbon burning and
possibly accompanied by a SN Iax. In X-rays, we observe the point source
associated with the merger product while the surrounding diffuse nebula is a
supernova remnant. IRAS 00500+6713 will likely terminate its evolution with
another peculiar Type I supernova, where the final core collapse to a neutron
star might be induced by electron captures.
