1720012

Back in 1997 McConnell et al. searched for 2.2 MeV emission from neutron capture processes with COMPTEL and reported one 3.7 sigma signal in their full sky survey. As exceptional as this discovery was as surprising it still is that no targeted follow-up observation of this source with a gamma-ray telescope has been performed up to now. In addition the only suggested counterpart to this 2.2 MeV signal is very peculiar and INTEGRAL/SPI is the only instrument capable of clarifying this 20 year old puzzle.The most promising candidate is claimed to be RE J0317-853 a fast rotating extremely hot and highly magnetised binary white dwarf (WD). This claim was barely investigated further only trying to connect radio emission as a possible proof of particle acceleration and flaring activity. In general the properties of such exceptional objects and extreme environments are widely debated.A detailed spectral analysis of the 2.2 MeV line from RE J0317-853 can confirm its correspondence to the signal found by McConnell et al. (1997). Furthermore this will provide basic insight in fundamental processes concerning X-ray binary feedback in WD systems. This includes neutron production and accretion in the presence of large magnetic fields as well as recombination physics in extreme environments. RE J0317-853 is the closest high-B WD only 27 pc awayat (l/b) = (299.85/-30.68) and the most massive WD known. This makes it also one of the best candidates to study a possible progenitor of a type Ia supernova in a double-degenerate WD-merger scenario. Its properties especially the extreme values of temperature magnetisation rotation and mass make it to one of the most promising INTEGRAL/SPI high-energy targets.We propose a dedicated 1.5 Ms observation of this serendipitous 2.2 MeV source with INTEGRAL/SPI in addition to the currently available not solar-flare contaminated or zenith-angle constrained 500 ks. [truncated! Please see actual data for full text]