A possible line-like emission feature at 8 keV in the Seyfert 1.2 UGC 3973

Two short X-ray exposures (<3600 s each) of the radio-quiet Seyfert 1.2 galaxy UGC 3973 (Mrk 79) were conducted with XMM-Newton as part of an active galactic nucleus snapshot survey. In this paper we concentrate on the significance and possible origin of a narrow 8-keV (rest frame) line-like emission feature detected with the pn instrument during the second observation. Simulations show that the feature is significant at 96.0-98.4 per cent confidence, depending on what a priori assumptions are made. The feature cannot be attributed to background contamination and appears to be variable (or transient) because it was not detected in the first observation of UGC 3973 six months earlier. However, a constant feature cannot be completely dismissed, based on the 90 per cent upper limit on the flux from the first observation. There is some indication that the feature is variable over the duration of the second observation as well. We discuss various models (e.g. Ni emission, recombination edges, outflows, disc lines) which could potentially produce an emission feature at such energies.