Reobserving the extreme-ultraviolet emission from Abell 2199: In situ measurement of background distribution by offset pointing

The EUV excess emission from the clusters A2199 and A1795 remains an unexpl ained astrophysical phenomenon. There has been many unsuccessful attempts t o "trivialize" the findings. In this Letter, we present direct evidence pro ving that the most recent of such attempts, which attributes the detected s ignals to a background nonuniformity effect, is likewise excluded. We addre ss the issue by a reobservation of A2199 that features a new filter orienta tion, usage of a more sensitive part of the detector, and, most crucially, the inclusion of a background pointing at similar to 2 degrees offset-the f irst in situ measurement of its kind. We first demonstrate quantitatively t wo facts: (1) the offset pointing provides an accurate background template for the cluster observation, while (2) data from other blank fields do not. We then performed a point-to-point subtraction of the in situ background f rom the cluster field, with an appropriate propagation of errors. The resul ting cluster radial profile is consistent with that obtained by our origina l method of subtracting a flat asymptotic background. The emission now exte nds to a radius of 20'; it confirms the rising prominence of EUV excess bey ond similar to 5' as previously reported.