Re-observing the EUV emission from Abell 2199: in-situ measurement of background distribution by offset pointing

摘要

The EUV excess emission from the clusters A2199 and A1795 remains anunexplained astrophysical phenomenon. There has been many unsuccessful attemptsto `trivialize' the findings. In this Letter we present direct evidence toprove that the most recent of such attempts, which attributes the detectedsignals to a background non-uniformity effect, is likewise excluded. We addressthe issue by a re-observation of A2199 which features a new filter orientation,usage of a more sensitive part of the detector and, crucially, includes abackground pointing at $sim$ 2$^o$ offset - the first {it in situ}measurement of its kind. We demonstrate quantitatively two facts: (a) theoffset pointing provides an accurate background template for the clusterobservation, while (b) data from other blank fields do not. We then performedpoint-to-point subtraction of the {it in situ} background from the clusterfield, with appropriate propagation of errors. The resulting cluster radialprofile is consistent with that obtained by our original method of subtractinga flat asymptotic background. The emission now extends to a radius of 20arcmin; it confirms the rising prominence of EUV excess beyond $sim$ 5 arcminas previously reported.