R. Lieu et al., Reobserving the extreme-ultraviolet emission from Abell 2199: In situ measurement of background distribution by offset pointing, ASTROPHYS J, 527(2), 1999, pp. L77-L80
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.