  We report the results of a search for an emission line from radiatively
decaying dark matter in the ultra-faint dwarf spheroidal galaxy Willman 1 based
on analysis of spectra extracted from XMM-Newton X-ray Observatory data. The
observation follows up our analysis of Chandra data of Willman 1 that resulted
in line flux upper limits over the Chandra bandpass and evidence of a 2.5 keV
feature at a significance below the 99% confidence threshold used to define the
limits. The higher effective area of the XMM-Newton detectors, combined with
application of recently developing methods for extended-source analysis, allow
us to derive improved constraints on the combination of mass and mixing angle
of the sterile neutrino dark matter candidate. We do not confirm the Chandra
evidence for a 2.5 keV emission line.
