  This short technical note addresses a large discrepancy between the
temperature profiles for the galaxy cluster A1835 derived by Schmidt et al.
(2001) using Chandra and by Majerowicz et al. (2002) using XMM. The causes of
this discrepancy may be instructive for the Chandra and XMM cluster analyses in
general. The observation used by Schmidt et al. was affected by a mild
background flare that could not be identified by the usual technique. This
flare biased upwards the measured temperatures at large radii. The remaining
discrepancy appears to be due to the XMM PSF scattering that was not taken into
account in the published analyses. While the XMM PSF is narrow, the surface
brightness of a typical cluster also declines very steeply with radius. For the
moderately distant, cooling flow cluster A1835, about 1/3 of the observed XMM
brightness at any radius is due to the PSF scattering from the smaller radii.
As a result, the contamination from the bright cool cluster center biases low
the measured temperatures near the core, and in general, any temperature
gradients are underestimated.
