Planck Intermediate Results. I. Further validation of new Planck
  clusters with XMM-Newton
  We present further results from the ongoing XMM-Newton validation follow-up
of Planck cluster candidates, detailing X-ray observations of eleven candidates
detected at a signal-to-noise ratio of 4.5<S/N<5.3 in the same 10-month survey
maps used in the construction of the Early SZ sample. The sample was selected
in order to test internal SZ quality flags, and the pertinence of these flags
is discussed in light of the validation results. Ten of the candidates are
found to be bona fide clusters lying below the RASS flux limit. Redshift
estimates are available for all confirmed systems via X-ray Fe-line
spectroscopy. They lie in the redshift range 0.19<z<0.94, demonstrating
Planck's capability to detect clusters up to high z. The X-ray properties of
the new clusters appear to be similar to previous new detections by Planck at
lower z and higher SZ flux: the majority are X-ray underluminous for their
mass, estimated using Y_X as mass proxy, and many have a disturbed morphology.
We find tentative indication for Malmquist bias in the Y_SZ-Y_X relation, with
a turnover at Y_SZ \sim 4 e-4 arcmin^2. We present additional new optical
redshift determinations with ENO and ESO telescopes of candidates previously
confirmed with XMM-Newton. The X-ray and optical redshifts for a total of 20
clusters are found to be in excellent agreement. We also show that useful lower
limits can be put on cluster redshifts using X-ray data only via the use of the
Y_X vs. Y_SZ and X-ray flux F_X vs. Y_SZ relations.
