Planck Collaboration, Aghanim, N., Arnaud, M., Ashdown, M., Aumont, J., Baccigalupi, C., et al. (2011). Planck early results - IX. XMM-Newton follow-up for validation of Planck cluster candidates. Astronomy & Astrophysics, 536: A9, pp. 1-20. doi:10.1051/0004-6361/201116460.