Spectral monitoring of RX J1856.5-3754 with XMM-Newton - Analysis of EPIC-pn dataN. Sartore, A. Tiengo, S. Mereghetti, A. De Luca, R. Turolla and F. HaberlA&A, 541 (2012) A66DOI: https://doi.org/10.1051/0004-6361/201118489