X-ray variability of the Narrow Line Seyfert 1 Galaxy PKS 0558-504 W. Brinkmann, P. Arévalo, M. Gliozzi and E. Ferrero A&A, 415 3 (2004) 959-969 Published online: 13 February 2004 DOI: 10.1051/0004-6361:20031705