RX J0440.9 + 4431: a persistent Be/X-ray binary in outburstC. Ferrigno, R. Farinelli, E. Bozzo, K. Pottschmidt, D. Klochkov and P. KretschmarA&A, 553 (2013) A103DOI: https://doi.org/10.1051/0004-6361/201321053