A large spin-up rate measured with INTEGRAL in the high mass X-ray binary pulsar SAX J2103.5+4545

The High Mass X-ray Binary Pulsar SAX J2103.5+4545 has been observed with INTEGRAL several times during the last outburst in 2002–2004. We report a comprehensive study of all INTEGRAL observations, allowing a study of the pulse period evolution during the recent outburst. We measured a very rapid spin-up episode, lasting ~130 days, which decreased the pulse period by ~1.8 s. The spin-up rate, $\dot{P}$ ~ -1.5 $\times$ 10-7 s s-1, is the largest ever measured for SAX J2103.5+4545, and it is among the fastest for an accreting pulsar. The pulse profile shows evidence for temporal variability, apparently not related to the source flux or to the orbital phase. The X-ray spectrum is hard and there is significant emission up to 150 keV. A new derivation of the orbital period, based on RXTE data, is also reported