Timing analysis and spin evolution of accreting millisecond X-ray pulsars: the strange case of XTE J1807—294.

The detailed timing analysis of the accreting X-ray millisecond pulsar XTE J1807—294 during the 2003 outburst observed by RXTE, will be summed up here. A timing technique was developed in order to obtain the complete set of orbital parameters. The phase delays derived from the pulse profile show a strong erratic behavior superposed to what appears as a global spin-up trend. The erratic behavior of the pulse phase delays is strongly anti-correlated to rapid variations of the flux. A separate analysis of the phase delays of the first and second harmonic of the spin frequency shows that the latter are far less affected by the erratic behavior. In the hypothesis that the second harmonic pulse phase delays are a good tracer of the spin frequency evolution, we derive a precise estimate of the spin frequency and its derivative. [ABSTRACT FROM AUTHOR]