Spin down of an Accreting Millisecond Pulsar, the case of XTE J1814-338

We report about a timing analysis performed on the data gathered by RXTE of the accreting millisecond pulsar XTE J1814-338 during its 2003 outburst. The first full orbital solution of this binary system is given. Moreover the evolution of the phase of the pulsed emission reveals that the rotating compact object is spinning down at a rate ν˙ = (-6.7 +/- 0.7) × 10-14 Hz/s, while accreting. This behavior is considered as a result of the braking effect due to the interaction between the magnetosphere and the inner parts of the accretion disc, in the case of an accretion rate low enough to allow the expansion of the magnetospheric radius to the corotation limit. In this context we derive an estimate of the superficial magnetic field, BS ~= 8 × 108 G, which lies in the plausible range for the accreting millisecond pulsars to be the progenitors of the radio millisecond pulsar. We also report about a modulation of the phase delays around the mean timing solution, which we interpret as a signature of movements of the accreting hotspot, resulting from a variable accretion rate.