The puzzling orbital period evolution of the LMXB AX J1745.6-2901

The orbital period evolution of X-ray binaries provides fundamental clues to understanding mechanisms of angular momentum loss from these systems. I will present an X-ray eclipse timing analysis of the transient low mass X-ray binary AX J1745.6-2901. This system shows full eclipses and thus is one of the few objects for which accurate orbital evolution studies using this method can be carried out. We report on XMM-Newton and ASCA observations covering 30 complete X-ray eclipses spanning an interval of more than 20 years. We improve the determination of the orbital period to a relative precision of 2 × 10-8, two orders of magnitudes better than previous estimates. We determine, for the first time, a highly significant rate of decrease of the orbital period Pdot_orb = -4.03 ± 0.32 × 10-11 s/s. This is at least one order of magnitude larger than expected from conservative mass transfer and angular momentum losses due to gravitational waves and magnetic braking, and might result from either non-conservative mass transfer or be the product of magnetic activity changing the quadrupole moment of the companion star. Imprinted on the long term evolution of the orbit, we observe highly significant eclipse leads-delays of ∼ 10 - 30 s, characterised by a clear state dependence in which, on average, eclipses occur earlier during the hard state.