The puzzling orbital period evolution of the low mass X-ray binary AX J1745.6-2901

TL;DR

This study analyzes 20 years of X-ray eclipse data from AX J1745.6-2901, revealing a surprisingly rapid orbital period decrease and state-dependent eclipse timing variations, challenging existing models of angular momentum loss.

Contribution

It provides the first precise measurement of the orbital period derivative and uncovers state-dependent eclipse timing variations in a low mass X-ray binary.

Findings

Orbital period decreasing at a rate much faster than predicted by conservative models.

Eclipse timing shows significant leads during the hard state.

Improved orbital period measurement with a relative precision of 2×10⁻⁸.

Abstract

The orbital period evolution of X-ray binaries provides fundamental clues to understanding mechanisms of angular momentum loss from these systems. We 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 $2\times10^{-8}$, two orders of magnitudes better than previous estimates. We determine, for the first time, a highly significant rate of decrease of the orbital period $\dot{P}_{orb}=-4.03\pm0.32\times10^{-11}$~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 breaking, and might result from non-conservative mass transfer. Imprinted on the long term evolution of the orbit, we observe highly significant eclipse leads-delays of ~10-20 s, characterised by a clear state dependence in which, on average, eclipses occur earlier during the hard state.