Precise Orbital Parameters and Anomalous Phase Variations of the Accretion-powered Millisecond Pulsar XTE J1807-294

TL;DR

This paper analyzes pulse variations and orbital parameters of the accretion-powered millisecond pulsar XTE J1807-294 during its 2003 outburst, revealing complex phase behavior, precise orbital measurements, and evidence of neutron star spin-up.

Contribution

It provides the first detailed orbital parameters and phase variation analysis of XTE J1807-294, highlighting the complex pulse behavior and potential hot spot movement on the neutron star surface.

Findings

Precise orbital parameters: P_orb=40.073601(8) min, a_x sin i=4.823(5) lt-ms.

Neutron star spun-up with rate b7 b7 10^{-13} Hz/s during the first 60 days.

Detected soft phase lags up to 500 5s between 2 to 20 keV.

Abstract

This study reports pulse variation analysis results for the forth discovered accretion-powered millisecond pulsar XTE J1807-294 during its 2003 outburst observed by {\it Rossi X-ray Timing Explorer}. The pulsation is significantly detected only in the first $\sim$90d out of $\sim$150d observations. The pulse phase variation is too complex to be described as an orbital motion plus a simple polynomial model. The precise orbital parameters with $P_{orb}=40.073601(8)$ min and ${\it a_x}\sin {\it i}=4.823(5)$ lt-ms were obtained after applying the trend removal to the daily observed 150s segments pulse phases folded with a constant spin frequency without Keplerian orbit included. The binary barycenter corrected pulse phases show smooth evolution and clear negative phase shifts coincident with the flares seen on the light curve and the enhancements of fractional pulse amplitude. The non-flare pulse phases for the first $\sim$60d data are well described as a fourth order polynomial implying that the neutron star was spun-up during the first $\sim$60d with a rate $\dot \nu=(1.7\pm0.3) \times 10^{-13}$ Hz/s at the beginning of the outburst. Significant soft phase lags up to $\sim$500 $\mu s$ ($\sim$10% cycle) between 2 to 20 keV were detected for the nonflare pulse phases. We conclude that the anomalous phase shifts are unlikely due to the accretion torque but could result from the ``hot spot'' moving on the surface of neutron star.