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

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 $a_x\sin i=4.823(5)$ 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 $\dot{\nu }=(1.7\pm 0.3)\times 10^{-13}$ 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.

26 pages, 8 figures, accepted by ApJ

References in corpus (21)

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• INTEGRAL spectroscopy of the accreting millisecond pulsar XTE J1807-294 in outburst
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• Timing of the Accreting Millisecond Pulsar XTE J1814-338
• Intermittent pulsations in an accretion-powered millisecond pulsar
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• Timing an Accreting Millisecond Pulsar: Measuring the Accretion Torque in IGR J00291+5934
• Timing an Accreting Millisecond Pulsar: Measuring the Accretion Torque in IGR J00291+5934
• XMM-Newton observation of the 5.25 ms transient millisecond pulsar XTE J1807-294 in outburst
• Studies of orbital parameters and pulse profile of the accreting millisecond pulsar XTE J1807-294
• Discovery of microsecond time lags in kilohertz QPO
• Discovery of 442-Hz Pulsations from an X-ray Source in the Globular Cluster NGC 6440
• Timing Features of the Accretion--driven Millisecond X-Ray Pulsar XTE J1807--294 in 2003 March Outburst

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• X-ray spectral properties of the accreting millisecond pulsar IGR J17498-2921 during its 2023 outburst
• Updated Spin and Orbital Parameters and Energy Dependent Pulse Behaviors of the Accreting Millisecond X-ray Pulsar IGR J17591$-$2342
• An apparent positive relation between spin and orbital angular momentum in X-ray binaries