The long-term evolution of the accreting millisecond X-ray pulsar Swift J1756.9-2508

TL;DR

This paper analyzes the long-term evolution of the accreting millisecond X-ray pulsar Swift J1756.9-2508, constraining its magnetic field, accretion torque, and pulse profile changes over multiple outbursts.

Contribution

It provides the first long-term timing analysis of this pulsar, constraining its magnetic field and accretion torque over a two-year baseline.

Findings

Magnetic field constrained to 0.4x10^8 G < B < 9x10^8 G

Upper limit on spin frequency derivative: |ν̇| < 3x10^-13 Hz/s

Pulse profile shape varies systematically with outburst phase

Abstract

We present a timing analysis of the 2009 outburst of the accreting millisecond X-ray pulsar Swift J1756.9-2508, and a re-analysis of the 2007 outburst. The source shows a short recurrence time of only ~2 years between outbursts. Thanks to the approximately 2 year long baseline of data, we can constrain the magnetic field of the neutron star to be 0.4x10^8 G < B < 9x10^8 G, which is within the range of typical accreting millisecond pulsars. The 2009 timing analysis allows us to put constraints on the accretion torque: the spin frequency derivative within the outburst has an upper limit of $|\dot{\nu}| < 3x10^-13 Hz/s at the 95% confidence level. A study of pulse profiles and their evolution during the outburst is analyzed, suggesting a systematic change of shape that depends on the outburst phase.