On the 2018 outburst of the accreting millisecond X-ray pulsar Swift J1756.9-2508 as seen with NICER

Abstract

We report on the coherent timing analysis of the 182 Hz accreting millisecond X-ray pulsar Swift J1756.9$-$2508 during its 2018 outburst as observed with the Neutron Star Interior Composition Explorer (NICER). Combining our NICER observations with Rossi X-ray Timing Explorer observations of the 2007 and 2009 outbursts, we also studied the long-term spin and orbital evolution of this source. We find that the binary system is well described by a constant orbital period model, with an upper limit on the orbital period derivative of $|{\dot{P}}_b|<7.4\times 10^{-13}$ s s${}^{-1}$. Additionally, we improve upon the source coordinates through astrometric analysis of the pulse arrival times, finding R.A. = $17^{\mathrm{h}}56^{\mathrm{m}}57.18^{\mathrm{s}}\pm 0.08^{\mathrm{s}}$ and Decl. = $-25^{\circ }06^{\prime }27.8^{\prime \prime }\pm 3.5^{\prime \prime }$, while simultaneously measuring the long-term spin frequency derivative as $\dot{\hat{I}}½=-7.3\times 10^{-16}$ Hz s${}^{-1}$. We briefly discuss the implications of these measurements in the context of the wider population of accreting millisecond pulsars.

9 pages, 4 figures, 2 tables. Accepted for publication in ApJ