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

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. = $17^{\rm h}56^{\rm m}57.18^{\rm s}\pm0.08^{\rm s}$ and Decl. = $-25^{\circ}06'27.8''\pm3.5''$, while simultaneously measuring the long-term spin frequency derivative as $\dot\nu = -7.3\times 10^{-16}$ Hz s$^{-1}$. We briefly discuss the implications of these measurements in the context of the wider population of accreting millisecond pulsars.