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× {10}^-13 s s^-1. Additionally, we improve upon the source coordinates through astrometric analysis of the pulse arrival times, finding R.A. = 17^h56^m57.ˢ18 ± 0.ˢ08 and decl. = -25°06‧27.″8 ± 3.″5, while simultaneously measuring the long-term spin frequency derivative as \dot{ν }=-7.3× {10}^-16 Hz s^-1. We briefly discuss the implications of these measurements in the context of the wider population of accreting millisecond pulsars.