NICER Detects Pulsations from Swift J1756.9-2508

Following the report of a new outburst of the accreting millisecond X-ray pulsar Swift J1756.9-2508 (ATel #11497), NICER performed pointed observations starting on 2018 April 3, collecting 9.4 ks of exposure over the ~30 hours between April 3 15:18 UTC and April 4 21:01 UTC. A source is clearly detected at ~30 ct/s (1-10 keV); the background level in this band is less than 1 ct/s. After barycenter-correcting the event times, we computed a power spectrum and detected a >5-sigma pulsation at 182.067 Hz, confirming that the active source is indeed Swift J1756.9-2508 (see Krimm et al. 2007, ApJ 668, L147, and the erratum in Krimm et al. 2009, ApJ 703, L183). The pulsar has a known binary period of 54.7 min. Propagating the best-known orbital solution (Patruno et al. 2010, MNRAS 403, 1426) under the assumption of a constant binary period, we calculated a current-epoch time of ascending node to be T_asc = MJD 58211.0170(2) TDB. The uncertainty on this predicted reference time is less than 0.5% of the orbital period. We then optimized our trial orbital solution by scanning a grid of T_asc values in steps of 1E-5 d. We found the best solution at T_asc = MJD 58211.01736 TDB, consistent with the prediction within 2 sigma (statistical uncertainty). Folding the data using this orbital ephemeris, we retrieved an improved pulsation detection (22 sigma) at frequency F0=182.065803(2) Hz. The pulse profile is non-sinusoidal, showing a fractional sinusoidal amplitude of 5.9% for the fundamental and 3.4% for the first overtone, both measured in the 1-10 keV band. The shape of the pulse profile is similar to those shown in Figure 1 of Patruno et al. (2010), albeit in a somewhat softer, overlapping energy band. A more detailed analysis is underway. The X-ray spectrum is consistent with an absorbed disk-blackbody plus power-law model (red. chi^2 = 1.18 for 840 d.o.f). We measured an absorption column density of N_H = 6.4(2)E22 cm^-2, a disk temperature of kT = 0.17(1) keV, and a power-law photon index of Gamma = 2.04(3). There is no evidence of an Fe line feature near ~6.4 keV in the spectrum. The unabsorbed 1-10 keV flux is 1.3e-9 erg/s/cm^2. All values are consistent with those of the previous outbursts, and suggest that the source is in a typical atoll-type island (hard) spectral state. Further NICER observations of this source are underway. Additional multiwavelength follow-up is encouraged.