ATel #11502: NICER Detects Pulsations from Swift J1756.9-2508

P. M. Bult, K. C. Gendreau, P. S. Ray, D. Altamirano, Z. Arzoumanian, D. Chakrabarty*, S. Guillot, G. K. Jaisawal, R. M. Ludlam, C. B. Markwardt, I. A. Mereminskiy, F. Ozel, A. Sanna, T. E. Strohmayer, M. T. Wolff

*Corresponding author for this work

Research output: Other contributionNet publication - Internet publicationResearch

Abstract

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.
Original languageEnglish
Publication date5 Apr 2018
Publication statusPublished - 5 Apr 2018
SeriesThe Astronomer's telegram
NumberATel #11502

Keywords

  • Neutron Star
  • Transient
  • Pulsar
  • X-ray

Cite this

Bult, P. M., Gendreau, K. C., Ray, P. S., Altamirano, D., Arzoumanian, Z., Chakrabarty, D., Guillot, S., Jaisawal, G. K., Ludlam, R. M., Markwardt, C. B., Mereminskiy, I. A., Ozel, F., Sanna, A., Strohmayer, T. E., & Wolff, M. T. (2018, Apr 5). ATel #11502: NICER Detects Pulsations from Swift J1756.9-2508. The Astronomer's telegram, No. ATel #11502 http://www.astronomerstelegram.org/?read=11502