We report on the temporal analysis of NICER and Fermi/GBM observations of the new transient Swift J005139.2-721704 located in the SMC (ATel #12209, #12219) that resulted in its identification with the known X-ray pulsar XTE J0052-723 (SXP 4.78).
We obtained 14.1 ksec of NICER data between 2018 November 14 at 22:11:16 UTC and 2018 November 16 at 16:20:39 UTC (ATel #12219). We analyzed the cleaned NICER event list in the 0.35-9.0 keV range. We applied a barycenter correction using the coordinates reported in ATel #12209. We computed the power spectrum (Leahy normalized) and identified a fundamental peak and its first harmonic. The fundamental peak is located at 0.20913 Hz (4.78 s). The detection significance is about 7.8 sigma. The pulse profile shape and amplitude resemble those derived from the RXTE data (Laycock et al 2003).
Following the detection of a periodic signal in the NICER data we performed a dedicated search in the Fermi/GBM data. On November 11, Fermi/GBM detected coherent pulsations from the direction of the SMC with a frequency of 209.1208±0.0013 mHz.
The spin period matches that of the known RXTE pulsar SXP 4.78 nominally located at ~4′ distance from the transient system. We note that such a large discrepancy between RXTE/PCA-derived coordinates and the actual position of X-ray systems is not atypical for SMC pulsars. For example, SXP 2.16 was reported to have a positional uncertainty better than 4′ while the actual position of the system was ~16′ away (ATel #8305; Vasilopoulos et al. 2017).
The detection of a coherent signal strongly suggests that Swift J005139.2-721704 and XTE J0052-723 are the same source (SXP 4.78), and that the spin period of the pulsar has shown little net change over the last 19 years. Furthermore, based on the improved Swift/XRT position, the optical counterpart of SXP 4.78 is very likely the B-type star [M2002] SMC 20671 recently reported in ATel #12209, and not the star [MA93]537 proposed by Laycock et al. (2003), nor the star AzV 129 suggested by Coe et al. (2005).
NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
Coe et al. 2005, MNRAS, 356, 502
Laycock et al. 2003, MNRAS, 339, 435
Vasilopoulos et al. 2017, MNRAS, 470, 1971
|Publication date||16 Nov 2018|
|Publication status||Published - 16 Nov 2018|
|Series||The Astronomer's telegram|
- Neutron Star