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NuSTAR observations of the transient Swift J005139.2-721704 (SXP4.78 = XTE J0052-723) in the Small Magellanic Cloud

ATel #12234; V. Antoniou (TTU/SAO), A. Zezas (UoC/SAO), J. Hong (Harvard/SAO), J. Kennea (PSU), J. Tomsick (UC Berkeley), F. Haberl (MPE)
on 21 Nov 2018; 05:54 UT
Credential Certification: Vallia Antoniou (vantoniou@cfa.harvard.edu)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

We report on the analysis of a 75ksec NuSTAR observation of Swift J005139.2-721704 in the SMC. The source was discovered serendipitously with Swift (ATel #12209), and follow-up NICER observations identified it with pulsar SXP 4.78 (XTE J0052-723) in the SMC (ATel #12222). Optical observations showed that it is associated with an early-type star ([M2002]20671= [2dFS]0811), which shows Hα emission (ATel #12224).

The NuSTAR observations were obtained between Nov. 15, 2018 and Nov 17, 2018. The spectrum is well fitted with a power-law model with a photon index of 1.1±0.1 and a high-energy cutoff at 6.2 keV. We also detect an Fe-Kα line at 6.3±0.2 keV. The spectrum shows tentative evidence for a cyclotron resonance scattering feature (CRSF) at 10.2±0.3keV, which would indicate a magnetic field of ~9×1011G. The derived absorbed luminosity is LX~1038erg s-1 (4-25 keV; 60 kpc).

The analysis of the NuSTAR light-curve yields a period of 4.7816072(73)s. The energy resolved pulse profile shows higher pulsed fraction at higher energies, and marginal evidence for spectral variations as a function of phase.