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Swift and NuSTAR observations of Swift J0243.6+6124

ATel #10866; A. Bahramian (Michigan State), J. A. Kennea (Penn State) and A. W. Shaw (U. Alberta)
on 19 Oct 2017; 02:15 UT
Credential Certification: Aarran Shaw (aarran@ualberta.ca)

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

Referred to by ATel #: 10907, 10946

We report on X-ray spectroscopy and timing analysis of the candidate Be/X-ray binary Swift J0243.6+6124 (Kennea et al., ATel #10809; Kouroubatzakis et al., ATel #10822). Starting at 15:51 UTC on October 5th, 2017, NuSTAR performed a ~14ks observation of the source, in coordination with a quasi-simultaneous 2ks observation with Swift.

The combined Swift/XRT and NuSTAR/FPMA+B data show that the spectrum is well fit (with reduced Χ2=0.95 for 1563 degrees of freedom) by an absorbed blackbody (NH=1.79 ± 0.05 × 1022 cm-2, kT=2.93 ± 0.04 keV) plus cut-off power-law model (Γ=1.07 ± 0.02 , Ecut=22.9 ± 0.9 keV). In addition the fit requires the inclusion of a line at 6.42 ± 0.07 keV (σ=0.3 ± 0.1 keV, EW=0.5). The NuSTAR spectra indicate a 3-79 keV flux of 8.68 ± 0.04 × 10-9 erg s-1cm2. Due to the high number of counts in our spectra, we must assume a systematic uncertainty of 2% (e.g. SWIFT-XRT-CALDB-09). There is no evidence of any absorption lines, such as those commonly associated with cyclotron resonance scattering features, in the NuSTAR spectrum.

The NuSTAR light curve shows the source flux rising throughout the observation. Extracting a power spectrum in the 3-78 keV band, we find multiple strong peaks, with the fundamental frequency corresponding to a periodicity of 9.846s and the remaining peaks at periodicities consistent with harmonics of this value. Thus we infer a modulation period of 9.846 s which is likely to be the pulsar period. This is consistent with measurements of the pulse period in earlier Swift observations (ATel #10809).

We thank the NuSTAR and Swift teams for rapidly scheduling our observations.