A flux state comparison of the transient X-ray pulsar SAX J2103.5+4545

We present the first NuSTAR observations of SAX J2103.5+4545, a Be X-ray binary with a history of X-ray flares occurring every 2-3 years. We carried out two Target of Opportunity observations in spring of 2016, as J2103 went into outburst with the strongest flux seen from this object since the launch of NuSTAR. We obtained high-quality X-ray spectra in both epochs, with one observation capturing the bright precursor flare for the first time. We fit the spectra with an NPEX (Negative and Positive power law with an EXponential cut-off) model with Gaussian emission lines to constrain the iron line complex and detect a highly ionized iron line at 6.9 keV for the first time. We perform pulse-phase spectroscopy and find that the model parameters do not vary significantly with pulse phase, which has implications for the geometry and orientation of the accretion flow. We also detect a weak absorption feature at ~12 keV that shows strong pulse phase dependence and could, with further study, be classified as a cyclotron resonance scattering feature. If this line is related to cyclotron scattering, it would imply that J2103 has an unusually low magnetic field (~1e12 G) and opens the possibility of using NuSTAR to detect similar features in other transient X-ray pulsars.