A Possible Phase-dependent Absorption Feature in the Transient X-Ray Pulsar SAX J2103.5+4545
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Brumback, M. C., R. C. Hickox, F. S. Fürst, K. Pottschmidt, P. Hemphill, J. A. Tomsick, J. Wilms, and R. Ballhausen. “A Possible Phase-Dependent Absorption Feature in the Transient X-Ray Pulsar SAX J2103.5+4545.” The Astrophysical Journal 852, no. 2 (January 2018): 132. https://doi.org/10.3847/1538-4357/aa9e91.
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We present an X-ray spectral and timing analysis of two Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the transient Be X-ray binary SAX J2103.5+4545 during its 2016 April outburst, which was characterized by the highest flux since NuSTAR's launch. These observations provide detailed hard X-ray spectra of this source during its bright precursor flare and subsequent fainter regular outburst for the first time. In this work, we model the phase-averaged spectra for these observations with a negative and positive power law with an exponential cutoff (NPEX) model and compare the pulse profiles at different flux states. We found that the broadband pulse profile changes from a three-peaked pulse in the first observation to a two-peaked pulse in the second observation, and that each of the pulse peaks has some energy dependence. We also perform pulse-phase spectroscopy and fit phase-resolved spectra with NPEX to evaluate how spectral parameters change with pulse phase. We find that while the continuum parameters are mostly constant with pulse phase, a weak absorption feature at ∼12 keV might, with further study, be classified as a cyclotron line, does show strong pulse-phase dependence.