The First NuSTAR Observation of 4U 1538-522: Updated Orbital Ephemeris and A Strengthened Case for an Evolving Cyclotron Line Energy

Abstract

We have performed a comprehensive spectral and timing analysis of the first NuSTAR observation of the high-mass X-ray binary 4U 1538-522. The observation covers the X-ray eclipse of the source, plus the eclipse ingress and egress. We use the new measurement of the mid-eclipse time to update the orbital parameters of the system and find marginally-significant evolution in the orbital period, with ${\dot{P}}_{\mathrm{orb}}/P_{\mathrm{orb}}=\left(-0.95\pm 0.37\right)\times 10^{-6}$ yr${}^{-1}$. The cyclotron line energy is found approximately 1.2 keV higher than RXTE measurements from 1997--2003, in line with the increased energy observed by Suzaku in 2012 and strengthening the case for secular evolution of 4U 1538-522's CRSF. We additionally characterize the behavior of the iron fluorescence and emission lines and line-of-sight absorption as the source moves into and out of eclipse.

arXiv comment18 pages, 11 figures

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