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arXiv · 1901.11071

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

Paul B. Hemphill·Richard E. Rothschild·Diana M. Cheatham·Felix Fürst·Peter Kretschmar·Matthias Kühnel·Katja Pottschmidt·Rüdiger Staubert·Jörn Wilms·Michael T. Wolff
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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 $\dot{P}_{\rm orb}/P_{\rm orb} = \left(-0.95 \pm 0.37\right) \times 10^{-6}$ 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.

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Paul B. Hemphill, Richard E. Rothschild, Diana M. Cheatham, Felix Fürst, Peter Kretschmar, Matthias Kühnel, Katja Pottschmidt, Rüdiger Staubert, Jörn Wilms, Michael T. Wolff. 2019-01-30. The First NuSTAR Observation of 4U 1538-522: Updated Orbital Ephemeris and A Strengthened Case for an Evolving Cyclotron Line Energy. https://doi.org/10.3847/1538-4357/ab03d3

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