The First NuSTAR Observation of 4U 1538-522: Updated Orbital Ephemeris and A Strengthened Case for an Evolving Cyclotron Line Energy. (arXiv:1901.11071v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Hemphill_P/0/1/0/all/0/1">Paul B. Hemphill</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Rothschild_R/0/1/0/all/0/1">Richard E. Rothschild</a> (2), <a href="http://arxiv.org/find/astro-ph/1/au:+Cheatham_D/0/1/0/all/0/1">Diana M. Cheatham</a> (3 and 4), <a href="http://arxiv.org/find/astro-ph/1/au:+Furst_F/0/1/0/all/0/1">Felix F&#xfc;rst</a> (5), <a href="http://arxiv.org/find/astro-ph/1/au:+Kretschmar_P/0/1/0/all/0/1">Peter Kretschmar</a> (5), <a href="http://arxiv.org/find/astro-ph/1/au:+Kuhnel_M/0/1/0/all/0/1">Matthias K&#xfc;hnel</a> (6), <a href="http://arxiv.org/find/astro-ph/1/au:+Pottschmidt_K/0/1/0/all/0/1">Katja Pottschmidt</a> (3 and 4), <a href="http://arxiv.org/find/astro-ph/1/au:+Staubert_R/0/1/0/all/0/1">R&#xfc;diger Staubert</a> (7), <a href="http://arxiv.org/find/astro-ph/1/au:+Wilms_J/0/1/0/all/0/1">J&#xf6;rn Wilms</a> (6), <a href="http://arxiv.org/find/astro-ph/1/au:+Wolff_M/0/1/0/all/0/1">Michael T. Wolff</a> (8) ((1) MIT Kavli Institute, (2) UCSD Center for Astrophysics and Space Science, (3) UMBC Center for Space Sciences and Technology, (4) NASA Goddard Spaceflight Center, (5) European Space Astronomy Centre, (6) Dr. Karl-Remeis-Sternwarte &amp; Erlangen Centre for Astroparticle Physics, (7) Institut f&#xfc;r Astronomie und Astrophysik, Universit&#xe4;t T&#xfc;bingen, (8) Space Science Division, Naval Research Laboratory)

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 $dot{P}_{rm orb}/P_{rm orb} = left(-0.95 pm
0.37right) times 10^{-6}$ 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.

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 $dot{P}_{rm orb}/P_{rm orb} = left(-0.95 pm
0.37right) times 10^{-6}$ 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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