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

## TL;DR

This study presents the first NuSTAR observations of 4U 1538-522, updating its orbital parameters and providing evidence for the secular evolution of its cyclotron line energy, with detailed spectral and timing analysis during eclipse phases.

## Contribution

It offers the first NuSTAR spectral and timing analysis of 4U 1538-522, updating orbital parameters and strengthening evidence for evolving cyclotron line energy.

## Key findings

- Marginal orbital period evolution detected.
- Cyclotron line energy increased since previous measurements.
- Characterized iron line behavior during eclipse ingress and egress.

## 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.