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Published May 21, 2016 | Published + Submitted
Journal Article Open

Evidence for an evolving cyclotron line energy in 4U 1538−522

Abstract

We have performed a full time and luminosity-resolved spectral analysis of the high-mass X-ray binary 4U 1538−522 using the available RXTE, INTEGRAL, and Suzaku data, examining both phase-averaged and pulse-phase-constrained data sets and focusing on the behaviour of the cyclotron resonance scattering feature (CRSF). No statistically significant trend between the energy of the CRSF and luminosity is observed in the combined data set. However, the CRSF energy appears to have increased by ∼1.5 keV in the ∼8.5 yr between the RXTE and Suzaku measurements, with Monte Carlo simulations finding the Suzaku measurement 4.6σ above the RXTE points. Interestingly, the increased Suzaku CRSF energy is much more significant and robust in the pulse-phase-constrained spectra from the peak of the main pulse, suggesting a change that is limited to a single magnetic pole. The seven years of RXTE measurements do not show any strongly significant evolution with time on their own. We discuss the significance of the CRSF's behaviour with respect to luminosity and time in the context of historical observations of this source as well as recent observational and theoretical work concerning the neutron star accretion column, and suggest some mechanisms by which the observed change over time could occur.

Additional Information

© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2016 February 25. Received 2016 February 4. In original form 2015 December 3. First published online March 3, 2016. This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC and the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory. Our analysis makes heavy use of a collection of ISIS scripts provided by ECAP/Remeis Observatory and MIT, which can be found at http://www.sternwarte.uni-erlangen.de/isis/. Support for VG was provided by NASA through the Smithsonian Astrophysical Observatory (SAO) contract SV3-73016 to MIT for Support of the Chandra X-Ray Center (CXC) and Science Instruments; CXC is operated by SAO on behalf of NASA under contract NAS8-03060.

Attached Files

Published - MNRAS-2016-Hemphill-2745-61.pdf

Submitted - 1603.02198v1.pdf

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August 20, 2023
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