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Published July 1, 2018 | Published + Accepted Version
Journal Article Open

Discovery of Pulsation Dropout and Turn-on during the High State of the Accreting X-Ray Pulsar LMC X-4

Abstract

Two Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the luminous X-ray pulsar LMC X-4 in 2015 October and November captured several bright accretion flares from this source, which has a long history of stable pulse and superorbital behavior. We present a timing analysis of these data in which we detect a rapid pulse "turn-on" in association with the accretion flares, during which the source reaches super-Eddington luminosities. Pulsations, which are normally seen from this source, are found to only occur for approximately one hour before and during the bright flares. Beyond one hour before and after the flares, we find pulsations to be weak or nonexistent, with fractional rms amplitudes of less than 0.05. At the onset of the flare, the pulse profiles exhibit a phase shift of 0.25 cycles that could be associated with a change in the emission geometry. This increase in pulse strength occurring well before the flare cannot be explained by the propeller effect, and potentially offers a connection between the magnetic properties of pulsars that accrete close to their Eddington limits and ultra-luminous X-ray pulsars.

Additional Information

© 2018 The American Astronomical Society. Received 2018 April 26; revised 2018 June 11; accepted 2018 June 16; published 2018 June 28. We would like to thank the anonymous referee for comments that substantially improved this Letter. We would also like to thank the NuSTAR Galactic Binaries Science Team for comments and contributions. M.C.B. acknowledges support from NASA grant Nos. NNX15AV32G and NNX15AH79H. This research made use of NuSTARDAS, developed by ASDC (Italy) and Caltech (USA) and of ISIS functions (ISISscripts) provided by ECAP/Remeis observatory and MIT (http://www.sternwarte.uni-erlangen.de/isis/).

Attached Files

Published - Brumback_2018_ApJL_861_L7.pdf

Accepted Version - 1806.09630.pdf

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August 19, 2023
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