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A Broadband X-Ray View of the Precessing Accretion Disk and Pre-eclipse Dip in the Pulsar Her X-1 with NuSTAR and XMM-Newton

Brumback, McKinley C. and Hickox, Ryan C. and Fürst, Felix S. and Pottschmidt, Katja and Tomsick, John A. and Wilms, Jörn and Staubert, Rüdiger and Vrtilek, Saeqa (2021) A Broadband X-Ray View of the Precessing Accretion Disk and Pre-eclipse Dip in the Pulsar Her X-1 with NuSTAR and XMM-Newton. Astrophysical Journal, 909 (2). Art. No. 186. ISSN 0004-637X. doi:10.3847/1538-4357/abe122.

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We present a broadband X-ray timing study of the variations in pulse behavior with the superorbital cycle in the low-mass X-ray binary Her X-1. This source shows a 35 day superorbital modulation in X-ray flux that is likely caused by occultation by a warped, precessing accretion disk. Our data set consists of four joint XMM-Newton and NuSTAR observations of Her X-1 which sample a complete superorbital cycle. We focus our analysis on the first and fourth observations, which occur during the bright "main-on" phase, because these observations have strongly detected pulsations. We added an archival XMM-Newton observation during the "short-on" phase of the superorbital cycle because our observations at that phase are lower in signal to noise. We find that the energy-resolved pulse profiles show the same shape at similar superorbital phases, and the profiles are consistent with expectations from a precessing disk. We demonstrate that a simple precessing accretion disk model is sufficient to reproduce the observed pulse profiles. The results of this model suggest that the similarities in the observed pulse profiles are due to reprocessing by a precessing disk that has returned to its original precession phase. We determine that the broadband spectrum is well fit by an absorbed power law with a soft blackbody component and show that the spectral continuum also exhibits a dependence on the superorbital cycle. We also present a brief analysis of the energy-resolved light curves of a pre-eclipse dip, which shows soft X-ray absorption and hard X-ray variability during the dip.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Brumback, McKinley C.0000-0002-4024-6967
Hickox, Ryan C.0000-0003-1468-9526
Fürst, Felix S.0000-0003-0388-0560
Pottschmidt, Katja0000-0002-4656-6881
Tomsick, John A.0000-0001-5506-9855
Wilms, Jörn0000-0003-2065-5410
Staubert, Rüdiger0000-0003-1498-1543
Vrtilek, Saeqa0000-0002-7521-9897
Additional Information:© 2021 The American Astronomical Society. Received 2020 August 6; revised 2021 January 26; accepted 2021 January 27; published 2021 March 17. We would like to thank the anonymous referee for comments and suggestions that greatly improved the clarity and substance of this paper. We thank Lorenzo Ducci and Matteo Bachetti for helpful discussions relating to the spectral and timing analyses, respectively. We would like to thank the NuSTAR Galactic Binaries Science Team for comments and contributions. M.C.B. acknowledges support from NASA grant numbers NNX15AV32G and NNX15AH79H. This research made use of NuSTARDAS, developed by ASDC (Italy) and Caltech (USA), XMM-SAS developed by ESA, ISIS functions (ISISscripts) provided by ECAP/Remeis observatory and MIT (, and the MAXI data provided by RIKEN, JAXA, and the MAXI team. Software: HEAsoft (v6.26.1; HEASARC 2014), NuSTARDAS, SAS (Gabriel et al. 2004), Stingray (Huppenkothen et al. 2019), Xspec (v12.10.1; Arnaud 1996), MaLTPyNT (Bachetti 2015).
Funding AgencyGrant Number
Subject Keywords:Pulsars; Low-mass x-ray binary stars
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Pulsars (1306); Low-mass x-ray binary stars (939)
Record Number:CaltechAUTHORS:20210325-134018670
Persistent URL:
Official Citation:McKinley C. Brumback et al 2021 ApJ 909 186
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:108560
Deposited By: Tony Diaz
Deposited On:25 Mar 2021 23:00
Last Modified:16 Nov 2021 19:12

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