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A NuSTAR study of the 55 ks hard X-ray pulse-phase modulation in the magnetar 4U 0142+61

Makishima, Kazuo and Murakami, Hiroaki and Enoto, Teruaki and Nakazawa, Kazuhiro (2019) A NuSTAR study of the 55 ks hard X-ray pulse-phase modulation in the magnetar 4U 0142+61. Publications of the Astronomical Society of Japan, 71 (1). Art. No. 15. ISSN 0004-6264. doi:10.1093/pasj/psy129.

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Archival NuSTAR data of the magnetar 4U 0142+61, acquired in 2014 March for a total time span of 258 ks, were analyzed. This is to reconfirm the 55 ks modulation in the hard X-ray pulse phases of this source, found with a Suzaku observation in 2009 (Makishima et al., 2014, Phys. Rev. Lett., 112, 171102). Indeed, the 10–70 keV X-ray pulsation, detected with NuSTAR at 8.68917 s, was found to be also phase-modulated (at >98% confidence) at the same ∼55 ks period, or half that value. Furthermore, a brief analysis of another Suzaku data set of 4U 0142+61, acquired in 2013, reconfirmed the same 55 ks phase modulation in the 15–40 keV pulses. Thus, the hard X-ray pulse-phase modulation was detected with Suzaku (in 2009 and 2013) and NuSTAR (in 2014) at a consistent period. However, the modulation amplitude varied significantly; A ∼ 0.7 s with Suzaku (in 2009), A ∼ 1.2 s with Suzaku (in 2013), and A ∼ 0.17 s with NuSTAR. In addition, the phase modulation properties detected with NuSTAR differed considerably between the first 1/3 and the latter 2/3 of the observation. In energies below 10 keV, the pulse-phase modulation was not detected with either Suzaku or NuSTAR. These results reinforce the view of Makishima et al. (2014, Phys. Rev. Lett., 112, 171102); the neutron star in 4U 0142+61 keeps free precession, under a slight axial deformation due probably to ultra-high toroidal magnetic fields of ∼10^(16) G. The wobbling angle of precession should remain constant, but the pulse-phase modulation amplitude varies on time scales of months to years, presumably as asymmetry of the hard X-ray emission pattern around the star’s axis changes.

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Enoto, Teruaki0000-0003-1244-3100
Additional Information:© 2018 The Author(s). Published by Oxford University Press on behalf of the Astronomical Society of Japan. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Received 2018 July 28; Accepted 2018 October 22. Published: 28 November 2018. The authors thank Toshio Nakano and Yoshihiro Furuta for their help in data analysis and participation in discussion. This work was supported partially by the Ministry of Education, Culture, Sports, Science and Technology Grant-in-Aid for Scientific Research (C), Grant No. 18K03694.
Subject Keywords:stars: neutron stars: magnetars stars: individual stars: magnetic fields
Issue or Number:1
Record Number:CaltechAUTHORS:20181029-101613379
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Official Citation:Kazuo Makishima, Hiroaki Murakami, Teruaki Enoto, Kazuhiro Nakazawa; A NuSTAR study of the 55 ks hard X-ray pulse-phase modulation in the magnetar 4U 0142+61, Publications of the Astronomical Society of Japan, Volume 71, Issue 1, 1 January 2019, 15,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:90462
Deposited By: Joy Painter
Deposited On:29 Oct 2018 17:39
Last Modified:16 Nov 2021 03:32

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