A1 Refereed original research article in a scientific journal
Temporal evolution of the circumstellar disc orientation in the transient X-ray pulsar GRO J1008−57; 
Authors: Hu, Yongfeng; Xiao, Hua; Tsygankov, Sergey S.; Ji, Long; Poutanen, Juri; Huang, Runting
Publisher: EDP Sciences
Publication year: 2026
Journal: Astronomy and Astrophysics
Article number: A259
Volume: 710
ISSN: 0004-6361
eISSN: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202659962
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://doi.org/10.1051/0004-6361/202659962
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/527097133
Self-archived copy's licence: CC BY
Self-archived copy's version: Publisher`s PDF
The transient X-ray pulsar GRO J1008−57 was previously found to exhibit Type I outbursts occurring at stable orbital phases before its first observed Type II outburst in 2012. We extend the study to investigate the phase evolution after several Type II outbursts using long-term Swift/BAT and MAXI/GSC observations. Our results reveal that the orbital phases of Type I outbursts follow a step-like evolution: they remain largely stable over many orbital periods, but undergo abrupt small-amplitude jumps coincident with each Type II outburst. This step-like behaviour is difficult to explain with the commonly proposed mechanisms involving a highly eccentric or precessing disc around a Be star. The energetics of Type I X-ray outbursts shows a systematic increase before Type II outbursts, followed by a rapid decline and a subsequent gradual recovery. This behaviour suggests cycles of disc depletion and reconstruction driven by Type II outbursts. Considering the small amplitude of each phase jumps, we propose that this step-like phase evolution might be related to the long orbital period of GRO J1008−57, implying infrequent neutron star–disc interactions. After disc depletion by Type II outbursts, the disc around the Be star has enough time to rebuild its density and restore a geometric structure similar to its pre–Type II outburst state. Consequently, the orbital phases of subsequent Type I outbursts not only change very slightly, but can also remain stable over many orbital periods until the next Type II-driven disc reconfiguration, yielding the observed step-like evolution.
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This work is based on observations obtained with Swift/BAT and MAXI/GSC. We gratefully acknowledge the teams of both missions for maintaining the instruments, providing high-quality data, and making these data publicly available, which were essential for the completion of this study. This work was supported by the Research Council of Finland Centre of Excellence in Neutron-Star Physics (project 374064). HX acknowledges support from the China Scholarship Council (CSC). LJ acknowledges support from the National Natural Science Foundation of China under grant Nos. 12173103 and 12261141691.