Temporal evolution of the circumstellar disk orientation in the transient X-ray pulsar GRO J1008-57
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. In this work, 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. Such a step-like behavior is difficult to explain with the commonly proposed mechanisms involving a highly eccentric or precessing disk around the Be star. The energetics of Type I X-ray outbursts show a systematic increase before Type II outbursts, followed by a rapid decline and a subsequent gradual recovery. This behavior suggests cycles of disk depletion and reconstruction driven by Type II outbursts. Considering the small amplitude of each phase jump, we propose that this step-like phase evolution may be related to the long orbital period of GRO J1008-57, implying infrequent neutron star-disk interactions. After disk depletion by Type II outbursts, the disk 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 disk reconfiguration, yielding the observed step-like evolution.