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Discovery of a bimodal luminosity distribution in persistent Be/X-ray pulsar 2RXP J130159.6-635806 (2504.11263v1)

Published 15 Apr 2025 in astro-ph.HE

Abstract: We present a comprehensive analysis of 2RXP J130159.6-635806, a persistent low-luminosity Be/X-ray pulsar, focusing on its transition to a spin equilibrium state and the discovery of a bimodal luminosity distribution revealing possibly a new accretion regime. Using data from NuSTAR, Swift, XMM-Newton, and Chandra observatories, we investigate changes in the pulsar's timing and spectral properties. After more than 20 years of continuous spin-up, the pulsar's spin period stabilized, marking the onset of spin equilibrium. This transition was accompanied by the emergence of a previously unobserved accretion regime at $L_{\rm bol} = (2.0_{-1.0}{+2.3})\times 10{34}$ erg s${-1}$, an order of magnitude lower than its earlier quiescent state. After that, the source occasionally switched between these regimes, remaining in each state for extended periods, with the transition time from a luminosity of $10{35}$ erg s${-1}$ to $10{34}$ erg s${-1}$ taking less than 2.3 day. The analysis of the spectral data collected during this new low-luminosity state revealed a two-hump shape which is different from the cutoff power-law spectra observed at higher luminosities. The discovery of pulsations in this state, together with the hard spectral shape, demonstrates ongoing accretion. We estimate the magnetic field strength to be $\sim 10{13}$ G based on indirect methods. Additionally, we report a hint of a previously undetected $\sim$90-day orbital period in the system.

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