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arXiv · 2610.23339

The Dancing Bonfire: An Unusual Persistent X-ray Source in M81 Exhibiting Recurrent Luminous Flares

Jiachang Zhang·Zhiyuan Li·Xiangdong Li·Tong Bao·Ziqian Hua·ShanShan Weng
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Abstract

Short-timescale X-ray variability provides a direct probe of unstable accretion onto compact objects, yet extragalactic X-ray binaries (XRBs) with recurrent flaring remain largely unexplored. Here we present a detailed X-ray study of M81F4 (2CXO J095524.2+690957), a persistently luminous X-ray source towards M81 that exhibits recurrent short X-ray flares. Based on multi-mission X-ray observations spanning two decades, it is found that M81F4 remains persistently bright at $L_{0.5-8.0\rm~keV}\sim10^{38}-10^{39}\rm~erg~s^{-1}$, while producing flares lasting a few hundred seconds and reaching peak luminosities of a few $10^{39}\rm~erg~s^{-1}$, with an observed mean (minimum) recurrence time of $\sim0.5$ day ($\sim$6 ks). The stacked flare profile shows a rapid rise followed by a slower decay, but no significant hardness evolution. Spectral analysis reveals that both the flare and non-flare states are dominated by a hard power-law continuum, with evidence of highly ionized Fe lines in the non-flare state spectrum. These properties disfavor classical Type I and Type II bursts and have no close analog among known XRBs. We favor a magnetized neutron star accreting from an intermediate mass donor at a persistently high rate with the flares arising from instabilities at the disk-magnetosphere boundary. If confirmed, M81F4 might represent a hidden population of XRBs in which sustained, magnetically-regulated accretion can lead to brief episodes of super-Eddington emission.

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Jiachang Zhang, Zhiyuan Li, Xiangdong Li, Tong Bao, Ziqian Hua, ShanShan Weng. 2026-10-16. The Dancing Bonfire: An Unusual Persistent X-ray Source in M81 Exhibiting Recurrent Luminous Flares. https://arxiv.org/abs/2610.23339

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