发表机构
INAF Osservatorio Astronomico di Roma; INAF Istituto di Astrofisica e Planetologia Spaziali; INAF Osservatorio Astronomico di Brera; Institut d’Estudis Espacials de Catalunya (IEEC)（意大利国家天体物理研究所罗马天文台; 意大利国家天体物理研究所天体物理与空间行星学研究所; 意大利国家天体物理研究所布雷拉天文台; 加泰罗尼亚空间研究所）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究利用IXPE探测到吸积毫秒脉冲星SAX J1808.4-3658在爆发期间的偏振X射线，揭示热点几乎与自转轴对齐，并在I型X射线爆发中观测到高偏振度发射。
AI 中文摘要
对吸积毫秒脉冲星（AMSPs）的X射线脉冲轮廓进行建模是约束中子星质量和半径的关键技术。为此，测量偏振X射线的特性为系统几何结构提供了至关重要的信息。我们报告了IXPE在2025年爆发衰减期间对2.5毫秒AMSP SAX J1808.4-3658的2-8 keV X射线线性偏振的显著探测，平均偏振度为$2.9\\%\pm0.5\\%$，偏振角为$46°\pm5°$，与光学波段观测到的方向大致一致。这一偏振水平与迄今已观测的该类源中唯一另一个源的水平相似。通过使用IXPE和NuSTAR获得的脉冲相位定时解，该解证实了脉冲星自转和轨道的长期演化，我们在Stokes参数中未发现任何显著的脉冲相位分辨变化。将偏振X射线解释为源自中子星表面的热点，表明它们几乎与自转轴对齐。我们还报告了在I型热核X射线爆发峰值期间（2-6 keV）线性偏振发射的迹象，偏振度为$16\\%\pm6\\%$，其角度与持续发射的角度几乎正交。如果得到证实，这一水平将超过薄吸积盘反射X射线的预期。
英文摘要
Modelling the X-ray pulse profiles of accreting millisecond pulsars (AMSPs) is a key technique to constrain the masses and radii of neutron stars. To this end, measuring the properties of polarised X-rays provides crucial information on the system geometry. We present the significant detection by IXPE of linearly polarised 2-8 keV X-rays during the decay of the 2025 outburst of the 2.5 ms AMSP SAX J1808.4-3658, with an average polarisation degree of $2.9\%\pm0.5\%$ and a polarisation angle of $46°\pm5°$, broadly consistent with the direction observed in the optical band. This level of polarisation is similar to that found in the only other source of this class that has been targeted so far. By using a pulse-phase timing solution obtained with IXPE and NuSTAR, which confirms the long-term evolution of the pulsar spin and orbit, we did not find any significant pulse phase-resolved variability in the Stokes parameters. Interpreting the polarised X-rays as originating from hot spots on the NS surface indicates they are almost aligned with the spin axis. We also report an indication for linearly polarised emission with a degree of $16\%\pm6\%$ (2-6 keV) during the peak of a type-I thermonuclear X-ray burst, with an angle almost orthogonal to that of the persistent emission. If confirmed, this level would exceed expectations for X-rays reflected by a thin accretion disc.
CommentsSubmitted as a Letter to A&A, 9 pages, 10 figures
