Recovery of the X-ray polarisation of Swift J1727.8−1613 after the soft-to-hard spectral transition,Astronomy & Astrophysics

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Recovery of the X-ray polarisation of Swift J1727.8−1613 after the soft-to-hard spectral transition
Astronomy & Astrophysics ( IF 5.4 ) Pub Date : 2024-06-12 , DOI: 10.1051/0004-6361/202450566
J. Podgorný , J. Svoboda , M. Dovčiak , A. Veledina , J. Poutanen , P. Kaaret , S. Bianchi , A. Ingram , F. Capitanio , S. R. Datta , E. Egron , H. Krawczynski , G. Matt , F. Muleri , P.-O. Petrucci , T. D. Russell , J. F. Steiner , N. Bollemeijer , M. Brigitte , N. Castro Segura , R. Emami , J. A. García , K. Hu , M. N. Iacolina , V. Kravtsov , L. Marra , G. Mastroserio , T. Muñoz-Darias , E. Nathan , M. Negro , A. Ratheesh , N. Rodriguez Cavero , R. Taverna , F. Tombesi , Y. J. Yang , W. Zhang , Y. Zhang

We report on the detection of X-ray polarisation in the black-hole X-ray binary Swift J1727.8−1613 during its dim hard spectral state by the Imaging X-ray Polarimetry Explorer (IXPE). This is the first detection of X-ray polarisation at the transition from the soft to the hard state in an X-ray binary. We find an averaged 2–8 keV polarisation degree of (3.3 ± 0.4)% and a corresponding polarisation angle of 3° ±4°, which matches the polarisation detected during the rising stage of the outburst, in September–October 2023, within 1σ uncertainty. The observational campaign complements previous studies of this source and enables comparison of the X-ray polarisation properties of a single transient across the X-ray hardness-intensity diagram. The complete recovery of the X-ray polarisation properties, including the energy dependence, came after a dramatic drop in the X-ray polarisation during the soft state. The new IXPE observations in the dim hard state at the reverse transition indicate that the accretion properties, including the geometry of the corona, appear to be strikingly similar to the bright hard state during the outburst rise despite the X-ray luminosities differing by two orders of magnitude.

中文翻译：

软到硬光谱转变后 Swift J1727.8−1613 X 射线偏振的恢复

我们报告了成像 X 射线偏振探测器 (IXPE) 在黑洞 X 射线双星 Swift J1727.8−1613 昏暗硬光谱状态下检测到的 X 射线偏振。这是首次检测到 X 射线双星中从软态到硬态转变时的 X 射线偏振。我们发现平均 2–8 keV 偏振度为 (3.3 ± 0.4)%，相应的偏振角为 3° ±4°，与 2023 年 9 月至 10 月爆发上升阶段检测到的偏振相匹配，误差在 1σ 以内不确定。此次观测活动补充了之前对该源的研究，并能够在 X 射线硬度-强度图上比较单个瞬态的 X 射线偏振特性。在软态期间 X 射线偏振急剧下降后，X 射线偏振特性（包括能量依赖性）完全恢复。新的 IXPE 观测结果表明，尽管 X 射线光度相差两个数量级，但逆跃迁暗淡硬态下的吸积特性（包括日冕的几何形状）似乎与爆发上升期间的明亮硬态惊人地相似。的数量级。