Title: Discovery of Cyclotron Resonance Features in the Soft Gamma Repeater SGR 1806-20 Show Chinese title

Title: 软伽马重复源 SGR 1806-20 中回旋共振特征的发现

Abstract: We report evidence of cyclotron resonance features from the Soft Gamma Repeater SGR 1806-20 in outburst, detected with the Rossi X-ray Timing Explorer in the spectrum of a long, complex precursor that preceded a strong burst. The features consist of a narrow 5.0 keV absorption line with modulation near its second and third harmonics (at 11.2 keV and 17.5 keV respectively). The line features are transient and are detected in the harder part of the precursor. The 5.0 keV feature is strong, with an equivalent width of ~ 500 eV and a narrow width of less than 0.4 keV. Interpreting the features as electron cyclotron lines in the context of accretion models leads to a large mass-radius ratio (M/R > 0.3 M_sun/km) that is inconsistent with neutron stars or that requires a low (5-7)x10^{11} G magnetic field that is unlikely for SGRs. The line widths are also narrow compared with those of electron cyclotron resonances observed so far in X-ray pulsars. In the magnetar picture, the features are plausibly explained as ion cyclotron resonances in an ultra-strong magnetic field that have recently been predicted from magnetar candidates. In this view, the 5.0 keV feature is consistent with a proton cyclotron fundamental whose energy and width are close to model predictions. The line energy would correspond to a surface magnetic field of 1.0x10^{15} G for SGR 1806-20, in good agreement with that inferred from the spin-down measure in the source. Show Chinese abstract

Abstract: 我们报告了在爆发期间从软伽马射线重复源SGR 1806-20检测到的回旋共振特征，这些特征通过罗西X射线时变探测器在一次长而复杂的前导脉冲的光谱中被观测到。这些特征包括一个5.0 keV的吸收线，在其第二和第三谐波附近有调制（分别为11.2 keV和17.5 keV）。这些线特征是瞬态的，并在前导脉冲较硬的部分被检测到。5.0 keV特征很强，等效宽度约为500 eV，宽度小于0.4 keV。将这些特征解释为吸积模型中的电子回旋线会导致一个大的质量半径比（M/R > 0.3 M_sun/km），这与中子星不一致，或者需要一个低（5-7）x10^{11} G的磁场，这对于SGRs来说不太可能。与迄今为止在X射线脉冲星中观测到的电子回旋共振相比，这些线宽也较窄。在磁星观点下，这些特征可以合理地解释为超强磁场中的离子回旋共振，这些已经被预测来自磁星候选体。在这种观点下，5.0 keV特征与质子回旋基频一致，其能量和宽度接近模型预测。该线能量对应于SGR 1806-20的表面磁场1.0x10^{15} G，与从源的自转减速测量推断出的磁场值相符。