Spectral modification of energetic magnetar flares by resonant cyclotron scattering (RCS) is considered. During energetic flares, photons emitted from the magnetically-trapped fireball near the stellar surface should resonantly interact with magnetospheric electrons or positrons. We show by a simple thought experiment that such scattering particles are expected to move at mildly relativistic speeds along closed magnetic field lines, which would slightly shift the incident photon energy due to the Doppler effect. We develop a toy model for the spectral modification by a single RCS that incorporates both a realistic seed photon spectrum from the trapped fireball and the velocity field of particles, which is unique to the flaring magnetosphere. We show that our spectral model can be effectively characterized by a single parameter; the effective temperature of the fireball, which enables us to fit observed spectra with low computational cost. We demonstrate that our single scattering model is in remarkable agreement with Swift/BAT data of intermediate flares from SGR 1900+14, corresponding to effective fireball temperatures of $T_{\rm eff}=6$-$7$ keV, whereas BeppoSAX/GRBM data of giant flares from the same source may need more elaborate models including the effect of multiple scatterings. Nevertheless, since there is no standard physically-motivated model for magnetar flare spectra, our model could be a useful tool to study magnetar bursts, shedding light on the hidden properties of the flaring magnetosphere.

中文翻译：

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共振回旋散射对磁星耀斑的光谱修正

考虑了共振回旋散射 (RCS) 对高能磁星耀斑的光谱修正。在高能耀斑期间，从靠近恒星表面的磁性俘获火球发出的光子应该与磁层电子或正电子发生共振。我们通过一个简单的思想实验表明，这种散射粒子预计会沿着闭合的磁场线以温和的相对论速度移动，这会由于多普勒效应而略微改变入射光子的能量。我们开发了一个用于通过单个 RCS 进行光谱修改的玩具模型，该模型结合了来自被困火球的真实种子光子光谱和粒子的速度场，这是燃烧磁层独有的。我们表明我们的光谱模型可以通过单个参数有效地表征；火球的有效温度，这使我们能够以较低的计算成本拟合观察到的光谱。我们证明我们的单一散射模型与 SGR 1900+14 中间耀斑的 Swift/BAT 数据非常一致，对应于 $T_{\rm eff}=6$-$7$ keV 的有效火球温度，而 BeppoSAX/GRBM来自同一来源的巨大耀斑的数据可能需要更精细的模型，包括多次散射的影响。然而，由于磁星耀斑光谱没有标准的物理驱动模型，我们的模型可能是研究磁星爆发的有用工具，揭示了耀斑磁层的隐藏特性。我们证明我们的单一散射模型与 SGR 1900+14 中间耀斑的 Swift/BAT 数据非常一致，对应于 $T_{\rm eff}=6$-$7$ keV 的有效火球温度，而 BeppoSAX/GRBM来自同一来源的巨大耀斑的数据可能需要更精细的模型，包括多次散射的影响。然而，由于磁星耀斑光谱没有标准的物理驱动模型，我们的模型可能是研究磁星爆发的有用工具，揭示了耀斑磁层的隐藏特性。我们证明我们的单一散射模型与 SGR 1900+14 中间耀斑的 Swift/BAT 数据非常一致，对应于 $T_{\rm eff}=6$-$7$ keV 的有效火球温度，而 BeppoSAX/GRBM来自同一来源的巨大耀斑的数据可能需要更精细的模型，包括多次散射的影响。然而，由于磁星耀斑光谱没有标准的物理驱动模型，我们的模型可能是研究磁星爆发的有用工具，揭示了耀斑磁层的隐藏特性。