AI 中文总结
该研究利用ALMA对GRB余辉毫米波偏振样本进行系统研究，通过多时期观测揭示偏振演化，发现不同GRB的偏振特性，现有模型难以完全解释，在斑块状场框架下解释观测结果，证明了射电/毫米偏振测量对GRB喷流相关特性的诊断能力。
AI 中文摘要
我们对当前可用的阿塔卡马大型毫米/亚毫米波阵列（ALMA）伽马射线暴（GRB）余辉毫米波偏振样本进行了系统研究。我们的样本包括对11次长暴的24次观测（20次为新观测），时间跨度约为暴后0.1 - 87天。我们在6次事件的8次观测中检测到显著的线性偏振，偏振度范围从\(\Pi_L\approx0.6\%\)到\(2.4\%\)。其余观测给出了深度上限（中位数\(\Pi_L\lesssim1\%\)）。多时期观测揭示了多样的偏振演化。GRB 190114C给出了迄今为止射电余辉最佳采样的\(\Pi_L\)，早期演化支持反向激波（RS）中存在斑块状磁场，后期偏振大致符合正向激波（FS）模型。GRB 220921A在1.9 - 6.8天内偏振从\(\Pi_L\lesssim0.4\%\)快速上升到\(2.4\%\)，与环形RS磁场模型不一致，但大致符合几种FS随机场模型。GRB 221009A在样本中给出了最高显著性的偏振检测（\(\Pi_L\approx1.4 - 1.6\%\)），然而现有的FS和RS偏振模型都无法同时重现观测到的偏振演化和从宽带余辉建模推断出的观测几何，RS中的斑块状磁场除外。GRB 171205A的深度上限排除了已发表的离轴几何的RS环形场模型，而GRB 190829A的单次强检测（如果以RS为主）对于环形场配置需要近乎轴上的几何。在斑块状场框架内解释观测结果意味着磁场相干尺度约为\(\theta_B\sim10^{-3} - 10^{-2}\)弧度。这些观测证明了射电/毫米偏振测量在探测相对论性GRB喷流的磁场结构、发射区域和观测几何方面的诊断能力。
英文摘要
We present a systematic study of the currently available ALMA millimeter polarimetric sample of gamma-ray burst (GRB) afterglows. Our sample comprises 24 observations (20 new) of 11 long-duration GRBs spanning $\approx0.1$-87 days after the burst. We detect significant linear polarization in 8 observations across 6 events, with polarization degrees ranging from $Π_L\approx0.6\%$ to $2.4\%$. For the remaining observations, we place deep upper limits (median $Π_L\lesssim1\%$). Multi-epoch observations reveal diverse polarization evolution. GRB 190114C yields the best-sampled $Π_L$ for a radio afterglow to date, with early evolution favoring patchy magnetic fields in the reverse shock (RS) and later polarization broadly consistent with forward-shock (FS) models. GRB 220921A exhibits a rapid rise in polarization from $Π_L\lesssim0.4\%$ to $2.4\%$ over 1.9-6.8 days, inconsistent with toroidal RS magnetic-field models but broadly consistent with several FS random-field models. GRB 221009A yields the highest-significance polarization detections in the sample ($Π_L\approx1.4$-$1.6\%$), yet neither existing FS nor RS polarization models reproduce both the observed polarization evolution and the viewing geometry inferred from broadband afterglow modeling, with the exception of patchy fields in the RS. Deep upper limits for GRB 171205A rule out RS toroidal-field models for the published off-axis geometry, while a strong, single-epoch detection of GRB 190829A, if RS-dominated, requires a nearly on-axis geometry for toroidal-field configurations. Interpreting the observations within a patchy-field framework implies magnetic-field coherence scales of order $θ_B\sim10^{-3}$-$10^{-2}$ rad. These observations demonstrate the diagnostic power of radio/mm polarimetry for probing the magnetic-field structure, emission region, and viewing geometry of relativistic GRB jets.
CommentsSubmitted to ApJ, comments welcome
