Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to around 1 TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization—the only range available until now—probe extended regions of the jet containing particles that left the acceleration site days to years earlier^1,2,3, and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian 501 (Mrk 501). We measure an X-ray linear polarization degree Π_X of around 10%, which is a factor of around 2 higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration and also implies that the plasma becomes increasingly turbulent with distance from the shock.

大部分来自耀变体的光都是由高能粒子产生的，这些耀变体是活跃的星系核，带有磁化等离子体射流，几乎指向视线方向，最高可达 1 TeV。尽管已知喷流最终由超大质量黑洞提供动力，但粒子如何加速到如此高的能量一直是一个悬而未决的问题。这个过程一定与磁场有关，磁场可以通过观察来自喷流的光的偏振来探测。无线电到光学偏振的测量——迄今为止唯一可用的范围——探测射流的扩展区域，其中包含几天到几年前离开加速点的粒子^1,2,3，因此不会像 X 射线测量那样直接探索加速机制。在这里，我们报告了从耀变体 Markarian 501 (Mrk 501) 检测到的 X 射线偏振。我们测得 X 射线线性偏振度Π _X约为 10%，这是光学波长值的 2 倍左右，偏振角平行于射电射流。这表明激波前缘是粒子加速的来源，也意味着等离子体随着远离激波而变得越来越湍流。