The brightest fast blue optical transients (FBOTs) are mysterious extragalactic explosions that may represent a new astrophysical phenomenon¹. Their fast time to maximum brightness of less than a week, decline over several months, and atypical optical spectra and evolution are difficult to explain within the context of the core collapse of massive stars, which are powered by radioactive decay of ⁵⁶Ni and evolve more slowly^2,3. AT2018cow (at a redshift of 0.014) is an extreme FBOT in terms of rapid evolution and high luminosity^4,5,6,7. Here we present evidence for a high-amplitude quasiperiodic oscillation of AT2018cow’s soft X-rays with a frequency of 224 Hz (at a 3.7σ significance level or false alarm probability of 0.02%) and fractional root-mean-squared amplitude of >30%. This signal is found in the average power density spectrum taken over the entire 60-day outburst and suggests a highly persistent signal that lasts for a billion cycles. The high frequency (rapid timescale) of 224 Hz (4.4 ms) argues for a compact object in AT2018cow, which could be a neutron star or black hole with a mass less than 850 solar masses. If the quasiperiodic oscillation is equivalent to the spin period of a neutron star, we can set limits on the star’s magnetic field strength. Our work highlights a new way of using high-time-resolution X-ray observations to study FBOTs.

最亮的快速蓝色光学瞬变 (FBOT) 是神秘的河外爆炸，可能代表一种新的天体物理现象¹。它们在不到一周的时间内达到最大亮度的快速时间，几个月内下降，以及非典型的光谱和演化很难在大质量恒星核心坍缩的背景下解释，这些恒星由⁵⁶ Ni 的放射性衰变提供动力，并且演化得更多慢慢地^2,3。^{AT2018cow（红移为 0.014）在快速演化和高亮度4,5,6,7}方面是一个极端的 FBOT 。在这里，我们提供了 AT2018cow 软 X 射线的高幅度准周期振荡的证据，频率为 224 Hz（在 3.7 σ显着性水平或 0.02% 的误报概率）和大于 30% 的分数均方根幅度。该信号存在于整个 60 天爆发的平均功率密度谱中，表明这是一个持续十亿个周期的高度持久的信号。224 赫兹（4.4 毫秒）的高频（快速时间尺度）表明 AT2018cow 中有一个致密天体，它可能是质量小于 850 个太阳质量的中子星或黑洞。如果准周期振荡相当于中子星的自旋周期，我们可以对恒星的磁场强度进行限制。我们的工作突出了一种使用高分辨率 X 射线观测来研究 FBOT 的新方法。