Fast radio bursts (FRBs) are astrophysical transients of still debated origin. So far several hundred events have been detected, mostly at extragalactic distances, and this number is expected to grow significantly over the next years. The radio signals from the burst experience dispersion as they travel through the free electrons along the line-of-sight characterised by the dispersion measure (DM) of the radio pulse. In addition, each photon also experiences a gravitational Shapiro time delay while travelling through the potentials generated by the large-scale structure. If the equivalence principle (EP) holds, the Shapiro delay is the same for photons of all frequencies. In case the EP is broken, one would expect an additional dispersion to occur which could be either positive or negative for individual sources. Here we suggest to use angular statistics of the DM fluctuations to put constraints on the EP parametrized by the post-Newtonian parameter $γ$. Previous studies suffer from the problem that the gravitational potential responsible for the delay diverges in a cosmological setting, which our approach avoids. We carry out a forecast for a population of FRBs observable within the next years and show that any significant detection of the DM angular power spectrum will place constraints on the EP that are by a few orders of magnitude more stringent than current limits.
Redshift estimates for fast radio bursts and implications on intergalactic magnetic fields
背景：快速射电暴是来自银河系外致密恒星源的瞬时射电脉冲。随着新望远镜每天探测到多个事件，需要统计方法来解释观测结果，并就天体物理学和宇宙学问题作出推论。目的：我们提出了一种使用快速射电暴可观测值的概率估计来获得基础模型的似然估计的方法。方法：考虑模型的所有区域沿视线，包括居间星系，我们执行蒙特卡罗模拟估计的色散措施，旋转措施和时间展宽的分布。使用贝叶斯统计，我们比较这些预测观测的快速射电暴。结果：应用贝叶斯定理，得到了河外DM $\gtrsim 400$ pc cm $^{-3}$的快速射电暴红移的下限。我们发现，干预星系不能占所有高度分散的快速射电暴在FRBcat，因此需要一个更密集和更动荡的环境比SGR 1935+2154类磁星。我们表明，$\gtrsim 10^3$非定域快速射电暴与相关的河外RM $\geq 1$拉德$^{-2}$的样本可以提高星系际磁场的强度的电流上限。
