Numerical Studies on the Radio Afterglows in TDE: Bow Shock

The origin of radio afterglows or delayed radio flares in tidal disruption events (TDEs) is not fully understood. They could be generated either by a forward shock propagating into diffuse circumnuclear medium (CNM), or a bow shock around a dense cloud, each of which is fundamentally different. To elucidate the distinctions between these two scenarios, we conducted two-fluid simulations incorporating relativistic electrons to investigate the spatial evolution of these electrons after being accelerated by shock. Based on their spatial distribution, we performed radiative transfer calculations to obtain the synchrotron spectra. In Paper I (Mou 2025), we reported the results for the forward shock scenario; in this article, we focus on the bow shock scenario. Compared to that from the forward shock whose peak frequency typically lies around GHz and decreases with time, the radio emission from the bow shock peaks at higher frequencies, typically $\sim$1-20 GHz, and its flux rises more steeply than $t^4$ across our explored parameter space. The radio flux from the bow shock also responds to fluctuations in the outflow. The combined effects of the bow shock and forward shock substantially alter radio spectra, causing significant deviations from the single-zone emission model, and in some cases producing multi-component feature in spectra. This study highlights the importance of the bow shock, and inspires a novel approach for probing dense gas on sub-parsec scales in galactic nuclei by decomposing the bow shock radio spectrum to reveal the conditions of circumnuclear dense gas.