Numerical Studies on the Radio Afterglows in TDE: Bow Shock

TL;DR

This study uses simulations to differentiate radio afterglows from bow shocks and forward shocks in tidal disruption events, revealing distinct spectral features and offering a new method to probe dense gas near galactic centers.

Contribution

It provides the first detailed simulation-based analysis of bow shock radio emissions in TDEs, highlighting their unique spectral signatures and implications for understanding circumnuclear dense gas.

Findings

Bow shock radio emission peaks at higher frequencies.

Flux from bow shocks rises steeply and declines rapidly.

Spectral features can be double-peaked or flat-top, deviating from simple models.

Abstract

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 (FS) propagating into diffuse circumnuclear medium (CNM), or a bow shock (BS) 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 FS scenario; in this article, we focus on the BS scenario. Compared to that from the FS, the radio emission from the BS exhibits a higher peak frequency, and its flux shows a much steeper rise and a more rapid decline. The radio flux from the BS also responds to fluctuations in the outflow. The combined effects of the BS and FS substantially alter radio spectra, causing significant deviations from the single-zone emission model, and in some cases producing double-peaked or flat-top features in spectra. This study highlights the importance of the BS, and inspires a novel approach for probing dense gas on sub-parsec scales in galactic nuclei by decomposing the BS radio spectrum to reveal the conditions of circumnuclear dense gas.