Numerical Studies on the Radio Afterglows in TDE: Bow Shock

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.

Figures (9)

• Figure 1: BS formed by outflow impacting a toroidal (model ABt) and spherical (model BBs) cloud. The BS of the toroidal cloud is 2D-shock in nature and appears more extended.
• Figure 2: The synthetic radio spectra along the polar direction (Pol) and the equatorial direction (Eqt) for model ABt and BBs.
• Figure 3: Radio flux along the polar/Z direction for pure BS models (aBt, bBs, cBsv).
• Figure 4: For a cloud located far away (model DBt), the BS apparently undergoes 3 distinct stages: sweeping up post-shock CNM (3.6 yr), post-shock outflow (4.0 yr), and unshocked outflow (5.5 yr). In the snapshot at $t=3.6$ yr, region A, B, C and D mark the CNM, post-shock CNM (by FS), post-shock outflow (by reverse shock) and unshocked outflow, respectively. The synthetic spectra shown are for these 3 epochs along the polar direction.
• Figure 5: Simulation including both the BS and the FS (model aBFt). The left and right panel share the same colorbars.