The Inner and Outer Shock Layers of Bow Shocks in Cataclysmic Variables

TL;DR

This study reveals that bow shocks around cataclysmic variables are layered structures with an inner wind shock and an outer forward shock, requiring multi-wavelength observations to fully understand their morphology.

Contribution

It demonstrates that the bow shocks in CVs are more complex than previously thought, showing a layered two-shock structure confirmed by multi-wavelength imaging.

Findings

Identification of a layered shock structure in CV bow shocks

First detection of the infrared sweep-up boundary of the forward shock

Reinterpretation of the bright optical arc as the wind termination shock

Abstract

Bow shocks around cataclysmic variables (CVs) have traditionally been identified with a single bright optical arc. This feature has been interpreted as the bow shock formed by the interaction between a sustained outflow and the interstellar medium (ISM). We show that this interpretation is incomplete. Generic wind-ISM interaction theory predicts a two-shock configuration consisting of an inner terminal wind (reverse) shock and an outer forward shock, separated by a hot, low-density shocked wind cavity. Using archival ultraviolet, optical, and infrared imaging of the nova-like systems BZ Cam and V341 Ara, and the polar 1RXS J052832.5+283824, we find that the nebulae around all three systems exhibit this layered structure. In each case, the previously identified bow shock bright in Halpha and [OIII] corresponds to a compact inner arc, while additional emission components reveal a more extended morphology. Specifically, each system shows an outer arc detected in mid-infrared images, and the region between the optical and infrared arcs is filled with faint Halpha emission and, where available, far ultraviolet emission. We identify this infrared arc, reported here for the first time in these systems, as the sweep-up boundary of the forward shock, while the bright inner optical arc corresponds to the terminal wind shock rather than the forward shock as previously assumed. These results reveal that the true extent and layered structure of bow shocks around CVs only become apparent when observations extend beyond the optical band.