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

Krystian Ilkiewicz, Christian Knigge, Simone Scaringi, Noel Castro Segura, Santiago del Palacio, Martina Veresvarska

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.

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

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.
Paper Structure (7 sections, 7 figures)

This paper contains 7 sections, 7 figures.

Table of Contents

  1. Introduction
  2. Data selection
  3. Results: layered bow-shock morphology
  4. BZ Cam: the predicted two-shock stratification
  5. RXJ0528+2838 and V341 Ara: the same stratification across accretion regimes
  6. Discussion and Conclusions
  7. WISE knots in the BZ Cam outer shell

Figures (7)

  • Figure 1: False-color composite image of BZ Cam: GALEX FUV (blue), H$\alpha$ (green; 2018PASP..130i4201B), and unWISE W2 (red). The morphology is stratified: a compact inner optical bow shock, an extended FUV/faint-H$\alpha$ interior, and an outermost W2 shell. The arrow indicates the Gaia proper-motion vector 2023AA...674A...1G corrected for Galactic rotation. The corrected proper motion is $(\mu_{\alpha},\mu_{\delta}) = (-0.69\pm0.02,\,-21.61\pm0.07)\ \mathrm{mas\ yr^{-1}}$. The corresponding tangential, radial, and total space velocities are $V_t = 38.4\pm0.4$ km s$^{-1}$, $V_r = -69\pm10$ km s$^{-1}$, and $V = 79.3\pm8.8$ km s$^{-1}$. North is up, east is left.
  • Figure 2: Schematic mapping between observed tracers and physical zones in the two-shock interpretation of CV bow shocks. The outermost layer (orange) represents the swept-up H ii/forward-shock boundary traced by infrared-emitting dust. The inner bow shock (green) represents the terminal/reverse shock front, which is bright in [O iii] and H$\alpha$. The region between the two bow shocks (blue) is the shocked-wind cavity expected to be bright in the far-ultraviolet and only faintly in H$\alpha$. The inner region (gray) indicates unshocked wind with no clear observational signatures. The arrow shows the CV proper-motion direction.
  • Figure 3: False-color images of RXJ0528+2838 (top) and V341 Ara (bottom). Blue: [O iii] $\lambda5007$; green: H$\alpha$; red: W2--W1 difference (to suppress stellar crowding; see text). Optical narrow-band images are from nature_bowshocks (RXJ0528+2838) and 2021MNRAS.501.1951C (V341 Ara). In both systems, the infrared emission marks the outermost bow shock layer. The arrows indicate the Gaia proper-motion vectors 2023AA...674A...1G corrected for Galactic rotation. For RXJ0528+2838, the corrected proper motion is $(\mu_{\alpha},\mu_{\delta}) = (39.31\pm0.06,\,-29.1\pm0.2)\ \mathrm{mas\ yr^{-1}}$, corresponding to $V_t = 52.2\pm 0.9$ km s$^{-1}$, $V_r = 117.4\pm 5.0$ km s$^{-1}$, and a total space velocity $V = 128.5\pm 4.6$ km s$^{-1}$. For V341 Ara, the corrected proper motion is $(\mu_{\alpha},\mu_{\delta}) = (-44.89\pm0.03,\,-60.2\pm0.1)\ \mathrm{mas\ yr^{-1}}$, with $V_t = 55.5\pm0.4$ km s$^{-1}$, $V_r = 47.1\pm2.0$ km s$^{-1}$, and $V = 72.7\pm1.3$ km s$^{-1}$. North is up, east is left.
  • Figure 4: Spectral energy distributions (left) and W1/W2 light curves (right) for representative WISEA clumps on the BZ Cam outer shell. Red curves show blackbody fits; shaded regions indicate fit uncertainties.
  • Figure 5: (Continued.)
  • ...and 2 more figures