A persistent bow shock in a diskless magnetised accreting white dwarf

Abstract

Stellar bow shocks are formed when an outflow interacts with the interstellar medium. In white dwarfs accreting from a binary companion, outflows are associated with either strong winds from the donor star, the accretion disk, or a thermonuclear runaway explosion on the white dwarf surface. To date, only six accreting white dwarfs are known to harbour disk-wind driven bow shocks that are not associated to thermonuclear explosions. Here, we report the discovery of a bow shock associated with a high-proper-motion disk-less accreting white dwarf, 1RXS J052832.5+283824. We show that the white dwarf has a strong magnetic field in the range B~42-45 MG, making RXJ0528+2838 the a bonafide known polar-type cataclysmic variable harbouring a bow shock. The resolved bow shock is shown to be inconsistent with a past thermonuclear explosion, or being inflated by a donor wind, ruling out all accepted scenarios for inflating a bow shock around this system. Modelling of the energetics reveals that the observed bow shock requires a persistent power source with a luminosity significantly exceeding the system accretion energy output. This implies the presence of a powerful, previously unrecognized energy loss mechanism - potentially tied to magnetic activity - that may operate over sufficiently long timescales to influence the course of binary evolution.

Figures (15)

• Figure 1: False-colour image of RXJ0528+2838 and its surrounding nebula. The red, green, and blue channels correspond to the H$\alpha$, [N II] 6548Å, and [O III] 5007Å lines, respectively, extracted using a top-hat filter from the MUSE datacube. The gray arrow indicates the proper motion of RXJ0528+28382021AA...649A...1G. North is up, East is left.
• Figure 2: MUSE observation of RXJ0528+2838 and its extended emission. Top left: white light image of the field with the CV marked with green lines and the direction of its proper motion with a blue arrow. Intensities of measured emission lines are displayed in the other panels. All colour scales are normalised to the maximum $H\alpha$ intensity in the MUSE datacube after being multiplied by a factor of 100 for display purposes.
• Figure 3: Properties of bow-shock nebulae around cataclysmic variables. The properties include the space velocity relative to the local interstellar medium ($v_{\mathrm{ISM}}$), bow-shock size, mechanical luminosity required to power the bow shock ($L_{b}$), and total bow-shock mass.
• Figure 4: The X-ray observing log of RXJ0528+2838.
• Figure 5: Folded TESS lightcurves of RXJ0528+2838 on the orbital period of 80.05169 minutes. Data is from sectors 43, 44, 45, and 71 (gray points). Binned observations with a phase bin width of 0.01 are shown as red lines, with the shaded regions indicating the standard error of the mean for each bin. Each bin contained at least 140 data points.