A persistent bow shock in a diskless magnetised accreting white dwarf

TL;DR

This paper reports the discovery of a persistent bow shock around a diskless, highly magnetic white dwarf, suggesting a new energy loss mechanism possibly linked to magnetic activity that affects binary evolution.

Contribution

It presents the first observation of a bow shock in a diskless accreting white dwarf with a strong magnetic field, challenging existing models of bow shock formation.

Findings

The white dwarf has a magnetic field of 42-45 MG.

The bow shock cannot be explained by thermonuclear explosions or donor winds.

A persistent energy source exceeds the system's accretion energy, indicating a new magnetic activity-related mechanism.

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.