Summary
ESO says observations with the Very Large Telescope reveal a long-lived bow shock around the white dwarf RXJ0528+2838, despite the system having no disc to drive the outflow. Its strong magnetic field may help explain the shock, but the observed field appears insufficient to account for its full inferred lifetime.
The European Southern Observatory (ESO) reports that its Very Large Telescope has mapped a bow shock around RXJ0528+2838, a white dwarf in a binary system 730 light-years away. The shock is a curved structure formed as material around the star interacts with gas in space. What makes it puzzling is that the system shows no disc, the usual reservoir from which outflows in these binaries are thought to be driven.
The observations suggest the white dwarf has been producing a powerful outflow for at least 1,000 years. ESO’s researchers say the star’s strong magnetic field may supply the energy, but the field appears able to account for only a few hundred years of the shock’s inferred duration.
How the bow shock was identified
RXJ0528+2838 is a white dwarf—the leftover core of a low-mass star—in a binary with a Sun-like companion. Material can transfer from the companion to the white dwarf. In many such systems, the transferred material forms a disc around the compact star; some of it can then be expelled into space. When outflowing material meets the surrounding interstellar gas, it can create a bow shock, a curved arc resembling the wave that gathers in front of a moving ship.
Astronomers first noticed unusual nebulosity around RXJ0528+2838 in images from Spain’s Isaac Newton Telescope. They then used MUSE, an instrument on ESO’s Very Large Telescope, to map the structure and analyse its composition. ESO says this work established that the bow shock originates from the binary system rather than from an unrelated nebula or interstellar cloud.
A magnetic field offers only part of the explanation
The system’s lack of a disc makes the outflow difficult to explain using the usual picture of accretion in these binaries. MUSE data confirmed that RXJ0528+2838 has a strong magnetic field. According to the researchers, the field channels material taken from the companion directly onto the white dwarf, without a disc forming. It is a candidate energy source for the outflow, but not yet a complete explanation for the observed structure.
The bow shock’s shape and size imply that the outflow has persisted for at least 1,000 years. The researchers estimate that the star’s current magnetic field could power a shock lasting only a few hundred years, so the field accounts for only part of the inferred history. The mechanism that sustained the longer-lived outflow remains unresolved.
The finding points to a way that some disc-free binary systems may still produce substantial outflows, challenging the standard account of how matter moves in these systems. ESO says its future Extremely Large Telescope could help map more such binaries, including fainter examples, and investigate the source of their outflows. The research paper, “A persistent bow shock in a diskless magnetised accreting white dwarf,” is to appear in Nature Astronomy.