An international team of astronomers has discovered an unexpected bow-shaped shock wave around a nearby binary system containing a dead star known as a white dwarf, a phenomenon that current theories cannot explain. The finding was announced by the European Southern Observatory (ESO).
“Our observations revealed a powerful outflow, which, according to current knowledge, should not be there,” said Krystian Iłkiewicz of the Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences in Warsaw and the Centre for Extragalactic Astronomy at Durham University in the UK, the lead author of the study.
The object, RXJ0528+2838, lies about 730 light-years from Earth and orbits the center of the Milky Way. As it moves through the interstellar medium, its interaction with surrounding gas has produced a bow shock - a curved arc of compressed material similar to a wave forming in front of a ship’s bow.
Such bow shocks are normally created by material flowing outward from a star. In the case of RXJ0528+2838, however, astronomers know of no mechanism that should be capable of producing the observed outflow.
RXJ0528+2838 is a white dwarf, the dense core left behind after a low-mass star exhausts its nuclear fuel. It forms a binary system with a Sun-like companion. In typical systems of this kind, gas from the companion star is pulled toward the white dwarf, forming an accretion disk. While most of this material feeds the white dwarf, some is expelled into space, generating strong outflows. No such disk has been detected around RXJ0528+2838.
The system first drew attention after observations with the Isaac Newton Telescope in Spain revealed a surrounding nebula with an unusual shape. This prompted follow-up observations with ESO’s Very Large Telescope (VLT) in Chile.
“Observations with the MUSE instrument enabled us to make a detailed map of the bow shock wave and analyse its composition. This was crucial for confirming that the structure actually comes from the binary system and not from an unrelated nebula or interstellar cloud,” Iłkiewicz said.
From the size and geometry of the bow shock, the researchers concluded that the white dwarf has been driving a massive outflow for at least 1,000 years. How a dead star could sustain such a long-lived outflow remains unclear.
One possible explanation involves a strong magnetic field, whose presence has been confirmed by observations. According to the team’s hypothesis, the magnetic field channels material taken from the companion star directly onto the white dwarf, bypassing the formation of an accretion disk. Iłkiewicz said the analysis suggests that such systems may be able to launch powerful outflows even without a disk, challenging standard models of mass transfer in extreme binary systems.
The observations point to a hidden energy source, most likely linked to magnetism. However, the strength of the magnetic field measured so far would be sufficient to power the bow shock for only a few hundred years, leaving the long duration of the outflow unexplained.
Astronomers hope that ESO’s Extremely Large Telescope (ELT), currently under construction, will allow them to identify and study more systems of this type, potentially revealing the missing energy source.
The results were published in the journal Nature Astronomy. The research was led by Krystian Iłkiewicz together with Simone Scaringi of Durham University and the Italian National Institute for Astrophysics (INAF). (PAP)
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