A cannibalistic collapsed star is growing so fast from the partner it is slowly devouring that it is likely to be the most massive neutron star yet measured. The observation suggests that neutron stars can grow much bigger than previously thought before collapsing to become a black hole.
The star in question is a “black widow” pulsar, a type of rotating neutron star that is highly magnetised. It rotates in tandem with a smaller partner, which its gravitational field slowly destroys.
Previously the most massive neutron star known was between 1.66 and 1.68 times as massive as the sun. Now Marten van Kerkwijk of the University of Toronto, Canada, and his colleagues estimate that the black widow pulsar B1957+20 (depicted below) is 2.4 solar masses (arxiv.org/abs/1009.5427).
This rules out the popular “Brown-Bethe” model, which says the maximum mass for a neutron star is about 1.5 solar masses before collapse into a black hole is inevitable, as well as other models.
“Two solar masses would exclude a whole bunch of models and 2.4 solar masses would blow just about anything away,” says co-author Shrinivas Kulkarni of the California Institute of Technology in Pasadena.