Neutron Stars with a Secret 'Doughnut' Inside express
Neutron stars are super-dense spheres of the remnants of exploded stars. Some of them we see as pulsars — cosmic lighthouses sending out radio bursts. What's inside them long remained a mystery. Physicists assumed that hidden fields could affect their structure, but it was believed that if the field is short-range, its effect is negligible. A new calculation overturned this idea. It turned out the field doesn't spread evenly but gathers into a ring — a real invisible doughnut, enclosed in an intermediate layer. This 'filling' changes the internal pressure so that the neutron star mimics a quark star — an object where matter has fused into a quark soup. From the outside, in binary system observations, the impostor is unnoticed. But most astonishing: the same star can have two different masses simultaneously, like two states in one body. The discovery explains anomalies in neutron star sizes, and the mystery predicted by Fritz Zwicky and discovered by Jocelyn Bell Burnell has deepened further.
Original abstract
as written by the authors arXiv:2605.26228
Heavy scalar fields ($$m_\phi\gtrsim10^{-9}$$ eV) in scalar-tensor gravity are expected to be hidden from neutron-star observations because their Compton wavelength is sub-stellar. We show that neutron stars can nevertheless scalarize by forming a shell-localized profile, suppressed at their center and exterior but peaked in their interior. This \emph{donutization} reshapes the effective equation of state, making hadronic stars mimic quark-star mass-radius behavior or hybrid-star behavior with split stable branches, and breaks the $$I$$--$$Q$$ relation, while remaining hidden from binary pulsar observations.
The 'doughnut' effect allows a neutron star to exist in two stable states with different masses at once, as if two different objects coexist in a single body.