Gas falls onto a neutron star — the ultra-dense remnant of an exploded star. This gas accumulates at the magnetic poles and forms a mountain. But a mountain on a neutron star is not like one on Earth: it is held not by rock but by magnetic field lines, like iron filings on a magnet.
When there is too much gas, the mountain begins to break apart. Turbulent flows inside it mix the material, and it seeps across the magnetic field lines. Previously, it was thought this would destroy the mountain. But calculations showed the opposite: the gas flow itself quenches the turbulence and preserves the mountain. It's like a sand dune that shifts shape in the wind but doesn't disappear.
Unless destroyed by electrical resistance, the mountain can survive for millions of years. That means many neutron stars have such invisible mountains. They are like quiet beacons sending signals across the universe.
as written by the authors arXiv:2604.13557
Hydromagnetic instabilities modify the structure of a magnetically confined mountain on an accreting neutron star, once the accreted mass exceeds a critical value. Ideal magnetohydrodynamics and flux freezing break down, and mass diffuses across magnetic field lines locally, wherever instabilities are excited. Here a self-consistent, iterative, numerical scheme is used to evolve an axisymmetric magnetic mountain through a quasistatic sequence of Grad-Shafranov equilibria as a function of the accreted mass, $$M_{\rm a}$$, modified by instability-driven cross-field mass transport obeying the semi-analytic, Kulsrud-Sunyaev recipe. The results are compared to an artificially stabilised mountain, in which flux freezing does not break down, and there is no cross-field mass transport. It is shown that cross-field mass transport prevents instabilities from demolishing the mountain. Instead, the mass-flux distribution adjusts locally to nullify the instabilities and preserve a nonzero mass quadrupole moment indefinitely in the absence of ohmic dissipation.
A mountain on a neutron star just a few centimeters high would weigh as much as all the mountains on Earth combined.