A spinning top whirls slower and slower until it stops. Neutron stars, the ultra-dense remnants of exploded stars, have their own 'stalling period.' In a new study, astronomers noticed that the rotation periods of two types of such stars, magnetars (with very strong magnetic fields) and dim X-ray neutron stars, cluster around remarkably close values: about 12 and 14 seconds.
The researchers took data from 30 magnetars with periods ranging from 0.33 to 11.78 seconds and from 8 dim neutron stars with periods from 3.45 to 12.76 seconds. They applied a statistical method to estimate the periods at which these stars are born and the limit to which they slow down. It turned out that for magnetars, the limit is about 11.8–12.0 seconds, and for dim neutron stars, about 12.8–14.9 seconds. For the combined sample of 38 objects, the narrowest range emerges: 12.8–12.9 seconds.
Such a coincidence is unlikely to be random. Over two decades of discoveries, the number of known magnetars has tripled, while the upper limit of periods has shifted only slightly — from 12 to 15 seconds. This strongly supports the idea of a physical cause: perhaps the magnetic field of these stars weakens over time, or a disk of matter forms around them, braking their rotation to equilibrium. If the mechanism is common, then magnetars and dim neutron stars may be links in the same evolutionary chain.
as written by the authors arXiv:2603.09608
The spin periods of magnetars and X-ray dim isolated neutron stars (XDINS) cluster within a remarkably narrow range. Using the current sample of 30 magnetars with measured periods (ranging from 0.33 to 11.78 s) and 8 XDINS (ranging from 3.45 to 12.76 s), we utilize the point-likelihood technique to constrain the birth and final periods of these sources, assuming a steady-state population. Employing a general braking law characterized by a constant braking index $$n$$, we find that for $$n > 2$$ the final (cut-off) period of magnetars is constrained to $$P_f \simeq 11.8 - 12.0$$ s and XDINS to $$P_f \simeq 12.8 - 14.9$$ s, at the 95 per cent confidence level, while the birth periods remains largely unconstrained for dipole spin-down ($$n=3$$) as in earlier work. The slight increase in the upper cutoff from $$\sim$$12 to $$\sim$$15 s over two decades of discoveries of new sources, yielding a threefold increase in the known magnetar population, and the extension of the minimum period to $$\sim 0.33$$ s strongly support a physical origin for this clustering. We discuss this result in the context of magnetic-field-decay models and fallback-disc torque-equilibrium scenarios. The combined magnetar and XDINS sample (38 sources) yields the tightest constraints on $$P_f\simeq 12.8-12.9$$ s, for $$n=3$$, suggesting possible evolutionary connections between these populations and pointing toward a common physical mechanism that terminates the observable phase of these neutron stars at periods near 14 s.
A rotation period of 12 seconds is like a star the size of a city making one full turn while you read this sentence.