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.
A rotation period of 12 seconds is like a star the size of a city making one full turn while you read this sentence.