The neutron star 1E 161348-5055 in the supernova remnant RCW 103 pulses with a period of 6.67 hours. That's very slow for such a compact object, and astronomers long puzzled over what was going on. They proposed that the star is not a perfect sphere but slightly flattened, and its rotation axis slowly wobbles, like a spinning top. This wobble, free precession, combines with the intrinsic rotation to produce the long brightness period.
To test this idea, the scientists analyzed archival data from the X-ray telescopes ASCA, XMM-Newton, and NuSTAR, collected from 1993 to 2017. They removed the effect of slow precession and saw short pulsations with a period of about 1.01 seconds. This period was consistent across all six observations, although the optimal energy range for detecting it varied.
The measured periods fell neatly on a straight line of slowdown: each second, the period increases by 1.097×10⁻¹² seconds. This means the star is losing rotational energy. From the slowdown rate, they estimated an age of about 14,700 years and a spin-down power of 4.2×10³⁴ erg/s, which is insufficient to explain the observed X-ray luminosity. The star has a strong magnetic field: a dipole component of about 4.6×10¹³ G and a toroidal component of about 7×10¹⁵ G. Such fields are typical of magnetars, but this object is not entirely like them.
The authors proposed an emission geometry that roughly explains the observations: a hot spot on the star's surface periodically appears and disappears due to precession. This is like a lighthouse that not only rotates but also slowly nods, so its beam sometimes hits the detector and sometimes veers away.
The 6.67-hour period is one of the longest among X-ray pulsars; most rotate in fractions of a second.
In Robert Wilson's novel Spin, neutron stars are used as precise clocks — here we see that even such clocks can 'tick' with unexpected interruptions.