Period change in GX 1+4. Left: GLS periodogram (blue). The frequency with the highest power is highlighted in red. This periodogram corresponds to the full K2 light curve of GX 1+4. There are other peaks around the main peak, which suggests that the period changed during the observations (see Sect. 3). The inset shows the light curve folded at the 180.426 s period and binned at 80 bins/cycle. Right: historical evolution of the spin period from measurements from various instruments, listed in Table B.1 of González-Galán et al. (2012), such as Rossi X-ray Timing Explorer (RXTE) and Compton Gamma Ray Observatory/Burst And Transient Source Experiment (CGRO/BATSE), among others (see text). Fermi/Gamma-ray Burst Monitor (GBM) measurements are shown as orange symbols, while K2 measurements are shown as black symbols. The inset shows the spin evolution during the K2 observations reported here; the red line shows a simple linear fit to the spin-down trend.