Top: long-term spin-down of GX 1+4 expressed in frequencies. Middle: residuals from the linear fit to the frequencies. The orange shaded area indicates the perigee passage time, including uncertainties, given by Hinkle et al. (2006). Yellow shaded areas indicate the perigee passage intervals extrapolated from this ephemeris. Bottom: observed X-ray fluxes in arbitrary scaling to the same mean by BATSE, RXTE/ASM, and Swift/BAT, averaged over 30-day intervals.