Top: Values of the spin period measured in the single NICER, Swift, Insight-HXMT/LE, and NuSTAR observations in 2021 as a function of time. The blue dashed line indicates the best-fitting model (for more details see Sect. 3.3). The post-fit residuals are shown at the bottom. Middle: Evolution of the background-subtracted pulsed fraction of Sw J1555 as a function of time. Bottom: Background-subtracted pulsed fraction as a function of energy for the simultaneous Swift (black circle) and NuSTAR (green diamond) observations. The upper limit is reported at 3σ c.l.