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tokens: fluctuation in GX 301–2. This implies that the observed fluctuations are highly significant. The source flux was estimated based on Neil GehrelsSwift Burst Alert Telescope (BAT) data (Krimm et al. 2013 ) 2 and the X-ray light curve was folded in the way similar to the frequency derivative to trace the corresponding flux. The flux was transformed to X-ray luminosity using three simultaneous observations by the Nuclear Spectroscopic TelescopeArray ( NuSTAR ). In the full energy band, 3–79 keV, NuSTAR observed absorbed luminosities of (0.24 ± 0.07) × 10 37 , (0.4 ± 0.1) × 10 37 , and (1.2 ± 0.3) × 10 37 erg s −1 on MJD 56959, 57299, and 58545 (Nabizadeh et al. 2019 ). The luminosities corrected for absorption in the interstellar medium and in the system are consistently about 20 per cent higher. The corresponding count rates measured by Swift / BAT on the same dates are 0.017 ± 0.003, 0.020 ± 0.003, and 0.06 ± 0.03 counts cm −2 s −1 , which imply an average conversion factor of (18 ± 1) × 10 37 erg s −1 (counts cm −2 s −1 ) -1 . With the computed luminosity [Fig. 1(b) ], we now obtain variations of the specific accreted angular momentum [Fig. 1(c) ] with the orbital phase using the equation where I = 10 45 g cm 2 is the moment of inertia of the NS and is the mass accretion rate, which is connected to luminosity L through where ε = 0.15 is the assumed efficiency. It is evident that the spin evolution during the preperiaston flare is complex, showing both spin-up and spin-down episodes. If the mass accretion rate was simply correlated with the transported angular momentum, the NS would be expected to strongly spin-up throughout the flare. However, the maximum spin-up is already observed before the observed peak of the flux, when maximal accretion torque is naively expected. The observed spin-up rate, however, starts to steadily decrease several days prior to the flux maximum even though the accretion rate continues to increase. In fact, zero spin-up rate is observed at the peak of the flare, and the NS is already spinning down right after the peak. The maximal spin-down rate is then reached several days after the peak of the flare, still at a high accretion rate. 3 DISCUSSION Here, we would like to focus on the discussion of the spin-frequency changes around the periastron. As discussed in the data analysis section, even considering uncertainties in the orbital parameters determined by Koh et al. ( 1997 ), the observed evolution of the frequency derivative over the orbit has a high statistical significance and therefore requires a physical explanation. First of all, it is important to emphasize that in GX 301–2, the observed orbital flux dependence is neither consistent with accretion from a transient accretion disc, nor from a smooth homogeneous wind. However, in an eccentric binary system, the wind may be enhanced in the periastron due to tidal interaction of the NS with the primary (Stevens 1988 ). Based on this result, the flaring has been