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Cyclotron line scattering features are detected in a few tens of X-raypulsars (XRPs) and used as direct indicators of a strong magnetic field at thesurface of accreting neutron stars (NSs). In a few cases, cyclotron lines areknown to be variable with accretion luminosity of XRPs. It is accepted that theobserved variations of cyclotron line scattering features are related tovariations of geometry and dynamics of accretion flow above the magnetic polesof a NS. A positive correlation between the line centroid energy and luminosityis typical for sub-critical XRPs, where the accretion results in hot spots atthe magnetic poles. The negative correlation was proposed to be a specificfeature of bright super-critical XRPs, where radiation pressure supportsaccretion columns above the stellar surface. Cyclotron line in spectra ofBe-transient X-ray pulsar GRO J1008-57 is detected at energies from $\sim 75-90$ keV, the highest observed energy of cyclotron line feature in XRPs. Wereport the peculiar relation of cyclotron line centroid energies withluminosity in GRO J1008-57 during the Type II outburst in August 2017 observedby Insight-HXMT. The cyclotron line energy was detected to be negativelycorrelated with the luminosity at $3.2\times 10^{37}\,\ergs<L<4.2\times10^{37}\,\ergs$, and positively correlated at $L\gtrsim 5\times10^{37}\,\ergs$. We speculate that the observed peculiar behavior of acyclotron line would be due to variations of accretion channel geometry.

Relation of cyclotron resonant energy and luminosity in a strongly magnetized neutron star GRO J1008-57 observed by Insight-HXMT