Luminosity dependent cyclotron line in Swift J1626.6-5156

Abstract: We studied the timing and spectral properties of the Be/X-ray pulsar Swift J1626.6-5156 using the \emph{NICER} observations of its 2021 outburst. The most important observation is the positive correlation of the centroid energy of the fundamental cyclotron line with the luminosity. This observation agrees with the usual positive correlation of the centroid energy cyclotron line with luminosity in the sub-critical regime. The correlation between the two quantities is verified using two different continuum models. The photon index decreases with an increase in flux. Thus, the spectrum is softer when the flux is low, which may be due to a decrease in the optical depth of the accretion column with a decrease in the flux.