Weak secondary cyclotron line in eclipsing high-mass X-ray binary Cen X-3

We report the time-resolved spectroscopy results from two observations of Centaurus X-3, over one binary orbit with AstroSat and two binary orbits with Nuclear Spectroscopic Telescope Array (NuSTAR). NuSTAR covered two intensity states where the light curve showed transition in count rate from first to second binary orbit by a factor of similar to 3. A phenomenological model comprising of partially absorbed power law with smoothed high energy cutoff, cyclotron absorption similar to 24 and 6.4 keV iron emission gave good fit for AstroSat observation. NuSTAR spectra required two additional emission components, a broad one similar to 5.7 keV and a narrow one similar to 6.9 keV. A weak secondary absorption feature at similar to 11.6 and similar to 14.5 keV was seen in the residuals of the spectral fit for AstroSat and NuSTAR data, respectively. The secondary absorption energy showed no correlation with the cutoff energy. Its strength varied within 0.1-0.6 keV with its width similar to 1.6 keV. Its energy and optical depth showed linear positive correlation with the fundamental cyclotron line energy and depth, respectively. The cyclotron line energy showed anticorrelation to flux described by a power law with negative index and the secondary absorption also showed similar trend to flux. Depth of secondary absorption was similar to 45 per cent and centroid energy was similar to 54 per cent of fundamental. Depth and energy ratio of secondary to fundamental lied within 2 sigma deviation from 0.5. We suggest this secondary absorption to be a redshifted dipolar cyclotron resonance feature exhibiting sub-harmonic behaviour.