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

Abstract

ABSTRACT 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 ∼3. A phenomenological model comprising of partially absorbed power law with smoothed high energy cutoff, cyclotron absorption ∼24 and 6.4 keV iron emission gave good fit for AstroSat observation. NuSTAR spectra required two additional emission components, a broad one ∼5.7 keV and a narrow one ∼6.9 keV. A weak secondary absorption feature at ∼11.6 and ∼ 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 ∼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 ∼45 per cent and centroid energy was ∼54 per cent of fundamental. Depth and energy ratio of secondary to fundamental lied within 2σ deviation from 0.5. We suggest this secondary absorption to be a redshifted dipolar cyclotron resonance feature exhibiting sub-harmonic behaviour.