Discovery of Two Cyclotron Resonance Scattering Features in X-ray Pulsar Cen X-3 by Insight-HXMT

Abstract

We present the results of the neutron star X-ray binary system Cen X-3 performed by $Insight$-HXMT with two observations during 2017 and 2018. During these two observations, the source reached a X-ray luminosity of $\sim 10^{38}$ erg s${}^{-1}$ from 2 -- 105 keV. The analysis of the broadband X-ray spectrum reports the presence of two cyclotron resonance scattering features (CRSFs) with the fundamental line at $\sim$ 28 keV and the harmonic line at $\sim 47$ keV. The multiple lines exist by fittings with different continuum models, like the absorbed NPEX model and a power-law with high energy exponential cutoff model. This is the first time that both fundamental and harmonic lines are detected in Cen X-3. We also show evidence of two cyclotron lines in the phase-resolved spectrum of Cen X-3. The CRSF and continuum spectral parameters show evolution with the pulse profile, and the two line centroid energy ratio does not change significantly and locates in a narrow value range of $1.6-1.7$ over the pulse phase. The implications of the discovering two cyclotron absorption features and phase-resolved spectral properties are discussed.

8 pages, MNRAS, 2023, 519, 5402-5409

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• An in-depth analysis of the variable cyclotron lines in GX 301$-$2
• Luminosity dependence of the multiple cyclotron lines in 4U 0115$+$63
• Revealing the state transition of Cen X-3 at high spectral resolution with Chandra
• Broadband study of the Be X-ray binary RX J0520.5-6932 during its outburst in 2024
• Studying the variations of the cyclotron line in Cen X-3 using Insight-HXMT
• Cyclotron lines in subcritical X-ray pulsars: Monte Carlo simulations reveal the origin of the observed variability