Luminosity dependence of the multiple cyclotron lines in 4U 011563
arXiv:2407.13869 · doi:10.1051/0004-6361/202450395
Abstract
The Be X-ray binary 4U 011563 underwent a giant outburst in 2023 with the X-ray luminosity of the source reaching 10 erg s. During the outburst, two target of opportunity observations were made with \textit{NuSTAR}. The main goal of this work is to model the multiple cyclotron scattering features (CRSFs) present in 4U 011563 and study their dependence on the luminosity of the source. The 379 keV broadband X-ray coverage of \textit{NuSTAR} allowed us to properly model the continuum and investigate the nature of the multiple cyclotron resonance scattering features present in the source spectrum. We used the epoch-folding technique to find the pulsation from the source and also studied the changes in the cyclotron line energy with an order of magnitude variation in the source luminosity. We detected five cyclotron lines during the 2023 outburst near 12, 16, 24, 34, and 47 keV. The 16 keV cyclotron line cannot be harmonically related to the other cyclotron lines at 12 keV and 24 keV. This indicates the presence of two fundamental lines in the spectrum of 4U 011563 at 12 keV and 16 keV. With the inclusion of the two latest \textit{NuSTAR} observations, we have expanded the data set of the CRSF line center to encompass a broad range of luminosity. This enables us to comprehensively investigate the relationship between the centroid energy of the cyclotron lines and luminosity. The CRSF line center shows no anticorrelation with luminosity, unlike previously reported. Instead, a weak positive correlation is found in four out of the five detected cyclotron lines of 4U 011563. The luminosity variation of the two fundamental CRSFs could be well explained with the prediction from the collisionless shock model.
Accepted for publication in Astronomy & Astrophysics
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