Luminosity dependence of the multiple cyclotron lines in4U 0115+63

                                         Kinjal Roy, Hemanth Manikantan, and Biswajit Paul

Raman Research Institute, Bangalore-560080, India.

The Be X-ray binary 4U 0115+63 underwent a giant outburst in 2023 with X-ray luminosity of the source reaching 1038 erg s−1. During the outburst, two target of opportunity observations were made with NuSTAR. The main goal of this work is to model the multiple cyclotsron scattering features present in the 4U 0115+63 and study their dependence on the luminosity of the source. The 3−79 keV broadband X-ray coverage of 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 pulsation from the source and studied the changes in the cyclotron line energy with an order of magnitude variation in 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 0115+63 at 12 keV and 16 keV. With the inclusion of the two latest NuSTAR observations, we have expanded the data set of 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 centre shows no anti-correlation with luminosity, unlike previously reported. Instead, a weak positive correlation is found in four out of the five detected cyclotron lines of 4U 0115+63.

Reference:

[1] Cyclotron lines in highly magnetized neutron stars, Staubert, R. et al. 2019, A&A, 622, A61.

[2] Luminosity dependence of the multiple cyclotron lines in 4U 0115+63, K. Roy et al, submitted to A&A journal.