2020ApJ...900...41L

In 2015 October, the Be/X-ray binary 4U 0115+63 underwent a type II outburst, reaching an X-ray luminosity of ∼10³⁸ erg s^–1. During the outburst, Nuclear Spectroscopic Telescope Array (NuSTAR) performed two Target of Opportunity observations. Using the broadband spectra from NuSTAR (3-79 keV), we have detected multiple cyclotron lines of the source, i.e., ∼12, 16, 22, and 33/35 keV. Obviously, the 16 keV line is not a harmonic component of the 12 keV line. As described by the phase-dependent equivalent widths of these cyclotron lines, the 16 keV and 12 keV lines are two different fundamental lines. In our work, we apply the two-poles cyclotron line model to the observation, i.e., the two line sets are formed at the same altitude (∼0.2 km over the NS surface) of different magnetic poles, with ∼1.1 x 10¹² and 1.4 x 10¹² G in two poles, respectively.