A peculiar cyclotron line near 16 keV detected in the 2015 outburst of 4U 0115+63?

Abstract

In 2015 October, the Be/X-ray binary 4U 0115+63 underwent a type II outburst, reaching an X-ray luminosity of $\\sim$ {\\DP{38}\\es}. During the outburst, Nuclear Spectroscopic Telescope Array ({\\it NuSTAR}) performed two Target of Opportunity observations. Using the broad-band spectra from {\\it NuSTAR} (3$-$79 keV), we have detected multiple cyclotron lines of the source, i.e., $\\sim$ 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 ($\\sim$ 0.2 km over the NS surface) of different magnetic poles, with $\\sim$ {1.1\\TDP{12}} and {1.4\\TDP{12}} G in two poles, respectively.