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

TL;DR

This paper reports the detection of multiple cyclotron lines, including a peculiar 16 keV line, in the 2015 outburst of 4U 0115+63, revealing complex magnetic field structures near the neutron star surface.

Contribution

It introduces a two-poles cyclotron line model explaining the non-harmonic 16 keV line as a fundamental line from different magnetic poles.

Findings

Detection of multiple cyclotron lines at 12, 16, 22, and 33/35 keV.

The 16 keV line is a fundamental, not harmonic, line.

Different magnetic field strengths at two poles are inferred.

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.