The first cyclotron harmonic of 4U 1538-52

Abstract

Context: Cyclotron resonant scattering features are an essential tool for studying the magnetic field of neutron stars. The fundamental line provides a measure of the field strength, while the harmonic lines provide information about the structure and configuration of the magnetic field. Until now only a handful of sources are known to display more than one cyclotron line and only two of them have shown a series of harmonics.
Aims: The aim of this work is to see the first harmonic cyclotron line, confirming the fundamental line at ~22 keV, thus increasing the number of sources with detected harmonic cyclotron lines.
Methods: To investigate the presence of absorption or emission lines in the spectra, we have combined RXTE and INTEGRAL spectra. We modeled the 3-100 keV continuum emission with a power law with an exponential cut off and look for the second absorption feature.
Results: We show evidence of an unknown cyclotron line at ~47 keV (the first harmonic) in the phase-averaged X-ray spectra of 4U 1538-52. This line is detected by several telescopes at different epochs, even though the S/N of each individual spectrum is low.
Conclusions: We conclude that the line-like absorption is a real feature, and the most straightforward interpretation is that it is the first harmonic, thus making 4U 1538-52 the fifth X-ray pulsar with more than one cyclotron line.