The first cyclotron harmonic of 4U 1538-52

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. The aim of this work is to see the first harmonic cyclotron line, confirming the fundamental line at around 22 keV, thus increasing the number of sources with detected harmonic cyclotron lines. 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. We show evidence of an unknown cyclotron line at around 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 signal-to-noise ratio of each individual spectrum is low. 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.
Comment: 6 pages, 5 figures, accepted for publication in Astronomy and Astrophysics