The first cyclotron harmonic of 4U 1538–52

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.Part of this work was supported by the Spanish Ministry of Education and Science Primera ciencia con el GTC: La astronomía española en vanguardia de la astronomía europea CSD200670 and Multiplicidad y evolución de estrellas masivas project number AYA200806166C0303. J.M.T. acknowledges the support by the Spanish Ministerio de Educación y Ciencia (MEC) under grant PR2007-0176. J.J.R.R. acknowledges the support by the Spanish Ministerio de Educación y Ciencia (MEC) under grant PR2009-0455