tokens: spectrum (Figs. 2 and 3 of Ho et al. 2003 ), while atomic hydrogen transitions may produce additional features. That may explain some of the observed multiple lines. In view of the limited significance of the line detections it appears difficult to make specific assignments. Another way to explain multiple lines is to invoke – beyond the radiation from the hot cap at the magnetic dipole – separate hot spots on the stellar surface having higher multipolar magnetic fields. The first strong evidence for this comes from the discovery of strongly phase-variable absorption lines which are visible only for about 20% of the neutron star rotation. The lines were detected in the X-ray spectra of RX J0720.4−3125 and RX J1308.6+2127 ( Borghese et al. 2015 , 2017 , respectively) and have energies a factor of ∼2.5 higher than the broad lines (Table 3 ). Such multipole fields must be quite young compared to the age of the NS. They may be formed by extracting magnetic energy from the toroidal field that resides in deep crustal layers, via Hall drift (e.g., Geppert Viganò 2014 ). We conclude that proton cyclotron lines produced in strong magnetic fields of isolated neutron stars can explain the absorption-type features observed in the soft X-ray spectra of young isolated neutron stars of the M7 class. More accurate measurements in the future in combination with further theoretical work on magnetized atmospheres and the magneto-thermal evolution of neutron stars are required to make further progress in this exciting field. 9. Summary With this contribution we attempt to provide a review on the current state of our knowledge about X-ray sources that show cyclotron line features in their spectra – both for electron cyclotron lines (this is by far the larger part) and for proton cyclotron lines. We concentrate on the observational aspects, providing a number of Tables that contain detailed informations on all sources that we consider reasonably well secured regarding the existence of cyclotron lines. In the case of electron cyclotron lines – observed from binary X-ray pulsars, highly magnetized accreting neutron stars in binaries – we also give a Table with candidate sources, mostly objects for which a detection of a cyclotron line was claimed some time ago, but had not been confirmed in subsequent observations, or sources that were observed rather recently, but only once and with somewhat peculiar properties. On the other hand: for some of the objects listed in the main Tables, the existence of a cyclotron line still needs confirmation (or not) through re-measurements (with a finite probability that they have to be moved into the candidates category). We try to address the basic physics underlying the cyclotron line phenomenon, including a large number of references to theoretical work, with some coverage of specific theoretical modeling. Since the discovery of the first CRSF in Her X-1 in 1976, a new field of Cyclotron Line Research has evolved. With the rather
ner_tags: O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O B-CelestialObject I-CelestialObject O B-CelestialObject I-CelestialObject O B-Citation I-Citation I-Citation I-Citation O B-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model I-Model O B-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O