Cyclotron Lines with NuSTAR -- First Results

Cyclotron Resonant Scattering Features (CRSF or cyclotron lines) are the only direct way to measure magnetic field strengths close to the surface of accreting neutron stars. Fundamental lines are typically observed between 10-60 keV, corresponding to magnetic fields of the order of 1e12 G, harmonics can appear at multiples of the fundamental energy. NuSTAR covers this energy range with unprecedented spectral resolution and sensitivity, making it the ideal instrument to study cyclotron lines. We present early results for two sources, Vela X-1 and Her X-1, which have lines around 57 keV (first harmonic) and 37 keV (fundamental), respectively. With NuSTAR we study the change of line shape, width, and energy as a function of luminosity and time, while at the same time obtaining a very well constrained measurement of the underlying continuum. These measurements feed back into our theoretical understanding of the formation of these lines in the accretion column of the neutron star.