2014ApJ...795..154T

We present spectral and timing analysis of NuSTAR observations of RX J0520.5-6932 in the 3-79 keV band collected during its outburst in 2014 January. The target was observed on two epochs and we report the detection of a cyclotron resonant scattering feature with central energies of E_CRSF = 3.13_–0.7^+0.8 keV and 31.5_-0.6^+0.7 keV during the two observations, respectively, corresponding to a magnetic field of B ~ 2x10¹² G. The 3-79 keV luminosity of the system during the two epochs, assuming a nominal distance of 50 kpc, was 3.667±0.007x10³⁸ erg/s and 3.983±0.007x10³⁸ erg/s. Both values are much higher than the critical luminosity of ~1.5x10³⁷ erg/s, above which a radiation-dominated shock front may be expected. This adds a new object to the sparse set of three systems that have a cyclotron line observed at luminosities in excess of 10³⁸ erg/s. A broad (σ ~ 0.45 keV) Fe emission line is observed in the spectrum at a central energy of 6.58_–0.05^+0.05 keV in both epochs. The pulse profile of the pulsar was observed to be highly asymmetric with a sharply rising and slowly falling profile of the primary peak. We also observed minor variations in the cyclotron line energy and width as a function of the rotation phase.