NuSTAR DISCOVERY OF A CYCLOTRON LINE IN THE BE/X-RAY BINARY RX J0520.5–6932 DURING OUTBURST

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 sub(CRSF) = 31.3 super(+0.8) sub(-0.7) keV and 31.5 super(+0.7) sub(-0.6)keV during the two observations, respectively, corresponding to a magnetic field of B approximately 2 x 10 super(12) G. The 3-79 keV luminosity of the system during the two epochs, assuming a nominal distance of 50 kpc, was 3.667 + or - 0.007 x 10 super(38) erg s super(-1) and 3.983 + or - 0.007 x 10 super(38) erg s super(-1). Both values are much higher than the critical luminosity of approximately 1.5 x 10 super(37)ergs super(-1), 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 10 super(38) erg s super(-1). A broad ([sigma] approximately 0.45 keV) Fe emission line is observed in the spectrum at a central energy of 6.58 super(+0.05) sub(-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.