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We present spectral and timing analysis of NuSTAR observations of RXJ0520.5$-$6932 in the 3-79 keV band collected during its outburst in January2014. The target was observed on two epochs and we report the detection of acyclotron resonant scattering feature with central energies of $E_\mathrm{CRSF}= 31.3_{-0.7}^{+0.8}$ keV and $31.5_{-0.6}^{+0.7}$ keV during the twoobservations, respectively, corresponding to a magnetic field of $B \approx 2\times10^{12}$ G. The 3-79 keV luminosity of the system during the two epochsassuming a nominal distance of 50 kpc was $3.667\pm0.007\times10^{38}\,\mathrm{erg\,s^{-1}}$ and$3.983\pm0.007\times10^{38}\,\mathrm{erg\,s^{-1}}$. Both values are much higherthan the critical luminosity of $\approx1.5\times10^{37}\,\mathrm{erg\,s^{-1}}$above which a radiation dominated shock front may be expected. This adds a newobject to the sparse set of three systems that have a cyclotron line observedat luminosities in excess of $10^{38}\,\mathrm{erg\,s^{-1}}$. A broad($\sigma\approx0.45$ keV) Fe emission line is observed in the spectrum at acentral energy of $6.58_{-0.05}^{+0.05}$ keV in both epochs. The pulse profileof the pulsar was observed to be highly asymmetric with a sharply rising andslowly falling profile of the primary peak. We also observed minor variationsin the cyclotron line energy and width as a function of the rotation phase.% Asin observations of other cyclotron absorption line sources, there is a small($\Delta\phi\lesssim0.1$) phase difference between the peak of the cyclotronenergy variation and the peak of the flux variation.

NuSTAR Discovery of a Cyclotron Line in the Be/X-ray Binary RX J0520.5-6932 During Outburst