Tendulkar, Shriharsh P.Fürst, FelixPottschmidt, KatjaBachetti, MatteoBhalerao, Varun B.Boggs, Steven E.Christensen, Finn E.Craig, William W.Hailey, Charles A.Harrison, Fiona A.Stern, DanielTomsick, John A.Walton, Dominic J.Zhang, William2023-09-062023-09-062014-10-13Tendulkar, Shriharsh P., Felix Fürst, Katja Pottschmidt, Matteo Bachetti, Varun B. Bhalerao, Steven E. Boggs, Finn E. Christensen, et al. “NuSTAR Discovery of a Cyclotron Line in the Be/X-Ray Binary RX J0520.5-6932 during Outburst.” The Astrophysical Journal 795, no. 2 (October 2014): 154. https://doi.org/10.1088/0004-637X/795/2/154.https://doi.org/10.1088/0004-637X/795/2/154http://hdl.handle.net/11603/29576We 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 = 31.3⁺⁰.⁸₋₀.₇ keV and 31.5⁺⁰.⁷₋₀.₆ keV during the two observations, respectively, corresponding to a magnetic field of B ≈ 2 × 10¹²  G. The 3–79 keV luminosity of the system during the two epochs, assuming a nominal distance of 50 kpc, was 3.667 ± 0.007 × 1038 erg s−1 and 3.983 ± 0.007 × 10³⁸ erg s⁻¹. Both values are much higher than the critical luminosity of ≈1.5 × 10³⁷ erg s⁻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 10³⁸ erg s⁻¹. 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.9 pagesen-USThis work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.Public Domain Mark 1.0http://creativecommons.org/publicdomain/mark/1.0/Text