NuSTAR Discovery of a Cyclotron Line in the Be/X-Ray Binary RX J0520.5–6932 during Outburst

Abstract

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) = 31.3_(-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 ≈ 2 × 10^(12) 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 × 10^(38) erg s^(–1) and 3.983 ± 0.007 × 10^(38) erg s^(–1). Both values are much higher than the critical luminosity of ≈1.5 × 10^(37) 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 10^(38) 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.