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⁺⁰.⁸₋₀.₇ 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.