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 January 2014. The target was observed on two epochs and we report the detection of a cyclotron 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 two observations, respectively, corresponding to a magnetic field of $B\approx 2\times 10^{12}$ G. The 3-79 keV luminosity of the system during the two epochs assuming a nominal distance of 50 kpc was $3.667\pm 0.007\times 10^{38}\mathrm{erg}{\mathrm{s}}^{-1}$ and $3.983\pm 0.007\times 10^{38}\mathrm{erg}{\mathrm{s}}^{-1}$. Both values are much higher than the critical luminosity of $\approx 1.5\times 10^{37}\mathrm{erg}{\mathrm{s}}^{-1}$ 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 $10^{38}\mathrm{erg}{\mathrm{s}}^{-1}$. A broad ($\ddot{I}ƒ\approx 0.45$ 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.% As in observations of other cyclotron absorption line sources, there is a small ($\hat{I}”\ddot{I}•\lesssim 0.1$) phase difference between the peak of the cyclotron energy variation and the peak of the flux variation.

9 pages, 4 figures, 6 tables. Accepted to ApJ