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

  • Tendulkar, Shriharsh P.
  • Fürst, Felix
  • Pottschmidt, Katja
  • Bachetti, Matteo
  • Bhalerao, Varun B.
  • Boggs, Steven E.
  • Christensen, Finn E.
  • Craig, William W.
  • Hailey, Charles A.
  • Harrison, Fiona A.
  • Stern, Daniel
  • Tomsick, John A.
  • Walton, Dominic J.
  • Zhang, William
Publication Date
Sep 17, 2014
Submission Date
Sep 17, 2014
DOI: 10.1088/0004-637X/795/2/154
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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 $B \approx 2 \times10^{12}$ G. The 3-79 keV luminosity of the system during the two epochs assuming a nominal distance of 50 kpc was $3.667\pm0.007\times 10^{38}\,\mathrm{erg\,s^{-1}}$ and $3.983\pm0.007\times10^{38}\,\mathrm{erg\,s^{-1}}$. Both values are much higher than 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 new object to the sparse set of three systems that have a cyclotron line observed at 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 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 ($\Delta\phi\lesssim0.1$) phase difference between the peak of the cyclotron energy variation and the peak of the flux variation.

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