NuSTAR discovery of an unusually steady long-term spin-up of the Be binary 2RXP J130159.6-635806

Roman A. Krivonos, Sergey S. Tsygankov, Alexander A. Lutovinov, John A. Tomsick, Deepto Chakrabarty, Matteo Bachetti, Steven E. Boggs, Masha Chernyakova, Finn Erland Christensen, William W. Craig, Felix Fürst, Charles J. Hailey, Fiona A. Harrison, George B. Lansbury, Farid Rahoui, Daniel Stern, William W. Zhang

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We present spectral and timing analysis of NuSTAR observations of the accreting X-ray pulsar 2RXP J130159.6-635806. The source was serendipitously observed during a campaign focused on the gamma-ray binary PSR B1259-63 and was later targeted for a dedicated observation. The spectrum has a typical shape for accreting X-ray pulsars, consisting of a simple power law with an exponential cutoff starting at ~7 keV with a folding energy of Efold=~18 keV. There is also an indication of the presence of a 6.4 keV iron line in the spectrum at the ~3σ significance level. NuSTAR measurements of the pulsation period reveal that the pulsar has undergone a strong and steady spin-up for the last 20 years. The pulsed fraction is estimated to be ~80%, and is constant with energy up to 40 keV. The power density spectrum shows a break towards higher frequencies relative to the current spin period. This, together with steady persistent luminosity, points to a long-term mass accretion rate high enough to bring the pulsar out of spin equilibrium.
Original languageEnglish
Article number140
JournalAstrophysical Journal
Number of pages8
Publication statusPublished - 2015


  • Pulsars: individual (2RXP J130159.6-635806)
  • Stars: emission-line, Be
  • X-rays: binaries

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