NuSTAR Discovery of an Unusually Steady Long-term Spin-up of the Be Binary 2RXP J130159.6-635806
Hailey, Charles J.; Chakrabarty, Deepto; Fürst, Felix; Stern, Daniel; Tsygankov, Sergey S.; Lutovinov, Alexander A.; Rahoui, Farid; Boggs, Steven E.; Harrison, Fiona A.; Christensen, Finn E.; Craig, William W.; Zhang, William W.; Tomsick, John A.; Bachetti, Matteo; Krivonos, Roman A.; Chernyakova, Masha; Lansbury, George B.
United States, Russia, Finland, France, Ireland, Denmark, United Kingdom, Germany
Abstract
We present spectral and timing analyses of Nuclear Spectroscopic Telescope Array (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 {E}{fold}≃ 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 toward 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.