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

TL;DR

NuSTAR observations reveal that the X-ray pulsar 2RXP J130159.6-635806 has experienced a consistent long-term spin-up over 20 years, with spectral features typical of accreting pulsars and a high, stable pulsed fraction.

Contribution

This study provides the first detailed spectral and timing analysis of 2RXP J130159.6-635806, highlighting its unusual steady long-term spin-up behavior.

Findings

The pulsar has undergone a steady spin-up for the past 20 years.

The spectrum shows a typical accreting pulsar shape with a possible iron line.

The pulsed fraction remains high (~80%) and constant up to 40 keV.

Abstract

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 E_fold=~18 keV. There is also an indication of the presence of a 6.4 keV iron line in the spectrum at the ~3 sigma 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.