2018ApJ...861...89A

PSR J2129-0429 is a 7.62 ms eclipsing millisecond pulsar (MSP) with a non-degenerate binary companion star that is likely in an early stage of the recycling process. It has one of the largest companion masses of a so-called "redback" (0.4 M_☉) and has an unusually high surface magnetic field (1.6 x 10⁹ G) for an MSP. We present here an X-ray and optical study of PSR J2129-0429 using new NuSTAR and LCOGT data of the system in addition to archival Swift and XMM-Newton data. Its X-ray light curve shows strong orbital variation from the intra-binary shock, about five times greater than is typical for other systems, and is clearly and persistently double-peaked. Its X-ray spectrum has a very hard power-law component (Γ = 1.1-1.2) which extends to ∼40 keV and exhibits an efficiency of up to a few percent in the X-ray band. The X-ray spectrum at the second peak of the light curve exhibits strong variability, while the spectrum of the first peak remains constant across observations. The hardness of the spectrum is suggestive of driven magnetic reconnection occurring at the shock. In addition, we observe the companion to be currently optically brightening after a multi-year period where it was dimming. The changes in color suggest that it has been continually cooling, implying that the companion is currently expanding.