Extending the Baseline for SMC X-1's Spin and Orbital Behavior with NuSTAR Stray Light

Abstract

StrayCats, the catalog of NuSTAR stray light observations, contains data from bright X-ray sources that fall within crowded source regions. These observations offer unique additional data with which to monitor sources such as X-ray binaries that show variable timing behavior. In this work, we present a timing analysis of stray light data of the high-mass X-ray binary SMC X-1, the first scientific analysis of a single source from the StrayCats project. We describe the process of screening stray light data for scientific analysis, verify the orbital ephemeris, and create both time- and energy-resolved pulse profiles. We find that the orbital ephemeris of SMC X-1 is unchanged and confirm a long-term spin-up rate of ν˙=(2.52±0.03)×10−11 Hz s-1. We also note that the shape of SMC X-1's pulse profile, while remaining double peaked, varies significantly with time and only slightly with energy.