Extending the baseline for SMC X-1's spin and orbital behavior with NuSTAR stray light

TL;DR

This study utilizes NuSTAR stray light observations to analyze the spin and orbital behavior of SMC X-1, confirming its long-term spin-up and stable orbital ephemeris while revealing pulse profile variations.

Contribution

First scientific analysis of a single source from the StrayCats stray light catalog, demonstrating its utility for monitoring X-ray binary timing behavior.

Findings

Confirmed unchanged orbital ephemeris of SMC X-1.

Measured a long-term spin-up rate of 2.52e-11 Hz/s.

Observed pulse profile shape varies with time, slightly with energy.

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 like 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 $\dot{\nu}=(2.52\pm0.03)\times10^{-11}$ 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.