NuSTAR’s Intentional Stray Light Observation of Scorpius X-1

Abstract We present the first spectral analysis of Scorpius X-1 (Sco X-1) using intentional stray light (SL) observations taken by NuSTAR. Unlike focused observations that have high telemetry load when observing bright sources, intentional SL observations can help reduce the telemetry and reduce the effect of dead time, thereby maximizing the on-source exposure time, all of which are critical for extremely bright sources that exhibit short timescale variability like Sco X-1. The intentional SL observation of Sco X-1, taken in 2023, captured the source primarily in the flaring branch (FB) of the Z track. We performed spectral modeling of the continuum and reprocessed emission. A combination of thermal and Comptonization components (modeled with thcomp ) provided a robust fit to the continuum. We test both scenarios for Comptonized emitting regions arising from the accretion disk and close to the neutron star, which provides comparable fit statistics. Reflection was modeled with the relxillNS model, enabling measurements of disk inclination consistent with prior radio and IXPE studies and comparison of the inner disk radius to the emission radii of the thermal components. Overall, the results from the intentional SL data provide comparable results to literature on the focused NuSTAR data of Sco X-1 in the FB or taken contemporaneously. The success of this observation demonstrates the capability of SL data to yield high-quality spectral constraints comparable to focused observations, offering a powerful avenue for studying bright X-ray binaries with NuSTAR.