NuSTAR's Intentional Stray Light Observation of Scorpious 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 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.

Figures (4)

• Figure 1: Detector images for NuSTAR FPMA (top) and FPMB (bottom) SL observations of Sco X-1. A circular source region (blue) was used for data extraction on both FPMs. An exclusion box region (red) was used to remove NuSTAR's shadow on FPMA. Region files are available on the StrayCats website.
• Figure 2: Top row shows the light curves of the NuSTAR focused (panels a, b, & d) and SL (panel c) observations of Sco X-1 in chronological order. The color bar denotes the elapsed time since the start of each respective observation. Note that the SL observation in 2023 occurred directly prior to the focused observation. The bottom row shows the hardness-intensity diagrams of the (panel e) focused and (panel f) SL data. Only one FPM from each observation is shown for clarity.
• Figure 3: SL spectrum for FPMA with different background components. The data are shown in black. The dash-dot line is the aCXB, dashed line is the fCXB, dotted line is the internal line emission, and the dot-dot-dashed line is the internal continuum. The overall background contribution is shown as the solid line.
• Figure 4: Spectra and ratio of overall model to data for (a) Model A, (b) Model B, (c) Model C, and (d) Model D. In all panels, the spectrum with different components are plotted in the top sub-panel. FPMA is plotted in dark grey and FPMB is plotted in mauve. The total model is plotted in solid line, the total background model is plotted in dotted line, the bbody component is plotted in dashed line, the diskbb component is plotted in dash-dash-dot line, the convoled component thcomp is plotted in dash-dot-dot line, and the relxillNS component is plotted in dash-dot line. Data were visually rebinned in the ratio panels for clarity.