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To 79 keV and Beyond: NuSTAR stray light analysis for bright X-ray sources

Presentation #103.77 in the session Missions and Instruments.

Published onJul 01, 2023
To 79 keV and Beyond: NuSTAR stray light analysis for bright X-ray sources

NuSTAR is the first focusing hard X-ray telescope in space. The X-ray optics have a precipitous drop in their response above the Pt K-Edge at 79 keV, effectively limiting the bandpass of the observatory even for bright X-ray sources. However, due to the structure of the NuSTAR telescope, photons at large off-axis (> 1◦) can reach the detectors directly (stray light), without passing through the instrument optics. At these off-axis angles NuSTAR essentially turns into a collimated instrument and the spectrum can extend to energies of up to 150 keV. These observations occur both serendipitously and intentionally as a method to reduce the telemetry load on the observatory when observing bright targets. Here, we present the results of a pilot study performing spectral analysis beyond 79 keV using a Cygnus X-1 stray light observation which occurred simultaneously with INTEGRAL. When the spectra are modeled together in the 30-120 keV energy band, we find that the NuSTAR stray light flux is well calibrated and constrained to be consistent with the INTEGRAL flux at the 90% confidence level. We will discuss best practices for dealing with stray-light backgrounds and discuss the tools for analyzing other stray light observations in the NuSTAR archive.

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