NuSTAR spectral analysis beyond 79 keV with stray light

TL;DR

This paper demonstrates the first spectral analysis of NuSTAR data beyond 79 keV using stray light observations, confirming flux calibration with INTEGRAL and addressing background treatment at high energies.

Contribution

It introduces a method for analyzing NuSTAR stray light data beyond 79 keV and validates flux calibration with simultaneous INTEGRAL observations.

Findings

NuSTAR stray light spectra are consistent with INTEGRAL flux within 90% confidence.

The paper provides a background treatment method for high-energy stray light spectral analysis.

First scientific spectral analysis of NuSTAR beyond 79 keV using stray light observations.

Abstract

Due to the structure of the NuSTAR telescope, photons at large off-axis (> 1deg) 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 above the Pt k-edge (79 keV) of the multi-layers, which limits the effective area bandpass of the optics. We present the first scientific spectral analysis beyond 79 keV using a Cygnus X-1 observation in StrayCats, the catalog of stray light observations. This serendipitous stray light observation occurred simultaneously with an INTEGRAL observation. 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. Furthermore, we explain how to treat the background of the stray light spectral analysis, which is especially important at high energies.