NuSTAR Spectral Analysis beyond 79 keV with Stray Light
Tomsick, J. A.; Wilms, J.; Buisson, D. J. K.; Grinberg, V.; Grefenstette, B. W.; Madsen, K. K.; García, J. A.; Ludlam, R. M.; Thalhammer, P.; Miyasaka, H.; Mastroserio, G.; Connors, R. M. T.; Brumback, M. C.
United States, Germany, Netherlands
Abstract
Due to the structure of the NuSTAR telescope, photons at a 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 above the Pt K edge (79 keV) of the multilayers, 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.