Presentation #106.41 in the session “AGN (Poster)”.
The cosmic X-ray background (CXB), in energies above 3 keV, is primarily attributed to the integrated emission of actively accreting super massive black holes (SMBHs) found in the centers of most galaxies. The various populations of these active galactic nuclei (AGN), which are differentiated by their levels of local obscuration, are highly sensitive to the CXB peak position, currently thought to be between ~10–30 keV. While historically all measurements in the energy range of ~0.1–100 keV can agree on the overall shape of the spectrum, normalization values and this peak position have varied up to ~10%. NuSTAR has the unique design that leaves an open pathway between the optics and detector benches that allows for stray light to fall on the detector planes creating a unique spatial gradient that is distinguishable from all other sources that comprise the background signal in energies below 20 keV. Since this light avoids the optics all together, systematic errors associated with any kind of mirror calibration can be avoided, while fitting this light spatially allows us to separate this stray light due to the CXB from its focused counterpart. Using archival data spanning 6 years of extragalactic observations, we report a measurement of the CXB based on ~8 Ms of observing time which covers an estimated ~20% of the sky while excluding any observation near the galactic ridge (|b| > 30°). At 10 keV the spectrum measured by NuSTAR is somewhat steeper than previously reported values, which can explain some the discrepancies between lower and higher energies of past measurements around this particular value. We’ll present an absolute flux measurement in the 3-30 keV band and discuss the implications for AGN population synthesis models in light of this measurement.