On joint analysing <i>XMM-NuSTAR</i> spectra of active galactic nuclei

Abstract

A recently released <i>XMM-Newton</i> note revealed a significant calibration issue between nuclear spectroscopic telescope array (<i>NuSTAR)</i> and <i>XMM-Newton</i> European Photon Imaging Camera (EPIC) and provided an empirical correction to the EPIC effective area. To quantify the bias caused by the calibration issue in the joint analysis of <i>XMM-NuSTAR</i> spectra and verify the effectiveness of the correction, in this work, we perform joint-fitting of the <i>NuSTAR</i> and EPIC-pn spectra for a large sample of 104 observation pairs of 44 X-ray bright active galactic nuclei (AGN). The spectra were extracted after requiring perfect simultaneity between the <i>XMM-Newton</i> and <i>NuSTAR</i> exposures (merging Good Time Intervals, GTIs from two missions) to avoid bias due to the rapid spectral variability of the AGN. Before the correction, the EPIC-pn spectra are systematically harder than the corresponding <i>NuSTAR</i> spectra by <inline-formula><tex-math id="M1">\begin{document}$\Delta \varGamma \sim 0.1 $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="JUSTC-2023-0160_M1.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="JUSTC-2023-0160_M1.png"/></alternatives></inline-formula>, subsequently yielding significantly underestimated cutoff energy <i>E</i><sub>cut</sub> and the strength of reflection component <i>R</i> when performing joint-fitting. We confirm that the correction is highly effective and can commendably erase the discrepancy in best-fit <i>Γ</i>, <i>E</i><sub>cut</sub>, and <i>R</i>. We thus urge the community to apply the correction when joint-fitting <i>XMM</i>-<i>NuSTAR</i> spectra, but note that the correction is limited to 3–12 keV and therefore not applicable when the soft X-ray band data are included. Besides, we show that as merging GTIs from two missions would cause severe loss of <i>NuSTAR</i> net exposure time, in many cases, joint-fitting yields no advantage compared with utilizing <i>NuSTAR</i> data alone. We finally present a technical note on filtering periods of high background flares for <i>XMM-Newton</i> EPIC-pn exposures in the Small Window mode.