Delving into the depths of NGC 3783 with XRISM II. Cross-calibration of X-ray instruments used in the large, multi-mission observational campaign

TL;DR

This paper tackles cross-calibration uncertainties across XRISM, Chandra, NuSTAR, and XMM-Newton using a 2024 NGC 3783 campaign. It introduces a model-independent, spline-based approach that leverages XRISM/Resolve's high-resolution data to derive energy-dependent corrections for inter-instrument flux and spectral shape discrepancies. The authors quantify contributions from background, pileup, energy-scale, redistribution tails, and time-coverage, and derive practical energy-dependent correction factors to enable robust joint spectral analyses. The work provides interim calibration adjustments and a framework for future multi-instrument studies, underscoring the importance of careful cross-calibration in extracting reliable astrophysical results from broad-band X-ray spectroscopy.

Abstract

Accurate X-ray spectroscopic measurements are fundamental for deriving basic physical parameters of the most abundant baryon components in the Universe. The plethora of X-ray observatories currently operational enables a panchromatic view of the high-energy emission of celestial sources. However, uncertainties in the energy-dependent calibration of the instrument transfer functions (e.g. the effective area, energy redistribution, or gain) can limit - and historically, did limit - the accuracy of X-ray spectroscopic measurements. We revised the status of the cross-calibration among the scientific payload on board four operation missions: Chandra, NuSTAR, XMM-Newton, and the recently launched XRISM. XRISM carries the micro-calorimeter Resolve, which yields the best energy resolution at energies above 2 keV. For this purpose, we used the data from a 10-day-long observational campaign targeting the nearby active galactic nucleus NGC 3783, carried out in July 2024. We present a novel model-independent method for assessing the cross-calibration status that is based on a multi-node spline of the spectra with the highest-resolving power (XRISM/Resolve in our campaign). We also estimated the impact of the intrinsic variability of NGC 3783 on the cross-calibration status due to the different time coverages of participating observatories and performed an empirical reassessment of the Resolve throughput at low energies. Based on this analysis, we derived a set of energy-dependent correction factors of the observed responses, enabling a statistically robust analysis of the whole spectral dataset. They will be employed in subsequent papers describing the astrophysical results of the campaign.

Figures (12)

• Figure 1: Overview of the observations. Horizontal lines indicate the time intervals when the instruments were observing NGC 3783. 'COS' indicates the Cosmic Origin Spectrograph on board HST. XRISM contains the Xtend and Resolve instruments, XMM-Newton the pn, mos1, mos2, RGS1, and RGS2 instruments; for Chandra, the HETGS was the only operational instrument.
• Figure 2: Relative residuals (data/model-1) against a common spline model applied to RGS, Resolve, and NuSTAR data (details in the text). Differences between residuals mainly indicate differences of the effective area with respect to the effective area calibrations. The residuals have been binned for clarity. The black and grey residuals refer to RGS1/2 below 3 keV and to FPMA/B above 3 keV.
• Figure 3: Residuals of the Xtend spectrum relative to the Resolve model (black). The red line is a simple analytical representation of these residuals. The dotted purple lines indicate the 1$\sigma$ combined statistical uncertainty of the Resolve spectra of NGC 3783, binned to the energy grid of the Xtend data. They represent a systematic uncertainty of the spline model for Resolve.
• Figure 4: Simple analytical approximation (red) used to reduce excess Resolve flux compared to Xtend and other CCD instruments at the low-energy end of the Resolve band. This expression was derived by approximating the transmission of $10^{24}$ atoms m$^{-2}$ (black).
• Figure 5: Residuals of the FPMA spectrum relative to the Resolve model (black). The red line is a simple analytical representation of these residuals.