Investigation on the X-ray emission of NGC 4051 during its 2009 optical/UV-X-ray dissociation phase

Abstract

This study investigates the X-ray characteristics of jet-associated radio-quiet AGNs across distinct optical/UV to X-ray correlation phases. Quasi-simultaneous optical/UV/X-ray observations of NGC 4051 from May-June 2009, obtained through Swift and XMM-Newton, reveal a temporal dichotomy: a strong optical/UV to X-ray correlation dominates the initial observation phase (before May 27), followed by an optical/UV flare event concurrent with X-ray flux suppression in the latter period. Our multi-method analysis of XMM-Newton data, incorporating short-term X-ray variability assessment, spectral decomposition, and RGS spectral analysis, identifies significant inter-phase X-ray emission disparities. During optical/UV flaring episodes, compared to the correlated phase, we observe: attenuated short-term X-ray variability amplitudes, enhanced soft X-ray absorption, suppressed intrinsic hard X-ray flux, and more prominent RGS emission-line features. Notably, these X-ray characteristics during optical/UV flaring intervals show no statistically significant deviations from pre-flare low-state X-ray emission patterns. These non-synchronous optical/UV-X-ray variations contradict predictions from both reprocessing models, starburst-driven emission scenarios, and the simplistic absorption models. While potential jet-related mechanisms remain ambiguous, our findings demonstrate strong consistency with predictions from the inhomogeneous accretion disk perturbation framework.

Figures (6)

• Figure 1: The light curves of NGC 4051. (Top panel) Long-term optical/UV and $0.3-10\,\rm keV$ X-ray light curves of Swift and XMM-Newton data observed between May and June 2009. (Middle and bottom panels) Short-term soft and hard X-ray light curves of each XMM-Newton observation. The red-, gray-shaded, and white regions correspond to different optical/UV-X-ray correlation periods.
• Figure 2: The hardness ratio versus X-ray luminosity correlation in NGC 4051's short-term hard X-ray light curves. Red and blue data points represent X-ray observations before and after May 27, 2009, respectively. Larger solid circular markers indicate median hardness ratios within each count-rate bin. The top and right 2D histograms show the distribution of hard X-ray photon count rate and hardness ratio, respectively.
• Figure 3: The example of the XMM-Newton hard X-ray spectrum fitting of NGC 4051.
• Figure 4: Best model ($phabs*zxipcf*(zedge*bbody+zgauss+relxill+zgauss)$) fitting for 15 XMM-Newton pn spectra of NGC 4051.
• Figure 5: The spectral fitting components flux variation of NGC 4051.