Intensive X-ray/UVOIR continuum reverberation mapping of the Seyfert AGN MCG+08-11-11

TL;DR

This study presents intensive, multiwavelength reverberation mapping of the Seyfert galaxy MCG+08-11-11, combining three months of high-cadence Swift X-ray/UV data with dense ground-based optical/IR monitoring and archival NuSTAR/XMM data. The authors derive a robust lag spectrum showing X-ray leading UV by about 1.7–1.8 days and lags that increase with wavelength, but with pronounced excess lags in the u and i bands that strongly point to reprocessing in the broad line region (BLR) rather than in a pure accretion disc. Detailed SED modelling reveals tensions between a canonical disc interpretation and the observed X-ray/UV luminosities, suggesting either substantial intrinsic reddening at the RM mass or a higher black hole mass with a lower Eddington ratio, with the latter aligning with BLR-dominated reprocessing. Physical lag modelling indicates disc-only scenarios cannot reproduce the full lag spectrum, whereas a radiation-pressure-confined BLR cloud model provides a natural explanation for the observed BLR-related features, implying the BLR is the dominant reprocessor and shaping the observed delays. The findings imply a complex, multi-component reprocessing geometry and have implications for BH mass estimates and accretion states in low-to-moderate luminosity AGN, challenging simplified disc RM assumptions.

Abstract

We present results from intensive (x3 daily), three-month-long X-ray, UV and optical monitoring of the bright Seyfert active galactic nucleus (AGN) MCG+08-11-11 with Swift, supported by optical-infrared ground-based monitoring. The 12 resultant, well-sampled, lightcurves are highly correlated; in particular, the X-ray to UV correlation r_max = 0.85 is, as far as we know, the highest yet recorded in a Seyfert galaxy. The lags increase with wavelength, as expected from reprocessing of central high-energy emission by surrounding material. Our lag spectrum is much shallower than that obtained from an optical monitoring campaign conducted a year earlier when MCG+08-11-11 was approximately 4 times brighter. After filtering out long-term trends in the earlier optical lightcurves we recover shorter lags consistent with our own - demonstrating concurrent reverberation signals from different spatial scales and the luminosity dependence of the measured lags. We use our lag spectrum to test several physical models, finding that disc reprocessing models cannot account for the observed 'excess' lags in the u and r-i-bands that are highly indicative of the Balmer and Paschen continua produced by reprocessing in the broad line region (BLR) gas. The structure seen in both the variable (rms) and lag spectra, and the large time delay between X-ray and UV variations (approximately 2 days) all suggest that the BLR is the dominant reprocessor. The hard X-ray spectrum (Gamma approximately 1.7) and faint, red, UV-optical spectrum both indicate that the Eddington accretion ratio is low: approximately 0.03. The bolometric luminosity then requires that the black hole mass is substantially greater than current reverberation mapping derived estimates.

Figures (13)

• Figure 1: Swift XRT (top panel) and UVOT (lower panels) lightcurves from the three-month period between 2021 February 5 and May 7.
• Figure 2: The combined Zowada and LCO lightcurves from the eight months between 2020 September and 2021 May. The beginning and end of the intensive three-month Swift monitoring period are indicated by vertical grey dashed lines. The Swift UVOT U-band lightcurve is shown in the top panel (offset by 1 mJy) for comparison.
• Figure 3: Flux-flux analysis of the Swift UVOT, LCO and Zowada data, used to separate the variable and constant components of the lightcurves. The left-hand panel shows the observed flux densities against a dimensionless lightcurve shape X($t$), which is normalised to mean 0 and rms 1. A linear regression extrapolates the flux-flux relations back to X$_\mathrm{gal}$, at which point the variable component is zero and the values of $F_\nu(\lambda,t)$ give the spectrum of the constant, host galaxy, component. The right-hand panel shows the galaxy component as a the deep red line and open diamonds. (The thin, purple line shows the template spectrum of a barred spiral 'Sb' galaxy, reddened for comparison with the other observed spectra.) The right-hand panel also shows the average SED, $A_\nu(\lambda)$, i.e. $F_\nu(\lambda,t)$ where $\mathrm{X}(t)=0$, as a blue line with open circles. The grey envelope, evaluating the linear model at X$_{\rm max}$ and X$_{\rm min}$, shows the AGN flux range seen during the campaign. The difference between the average SED, $A_\nu(\lambda)$, and the galaxy component represents the average spectrum of the observed variable AGN component, shown as a solid pink line and pink diamonds. The dashed pink line shows the average AGN component corrected for Galactic reddening ($A_\mathrm{V}=0.568$ mag). The rms of the variable AGN component, $S_\nu(\lambda)$, is shown as an orange line with open squares. The dotted grey line shows a canonical optical accretion disc spectrum with $F_\nu\propto\nu^{1/3}$ scaled to match the dereddened AGN spectrum in the IR.
• Figure 4: Results from the time series analyses of the MCG $+$08$-$11$-$11 intensive continuum reverberation mapping campaign. All measurements are relative to the Swift UVW2 band (2084 Å). The left-hand panels show the interband interpolated cross-correlation functions (ICCFs), with their maxima $r_\mathrm{max}$ given in the inset text. The corresponding right-hand panels contain the cross-correlation centroid distributions (CCCDs), shown as solid histograms. Also shown are the javelin-derived lag distributions (open histograms). Filled and open triangles indicate the centroid lag $\tau$ for the ICCF and javelin methods, respectively; the inset text quotes each lag and its uncertainty.
• Figure 5: Swift X-ray and Wise Observatory narrow band optical lightcurves of MCG $+$08$-$11$-$11 in the period 2019 October to 2020 April. Top: The Swift XRT lightcurve (left) and hardness ratio (right; using 0.3--1.5 and 1.5--10 keV bands) are shown for comparison. Left: The original lightcurves presented by Fian23 in which long-term trend (shown in red) has been estimated using the LOWESS method with a 45 day smoothing window. (The optical fluxes have been normalised to have a mean of 0 and standard deviation of 1.) Centre: The filtered lightcurves, from which the long-term trend has been subtracted. Right: The interpolated cross-correlation function (ICCF) and lag distribution for each filtered lightcurve relative to the 4250 Å band. Solid and open histograms show the ICCF centroid javelin lag distributions, respectively.