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

ABSTRACT We present results from intensive ($\times 3$ daily) 3-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 light curves are highly correlated; the X-ray-to-UV correlation $r_\mathrm{max}=0.85$ is 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 four times brighter. After filtering out long-term trends in the earlier optical light curves 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 ($\approx 2$ d) suggest that the BLR is the dominant reprocessor. The hard X-ray spectrum ($\Gamma \approx 1.7$) and faint red UV–optical spectrum indicate that the Eddington accretion ratio is low: $\dot{m}_\mathrm{E}\sim 0.03$. The bolometric luminosity then requires that the black hole mass is substantially greater than current reverberation mapping estimates.