Inconsistent conclusions are obtained from recent active galactic nuclei (AGNs) accretion disk inter-band time-lag measurements. While some works show that the measured time lags are significantly larger (by a factor of ∼3) than the theoretical predictions of the Shakura & Sunyaev disk (SSD) model, others find that the time-lag measurements are consistent with (or only slightly larger than) that of the SSD model. These conflicting observational results might be symptoms of our poor understanding of AGN accretion physics. Here we show that sources with larger-than-expected time lags tend to be less luminous AGNs. Such a dependence is unexpected if the inter-band time lags are attributed to the light-travel-time delay of the illuminating variable X-ray photons to the static SSD. If, instead, the measured inter-band lags are related not only to the static SSD but also to the outer broad emission-line regions (BLRs; e.g., the blended broad emission lines and/or diffuse continua), our result indicates that the contribution of the non-disk BLR to the observed ultraviolet/optical continuum decreases with increasing luminosity (L), i.e., an anti-correlation resembling the well-known Baldwin effect. Alternatively, we argue that the observed dependence might be a result of coherent disk thermal fluctuations as the relevant thermal timescale, τ TH ∝ L 0.5. With future accurate measurements of inter-band time lags, the above two scenarios can be distinguished by inspecting the dependence of inter-band time lags upon either the BLR components in the variable spectra or the timescales.

We present our observations of the J = 1 − 0 rotation transitions in molecular isotopes C 18 O and C 17 O toward a sample of molecular clouds with different galactocentric distances, using the Delingha 13.7 m (DLH 13.7 m) telescope, administered by Purple Mountain Observatory, and its 9-beam SIS receiver. Complementary observations toward several sources with large galactocentric distance are obtained with the IRAM 30m and Mopra 22m telescopes. C 18 O/C 17 O abundance ratios reflecting the 18 O/ 17 O isotope ratios are obtained from integrated intensity ratios of C 18 O and C 17 O. We derived the ratio value for 13 sources covering a galactocentric distance range of 3 kpc to 16 kpc. In combination with our mapping results that provide a ratio value of 3.01±0.14 in the Galactic center region, it shows that the abundance ratio tends to increase with galactocentric distance, i.e., it supports a radial gradient along the Galactic disk for the abundance ratio. This is consistent with the inside-out formation scenario of our Galaxy. However, our results may suffer from small samples with large galactocentric distance. Combining our data with multitransition lines of C 18 O and C 17 O will be helpful for constraining opacities and abundances and further confirming the Galactic radial gradient shown by the isotope ratio 18 O/ 17 O.