This paper reports results of the third-year campaign of monitoring super-Eddington accreting massive black holes (SEAMBHs) in active galactic nuclei (AGNs) between 2014 and 2015. Ten new targets were selected from the quasar sample of the Sloan Digital Sky Survey (SDSS), which have generally been more luminous than the SEAMBH candidates in the last two years. Hβ lags (t b H ) in five of the 10 quasars have been successfully measured in this monitoring season. We find that the lags are generally shorter, by large factors, than those of objects with same optical luminosity, in light of the well-known R Hβ -L 5100 relation. The five quasars have dimensionless accretion rates of Ṁ = 10-10 3 . Combining these with measurements of the previous SEAMBHs, we find that the reduction of Hβ lags depends tightly on accretion rates,, where t -R L is the Hβ lag from the normal R Hβ -L 5100 relation. Fitting 63 mapped AGNs, we present a new scaling relation for the broad-line region: . Implications of this new relation are briefly discussed.
NGC 5548 is the best-observed reverberation-mapped active galactic nucleus with long-term, intensive monitoring. Here we report results from a new observational campaign between January and July, 2015. We measure the centroid time lag of the broad Hβ emission line with respect to the 5100Å continuum and obtain τ cent = 7.20 +1.33 −0.35 days in the rest frame. This yields a black hole mass of M • = 8.71 +3.21 −2.61 × 10 7 M ⊙ using a broad Hβ line dispersion of 3124 ± 302 km s −1 and a virial factor of f BLR = 6.3 ± 1.5 for the broad-line region (BLR), consistent with the mass measurements from previous Hβ campaigns. The high-quality data allow us to construct a velocity-binned delay map for the broad Hβ line, which shows a symmetric response pattern around the line center, a plausible kinematic signature of virialized motion of the BLR. Combining all the available measurements of Hβ time lags and the associated mean 5100Å luminosities over 18 campaigns between 1989 and 2015, we find that the Hβ BLR size varies with the mean optical luminosity, but, interestingly, with a possible delay of 2.35 +3.47 −1.25 yrs. This delay coincides with the typical BLR dynamical timescale of NGC 5548, indicating that the BLR undergoes dynamical changes, possibly driven by radiation pressure.
As a natural consequence of cosmological hierarchical structure formation, sub-parsec supermassive black hole binaries (SMBHBs) should be common in galaxies but thus far have eluded spectroscopic identification. Based on four decades of optical spectroscopic monitoring, we report that the nucleus of NGC5548, a nearby Seyfert galaxy long suspected to have experienced a major merger about 1 billion yr ago, exhibits long-term variability with a period of ∼14 yr in the optical continuum and broad Hβ emission line. Remarkably, the double-peaked profile of Hβ shows systematic velocity changes with a similar period. These pieces of observations plausibly indicate that an SMBHB resides in the center of NGC5548. The complex, secular variations in the line profiles can be explained by orbital motion of a binary with equal mass and a semimajor axis of ∼22 light-days (corresponding to ∼18 milli-parsec). At a distance of 75Mpc, NGC5548 is one of the nearest sub-parsec SMBHB candidates that offers an ideal laboratory for gravitational wave detection.
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