Recently, postdoc LI Shasha, Dr. FENG Haicheng, from Yunnan Observatories of the Chinese Academy of Sciences, and their collaborators, found the evidence for kinematic evolution of the broad-line region (BLR). To investigate the geometry and kinematics of the BLR, and measure the mass of supermassive black hole (SMBH) in Changing-look (CL) Active Galactic Nucleus (AGN) NGC 4151, they performed a seven-month photometric and spectroscopic monitoring program in 2020-2021, using the 2.4 m telescope at Lijiang Observatory. Their research was published online in The Astrophysical Journal.

Broad emission lines are one of the most prominent features in AGNs which are generally believed to emit from the BLR. The gas in BLR is very close to the SMBH, so it is important to study the properties of SMBH and AGN center. Although broad emission lines allow to study the geometry and kinematics of the BLR, and measure the mass of SMBH, the BLR is still not yet fully understood.

Reverberation mapping (RM) is an effective way of investigating the BLR and SMBH, which subtly uses the time resolution of the telescope to substitute the spatial resolution. In principle, the continuum radiated from the accretion disk acts as ionizing photons to ionize the gas of the BLR, and this photoionization process generates the emission lines that are broadened by the BLR cloud motion into the corresponding broad emission lines.

The characteristic size of the BLR can be inferred from monitoring the responses of broad emission lines to continuum variations and measuring the time delays between broad emission-line and continuum light curves. The geometry and kinematics of BLR can be inferred from velocity-resolved RM analysis utilizing high-quality spectral data. The details are derived by measuring time lag as a function of line-of-sight velocity for the broad emission line. Therefore, to investigate the kinematic evolution of BLR, multiple velocity-resolved time delay measurements are necessary. However, only ~30 AGNs have reported the velocity-resolved time delay caused by the prohibitive observation time.

To investigate the kinematic evolution of BLR, NGC 4151 was selected for RM monitoring in this work. As one of the brightest and earliest Seyfert galaxies to be discovered, NGC 4151 was one of the best-studied AGNs. This target has been monitored by multiple RM projects in the last three decades including optical RM campaigns, and ultraviolet RM campaigns. And these results provide a suitable condition for investigating the kinematic evolution of BLR. Furthermore, it is a CL AGN which usually shows large variability amplitudes in both continuum and emission lines. Historical data of NGC 4151 show an extreme outburst lasting for more than 10 years, and it recently arrived at the second outburst stage. Such unusual variability of continuum should lead to dramatic changes in the radiation pressure on BLR, and then in the properties of geometry and kinematics of BLR, which provide a good opportunity to investigate the nature of BLR during this period.

The researchers successfully measured time lags of multiple broad emission lines, and obtained their velocity-resolved maps, respectively. In comparison with previous velocity-delay maps, the BLR kinematics of NGC 4151 changed, implying evolutionary kinematics of BLR. Furthermore, they also calculated the black hole mass and dimensionless accretion rate of NGC 4151, and the result indicated a sub-accretor.

Contact:
FENG Haicheng
Yunnan Observatories, CAS
Email: hcfeng@ynao.ac.cn