PhD student FENG Haicheng, Prof. LIU Hongtao, and their cooperators, from Yunnan observatories of the Chinese Academy of Sciences, investigated the double-peaked broad emission line and changing-look active galactic nucleus (AGN) NGC 3516. Recently, their research was published online in The Astrophysical Journal.

It is generally believed that the double-peaked broad emission lines are from an elliptical disk. Appearance or disappearance of the broad Balmer emission lines correspond to type 1 and type 2 AGNs, respectively. That’s how the name of changing-look AGN comes. The physical origin of changes of spectral type is still debated. Reverberation mapping (RM) observations can shed light on this issue.

From 2018 November to 2019 May, the researchers successfully performed spectroscopic RM observations of NGC 3516 with the 2.4 m optical telescope located at Lijiang Observatory of Yunnan Observatories. Their intensive and homogeneous RM monitoring campaign was run during a low-activity state of NGC 3516. The sampling is 2 days in most nights, and the average sampling is about 3 days.

The researches find that the time lags increase towards negative velocity for both Hα and Hβ in the velocity-resolved delay diagrams. Also, the velocity-resolved RM of Hα is done for the first time. Interestingly, the radius of broad-line region (BLR) in NGC 3516 is basically unchanged for about 30 years. However, most RM observations of AGNs show that the radii of BLRs change with the AGN brightness, i.e., the well-known “breathing” effect of BLR. Thus, the researchers think that the BLR in NGC 3516 is likely a narrow elliptical ring. Besides, the changing-look model of changes of accretion rate seems to be favored by the long-term Hβ variability and RM observations of NGC 3516.

The researchers obtained a time lag of 7.5 days, derived from cross-correlation functions between the light curves of the optical continuum and broad emission lines. Furthermore, this lag is equal to the light travelling time of the continuum photons from their generation region to BLR. So, a radius of BLR is 7.5 light days. Based on the assumption of virialized motion of gas clouds in the BLR, a virial mass of 24 million solar masses is obtained for the central supermassive black hole in NGC 3516.

Their new findings enable people to better understand the central physical structure and the changing-look mechanism of the double-peaked broad emission line changing-look active galactic nuclei.

Contact:

LIU Hongtao

Yunnan Observatories, CAS

htliu@ynao.ac.cn