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Researchers Find Unchanged Broad-line Region for Changing-look Active Galactic Nuclei

Mar 05, 2021

PhD student FENG Haicheng, Prof. LIU Hongtao, and their collaborators, 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. The study 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. This is 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 November 2018 to May 2019, the researchers performed spectroscopic RM observations of NGC 3516 with the 2.4 m optical telescope located at Lijiang Observatory of Yunnan Observatories. The intensive and homogeneous RM monitoring campaign was implemented during a low-activity state of NGC 3516. The sampling is two days in most nights, and the average sampling is about three days.

The researchers found that the time lags increase towards negative velocity for both Hα and Hβ in the velocity-resolved delay diagrams, and 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, they thought that the BLR in NGC 3516 is likely a narrow elliptical ring. 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, which is measured with cross-correlation functions between the light curves of the optical continuum and broad emission lines. This lag is equal to the light travelling time of the continuum photons from their generation region to BLR. Therefore, a radius of BLR is 7.5 light days. Based on the assumption of virial 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 findings of this study 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

E-mail:

Reverberation Mapping of Changing-look Active Galactic Nucleus NGC 3516

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