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.
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