Probing the origin of the two-component structure of broad line region by reverberation mapping of an extremely variable quasar

Shumpei Nagoshi, Fumihide Iwamuro, Satoshi Yamada, Yoshihiro Ueda, Yuto Oikawa, Masaaki Otsuka, Keisuke Isogai, Shin Mineshige

Published 2023-06-24Version 1

The physical origins of components in the unified model of quasars such as broad line region (BLR), dust torus, and narrow line region are unresolved. To learn more about them, we focus on studying Changing-State Quasars (also known as Changing-Look Quasars) as they offer the opportunity to observe structural changes associated with state transitions. This can give us insight into the origins of each quasar structure. We aimed to understand the central core structure of one of the most variable Changing-State Quasars, SDSS J125809.31+351943.0, and how it changes before and after the state transition. We performed reverberation mapping with optical spectroscopy to investigate the structure of the BLR and to measure the black hole mass. The results of the reverberation mapping show that the Eddington ratio crossed the value of 0.01 before and after state transition for the black hole mass of $10^{9.46^{+0.15}_{-0.19}}\rm{M_\odot}$. In addition, we compared optical to X-ray spectral indices ($\alpha_{\rm{ox}}$) before and after the state transition to investigate the structure difference of the accretion disk. These variations in $\alpha_{\rm{ox}}$ and the Eddington ratio were found to behave similarly to the state transition seen in X-ray binary systems. We also measured the time-lag between the mid-IR and the optical light curve to estimate the size of the dust torus. From all the acquired information about the BLR and dust torus, we confirmed the existence of two distinct rotating/inflowing BLR components located near the dust torus, probably generated by different processes, which are the origins of the BLRs.