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Astronomy Colloquium (2019 Fall Semester)

* Date
Nov. 7
* Speaker
Prof. Hirofumi NODA (OSAKA Univ.)
* Title
Structure and State Transition of Supermassive Black Hole Accretion Flow Studied by Multi-Wavelength Observations
* Abstract
 

It is under debate if a mass accretion onto a supermassive black hole (SMBH) can be explained by scaling that in a galactic black hole binary (BHB) with mass difference. One of interesting phenomena seen in a BHB accretion is the ``state transition'' with which a standard disk may evaporate into a hot flow, or a hot flow may condense into a disk, following accretion rate change. However, no clear state transition has been confirmed in a SMBH accretion so far, and hence, to explore it, we focused on a ``changing-look AGN (CLAGN)'' which changes its type between type 1 and 2 with drastic luminosity variation. We examined optical, UV, and X-ray continua of a CLAGN Mrk 1018 observed by XMM-Newton and Swift through its changing-look (CL) process from type 1 to 2. As a result, we found dramatic spectral variation like the soft-to-hard transition in a BHB accretion, suggesting that the CL process involves the state transition in a SMBH accretion (Noda & Done 2018, MNRAS, 480, 3898). Furthermore, we performed X-ray and optical monitoring on another CLAGN NGC 3516 at its type-2 phase in 2013–14 with Suzaku and ground-based telescopes. As a result, we found that their correlation and an optical time delay can be explained with disc reverberation by a hot and geometrically-thick accretion flow, supporting that the CLAGN type-2 phase corresponds to the BHB low/hard state (Noda et al. 2016, ApJ, 828, 78).