TY - JOUR
T1 - Explaining changing-look AGN with state transition triggered by rapid mass accretion rate drop
AU - Noda, Hirofumi
AU - Done, Chris
N1 - Funding Information:
We thank the referee for his/her careful read of this manuscript and valuable comments. HN is supported by Program for Establishing a Consortium for the Development of Human Resources in Science and Technology, Japan Science and Technology Agency (JST). CD acknowledges the Science and Technology Facilities Council (STFC) through grant ST/P000541/1 for support. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.
Publisher Copyright:
© 2018 The Author(s).
PY - 2018
Y1 - 2018
N2 - We model the broad-band (optical/UV and X-ray) continuum spectrum of the 'changing-look' active galactic nucleus (AGN) Mrk 1018, as it fades from Seyfert 1 to 1.9 in ~ 8 years. The brightest spectrum, with Eddington ratio L/LEdd ~ 0.08 has a typical type 1 AGN continuum, with a strong 'soft X-ray excess' spanning between the UV and soft X-rays. The dimmest spectrum, at L/LEdd ~ 0.006, is very different in shape as well as luminosity, with the soft excess dropping by much more than the hard X-rays. The soft X-ray excess produces most of the ionizing photons, so its dramatic drop leads to the disappearance of the broad-line region, driving the 'changing-look' phenomena. This spectral hardening appears similar to the soft-to-hard state transition in black hole binaries at L/LEdd ~ 0.02, where the inner disc evaporates into an advection dominated accretion flow, while the overall drop in luminosity appears consistent with the hydrogen ionization disc instability. None the less, both processes happen much faster in Mrk 1018 than predicted by disc theory. We critically examine scaling from Galactic binary systems and show that amajor difference is that radiation pressure should be much more important in AGNs, so that the sound speed is much faster than expected from the gas temperature. Including magnetic pressure to stabilize the disc shortens the time-scales even further. We suggest that all changing-look AGNs are similarly associated with the state transition at L/LEdd ~ a few per cent.
AB - We model the broad-band (optical/UV and X-ray) continuum spectrum of the 'changing-look' active galactic nucleus (AGN) Mrk 1018, as it fades from Seyfert 1 to 1.9 in ~ 8 years. The brightest spectrum, with Eddington ratio L/LEdd ~ 0.08 has a typical type 1 AGN continuum, with a strong 'soft X-ray excess' spanning between the UV and soft X-rays. The dimmest spectrum, at L/LEdd ~ 0.006, is very different in shape as well as luminosity, with the soft excess dropping by much more than the hard X-rays. The soft X-ray excess produces most of the ionizing photons, so its dramatic drop leads to the disappearance of the broad-line region, driving the 'changing-look' phenomena. This spectral hardening appears similar to the soft-to-hard state transition in black hole binaries at L/LEdd ~ 0.02, where the inner disc evaporates into an advection dominated accretion flow, while the overall drop in luminosity appears consistent with the hydrogen ionization disc instability. None the less, both processes happen much faster in Mrk 1018 than predicted by disc theory. We critically examine scaling from Galactic binary systems and show that amajor difference is that radiation pressure should be much more important in AGNs, so that the sound speed is much faster than expected from the gas temperature. Including magnetic pressure to stabilize the disc shortens the time-scales even further. We suggest that all changing-look AGNs are similarly associated with the state transition at L/LEdd ~ a few per cent.
KW - Galaxies: Seyfert
KW - Galaxies: active
KW - Galaxies: individual: Mrk 1018
KW - X-rays: galaxies
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U2 - 10.1093/MNRAS/STY2032
DO - 10.1093/MNRAS/STY2032
M3 - Article
AN - SCOPUS:85055168266
VL - 480
SP - 3898
EP - 3906
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
SN - 0035-8711
IS - 3
ER -