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First detection of the outer edge of an AGN accretion disc: very fast multiband optical variability of NGC 4395 with GTC/HiPERCAM and LT/IO:O.

Authors :
McHardy, I M
Beard, M
Breedt, E
Knapen, J H
Vincentelli, F M
Veresvarska, M
Dhillon, V S
Marsh, T R
Littlefair, S P
Horne, K
Glew, R
Goad, M R
Kammoun, E
Emmanoulopoulos, D
Source :
Monthly Notices of the Royal Astronomical Society; Mar2023, Vol. 519 Issue 3, p3366-3382, 17p
Publication Year :
2023

Abstract

We present fast (∼200 s sampling) |$\it ugriz$| photometry of the low -mass AGN NGC 4395 with the Liverpool Telescope, followed by very fast (3 s sampling) u<subscript>s</subscript> , g<subscript>s</subscript> , r<subscript>s</subscript> , i<subscript>s</subscript> , and z<subscript>s</subscript> simultaneous monitoring with HiPERCAM on the 10.4m GTC. These observations provide the fastest ever AGN multiband photometry and very precise lag measurements. Unlike in all other AGN, g<subscript>s</subscript>  lags u<subscript>s</subscript>  by a large amount, consistent with disc reprocessing but not with reprocessing in the broad-line region (BLR). There is very little increase in lag with wavelength at long wavelengths, indicating an outer edge (R<subscript>out</subscript>) to the reprocessor. We have compared truncated disc reprocessing models to the combined HiPERCAM and previous X-ray/UV lags. For the normally accepted mass of 3.6 × 10<superscript>5</superscript> M <subscript>⊙</subscript>, we obtain reasonable agreement with zero spin, R<subscript>out</subscript> ∼ 1700 R<subscript>g</subscript>  and the done physically motivated temperature-dependent disc colour-correction factor (f |$\rm _{col}$|⁠). A smaller mass of 4 × 10<superscript>4</superscript> M <subscript>⊙</subscript> can only be accommodated if f |$\rm _{col}=2.4$|⁠ , which is probably unrealistically high. Disc self gravity is probably unimportant in this low-mass AGN but an obscuring wind may provide an edge. For the small mass, the dust sublimation radius is similar to R<subscript>out</subscript> so the wind could be dusty. However, for the more likely large mass, the sublimation radius is further out so the optically thick base of a line-driven gaseous wind is more likely. The inner edge of the BLR is close to R<subscript>out</subscript> in both cases. These observations provide the first good evidence for a truncated AGN disc and caution that truncation should be included in reverberation lag modelling. [ABSTRACT FROM AUTHOR]

Details

Language :
English
ISSN :
00358711
Volume :
519
Issue :
3
Database :
Complementary Index
Journal :
Monthly Notices of the Royal Astronomical Society
Publication Type :
Academic Journal
Accession number :
161585780
Full Text :
https://doi.org/10.1093/mnras/stac3651