EUCLIA - Exploring the UV/optical continuum lag in active galactic nuclei. I. a model without light echoing

TL;DR

This paper proposes a new inhomogeneous accretion disk model with large-scale temperature fluctuations to explain the UV/optical continuum lags in active galactic nuclei, challenging traditional reprocessing scenarios.

Contribution

It introduces a model where large-scale temperature fluctuations produce observed inter-band lags without light echoing, aligning well with observations of NGC 5548.

Findings

The model reproduces the tight inter-band correlation and lag.

It explains the timescale-dependent color variations.

Requires propagation speeds up to 15% of light speed.

Abstract

The tight inter-band correlation and the lag-wavelength relation among UV/optical continua of active galactic nuclei have been firmly established. They are usually understood within the widespread reprocessing scenario, however, the implied inter-band lags are generally too small. Furthermore, it is challenged by new evidences, such as the X-ray reprocessing yields too much high frequency UV/optical variations as well as it fails to reproduce the observed timescale-dependent color variations among {\it Swift} lightcurves of NGC 5548. In a different manner, we demonstrate that an upgraded inhomogeneous accretion disk model, whose local {\it independent} temperature fluctuations are subject to a speculated {\it common} large-scale temperature fluctuation, can intrinsically generate the tight inter-band correlation and lag across UV/optical, and be in nice agreement with several observational properties of NGC 5548, including the timescale-dependent color variation. The emergent lag is a result of the {\it differential regression capability} of local temperature fluctuations when responding to the large-scale fluctuation. An average speed of propagations as large as $\gtrsim 15\%$ of the speed of light may be required by this common fluctuation. Several potential physical mechanisms for such propagations are discussed. Our interesting phenomenological scenario may shed new light on comprehending the UV/optical continuum variations of active galactic nuclei.