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Interpreting broad emission-line variations - I. Factors influencing the emission-line response

journal contribution
posted on 13.03.2015, 16:53 by Michael R. Goad, K. T. Korista
We investigate the sensitivity of the measured broad emission-line responsivity dlog fline/dlog fcont to continuum variations in the context of straw-man broad emission-line region (BLR) geometries of varying size with fixed BLR boundaries, and for which the intrinsic emission-line responsivity is known a priori. We find for a generic emission line that the measured responsivity ηeff, delay and maximum of the cross-correlation function are correlated for characteristic continuum variability time-scales Tchar less than the maximum delay for that line τmax(line) for a particular choice of BLR geometry and observer orientation. The above correlations are manifestations of geometric dilution arising from reverberation effects within the spatially extended BLR. When present, geometric dilution reduces the measured responsivity, delay and maximum of the cross-correlation function. Conversely, geometric dilution is minimized if Tchar ≥ τmax(line). We also find that the measured responsivity and delay show a strong dependence on light-curve duration, with shorter campaigns resulting in smaller than expected values, and only a weak dependence on sampling rate (for irregularly sampled data). The observed strong negative correlation between continuum level and line responsivity found in previous studies cannot be explained by differences in the sampling pattern, light-curve duration or in terms of purely geometrical effects. To explain this and to satisfy the observed positive correlation between continuum luminosity and BLR size in an individual source, the responsivity-weighted radius must increase with increasing continuum luminosity. For a BLR with fixed inner and outer boundaries this requires radial surface emissivity distributions which deviate significantly from a simple power law, and in such a way that the intrinsic emission-line responsivity increases towards larger BLR radii, in line with photoionization calculations.

History

Citation

Monthly Notices of the Royal Astronomical Society, 2014, 444 (1), pp. 43-61

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

VoR (Version of Record)

Published in

Monthly Notices of the Royal Astronomical Society

Publisher

Oxford University Press on behalf of the Royal Astronomical Society

issn

0035-8711

eissn

1365-2966

Copyright date

2014

Available date

13/03/2015

Publisher version

http://mnras.oxfordjournals.org/content/444/1/43

Notes

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2014, The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

Language

en