Space Telescope and Optical Reverberation Mapping Project. I. Ultraviolet Observations of the Seyfert 1 Galaxy NGC 5548 with the Cosmic Origins Spectrograph on Hubble Space Telescope

TL;DR

This study presents ultraviolet reverberation mapping of NGC 5548, revealing correlated variability between the continuum and emission lines, with insights into the structure and dynamics of the broad-line region using Hubble Space Telescope data.

Contribution

First ultraviolet reverberation-mapping experiment on NGC 5548, providing detailed measurements of emission-line lags and velocity-resolved structure with Hubble's Cosmic Origins Spectrograph.

Findings

Emission lines lag behind the continuum by 2.5 to 6 days.

High-velocity wings of C IV respond faster than line cores.

Emission-line lags increased during the campaign, indicating changing BLR structure.

Abstract

We describe the first results from a six-month long reverberation-mapping experiment in the ultraviolet based on 170 observations of the Seyfert 1 galaxy NGC 5548 with the Cosmic Origins Spectrograph on the Hubble Space Telescope. Significant correlated variability is found in the continuum and broad emission lines, with amplitudes ranging from ~30% to a factor of two in the emission lines and a factor of three in the continuum. The variations of all the strong emission lines lag behind those of the continuum, with He II 1640 lagging behind the continuum by ~2.5 days and Lyman alpha 1215, C IV 1550, and Si IV 1400 lagging by ~5-6 days. The relationship between the continuum and emission lines is complex. In particular, during the second half of the campaign, all emission-line lags increased by a factor of 1.3-2 and differences appear in the detailed structure of the continuum and emission-line light curves. Velocity-resolved cross-correlation analysis shows coherent structure in lag versus line-of-sight velocity for the emission lines; the high-velocity wings of C IV respond to continuum variations more rapidly than the line core, probably indicating higher velocity BLR clouds at smaller distances from the central engine. The velocity-dependent response of Lyman alpha, however, is more complex and will require further analysis.