Ultraviolet observations of the seyfert 1 galaxy ngc 5548 with the cosmic origins spectrograph on hubble space telescope

We describe the first results from a six-month long reverberation-mapping experiment in the ultraviolet based on 171 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 iiλ1640 lagging behind the continuum by ∼ 2.5 days and Lyαλ1215, C ivλ1550, and Si ivλ1400 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-ofsight 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 Lyα, however, is more complex and will require further analysis. Subject headings: galaxies: active — galaxies: individual (NGC 5548) — galaxies: nuclei — galaxies: Seyfert 1 Department of Astronomy, The Ohio State University, 140 W 18th Ave, Columbus, OH 43210 2 Center for Cosmology and AstroParticle Physics, The Ohio State University, 191 West Woodruff Ave, Columbus, OH 43210 3 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 4 Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218 5 Department of Physics and Astronomy, Georgia State University, 25 Park Place, Suite 605, Atlanta, GA 30303 6 SUPA Physics and Astronomy, University of St. Andrews, Fife, KY16 9SS Scotland, UK 7 Department of Physics, Western Michigan University, 1120 Everett Tower, Kalamazoo, MI 49008-5252 8 School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel 9 Instituto de F́ısica y Astronomı́a, Facultad de Ciencias, Universidad de Valparáıso, Gran Bretana N 1111, Playa Ancha, Valparáıso, Chile 10 Department of Physics and Astronomy, 4129 Frederick Reines Hall, University of California, Irvine, CA 92697 11 Department of Astronomy and Astrophysics, Eberly College of Science, Penn State University, 525 Davey Laboratory, University Park, PA 16802 12 Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK 13 Department of Statistics, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand 14 Dipartimento di Fisica e Astronomia “G. Galilei,” Università di Padova, Vicolo dell’Osservatorio 3, I-35122 Padova, Italy 15 INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5 I-35122, Padova, Italy 17 Department of Physics and Astronomy, Ohio University, Athens, OH 45701 18 Department of Earth, Environment, and Physics, Worcester State University, 486 Chandler Street, Worcester, MA 01602 19 Department of Astronomy, University of Maryland, College Park, MD 20742-2421 20 University of Leicester, Department of Physics and Astronomy, Leicester, LE1 7RH, UK 21 Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771 22 Spectral Sciences Inc., 4 Fourth Ave., Burlington, MA 01803 23 Eureka Scientific Inc., 2452 Delmer St. Suite 100, Oakland, CA 94602 24 Space Science Center, Morehead State University, 235 Martindale Dr., Morehead, KY 40351 25 Department of Physics and Astronomy, York University, Toronto, ON M3J 1P3, Canada 26 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands 27 Department of Physics and Astronomy, Univeristeit Utrecht, P.O. Box 80000, 3508 Utrecht, The Netherlands 28 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands 29 Department of Physics, University of California, Santa Barbara, CA 93106 30 Departamento de Astronomia, Universidad de Chile, Camino del Observatorio 1515, Santiago, Chile 31 University of Southampton, Highfield, Southampton, SO17 1BJ, UK 32 Department of Physics and Institute of Theoretical and Computational Physics, University of Crete, GR-71003 Heraklion, Greece 33 IESL, Foundation for Research and Technology, GR-71110 Heraklion, Greece