AGN STORM 2. II. Ultraviolet Observations of Mrk 817 with the Cosmic Origins Spectrograph on the Hubble Space Telescope

Y. Homayouni, Gisella De Rosa, Rachel Plesha, Gerard A. Kriss, Aaron J. Barth, Edward M. Cackett, Keith Horne, Erin A. Kara, Hermine Landt, Nahum Arav, Benjamin D. Boizelle, Misty C. Bentz, Thomas G. Brink, Michael S. Brotherton, Doron Chelouche, Elena Dalla Bontà, Maryam Dehghanian, Pu Du, Gary J. Ferland, Laura FerrareseCarina Fian, Alexei V. Filippenko, Travis Fischer, Ryan J. Foley, Jonathan Gelbord, Michael R. Goad, Diego H. González Buitrago, Varoujan Gorjian, Catherine J. Grier, Patrick B. Hall, Juan V. Hernández Santisteban, Chen Hu, Dragana Ilić, Michael D. Joner, Jelle Kaastra, Shai Kaspi, Christopher S. Kochanek, Kirk T. Korista, Andjelka B. Kovačević, Daniel Kynoch, Yan Rong Li, Ian M. McHardy, Jacob N. McLane, Missagh Mehdipour, Jake A. Miller, Jake Mitchell, John Montano, Hagai Netzer, Christos Panagiotou, Ethan Partington, Richard W. Pogge, Luka Č. Popović, Daniel Proga, Daniele Rogantini, Thaisa Storchi-Bergmann, David Sanmartim, Matthew R. Siebert, Tommaso Treu, Marianne Vestergaard, Jian Min Wang, Martin J. Ward, Tim Waters, Peter R. Williams, Fatima Zaidouni, Ying Zu

Research output: Contribution to journalArticlepeer-review


We present reverberation mapping measurements for the prominent ultraviolet broad emission lines of the active galactic nucleus Mrk 817 using 165 spectra obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. Our ultraviolet observations are accompanied by X-ray, optical, and near-infrared observations as part of the AGN Space Telescope and Optical Reverberation Mapping Program 2 (AGN STORM 2). Using the cross-correlation lag analysis method, we find significant correlated variations in the continuum and emission-line light curves. We measure rest-frame delayed responses between the far-ultraviolet continuum at 1180 Å and Lyα λ1215 Å ( 10.4 − 1.4 + 1.6 days), N v λ1240 Å ( 15.5 − 4.8 + 1.0 days), Si iv + ]O iv λ1397 Å ( 8.2 − 1.4 + 1.4 days), C iv λ1549 Å ( 11.8 − 2.8 + 3.0 days), and He ii λ1640 Å ( 9.0 − 1.9 + 4.5 days) using segments of the emission-line profile that are unaffected by absorption and blending, which results in sampling different velocity ranges for each line. However, we find that the emission-line responses to continuum variations are more complex than a simple smoothed, shifted, and scaled version of the continuum light curve. We also measure velocity-resolved lags for the Lyα and C iv emission lines. The lag profile in the blue wing of Lyα is consistent with virial motion, with longer lags dominating at lower velocities, and shorter lags at higher velocities. The C iv lag profile shows the signature of a thick rotating disk, with the shortest lags in the wings, local peaks at ±1500 km s−1, and a local minimum at the line center. The other emission lines are dominated by broad absorption lines and blending with adjacent emission lines. These require detailed models, and will be presented in future work.

Original languageEnglish
Article number85
JournalAstrophysical Journal
Issue number2
StatePublished - 1 May 2023

Bibliographical note

Funding Information:
We thank the anonymous reviewer for suggestions that improved this work. This paper is the second in a planned series of papers by the AGN STORM 2 Collaboration. Our project began with the successful Cycle 28 HST proposal 16196 (Peterson et al. ). Support for Hubble Space Telescope program GO-16196 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. We are grateful to the dedication of the Institute staff who worked hard to review and implement this program. We particularly thank the Program Coordinator, W. Januszewski, who made sure the intensive monitoring schedule and coordination with other facilities continued successfully.

Funding Information:
Y.H. acknowledges support from NASA grant HST-GO-16196, and was also supported as an Eberly Research Fellow by the Eberly College of Science at the Pennsylvania State University. Research at UC Irvine has been supported by NSF grant AST-1907290. Research at Wayne State University was supported by NSF grant AST 1909199, and NASA grants 80NSSC21K1935 and 80NSSC22K0089. E.K. acknowledges support from NASA grants 80NSSC22K0570 and GO1-22116X. M.C.B. gratefully acknowledges support from the NSF through grant AST-2009230. T.T. and P.R.W. acknowledge support by NASA through grant HST-GO-16196, by NSF through grant NSF-AST 1907208, and by the Packard Foundation through a Packard Research Fellowship to T.T. G.J.F. and M.D. acknowledge support by NSF (1816537, 1910687), NASA (ATP17-ATP17-0141, 19-ATP19-0188), and STScI (HST-AR- 15018 and HST-GO-16196.003-A). P.B.H. is supported by NSERC grant 2017-05983. M.V. gratefully acknowledges financial support from the Independent Research Fund Denmark via grant number DFF 8021-00130. D.H.G.B. acknowledges CONACYT support #319800 and of the researchers’ program for Mexico. D.C. acknowledges support by the ISF (2398/19) and the D.F.G. (CH71-34-3). A.V.F. was supported by the U.C. Berkeley Miller Institute of Basic Research in Science (where he was a Miller Senior Fellow), the Christopher R. Redlich Fund, and numerous individual donors. The UCSC team is supported in part by the Gordon and Betty Moore Foundation, the Heising-Simons Foundation, and by a fellowship from the David and Lucile Packard Foundation to R.J.F.

Publisher Copyright:
© 2023. The Author(s). Published by the American Astronomical Society.

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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