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Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the UV anomaly in NGC 5548 with X-Ray Spectroscopy
journal contributionposted on 2017-08-02, 11:56 authored by S. Mathur, A. Gupta, K. Page, R. W. Pogge, Y. Krongold, M. R. Goad, S. M. Adams, M. D. Anderson, P. Arévalo, A. J. Barth, C. Bazhaw, G. De Rosa, M. Dietrich, R. Edelson, J. Ely, C. J. Grier, M. Eracleous, P. A. Evans, M. M. Fausnaugh, G. J. Ferland, A. V. Filippenko, Z. Ma, K. Flatland, O. D. Fox, E. L. Gates, N. Gehrels, S. Geier, D. Grupe, P. B. Hall, C. B. Henderson, S. Hicks, E. Holmbeck, F. MacInnis, T. W.-S. Holoien, H. Netzer, D. Horenstein, Keith Horne, T. Hutchison, M. Im, J. J. Jensen, C. A. Johnson, M. D. Joner, J. Jones, E. R. Manne-Nicholas, J. Kaastra, S. Kaspi, M. L. Nguyen, B. C. Kelly, P. L. Kelly, J. A. Kennea, M. Kim, S. Kim, S. C. Kim, A. King, M. A. Malkan, S. A. Klimanov, C. S. Kochanek, K. T. Korista, R. P. Norris, G. A. Kriss, M. W. Lau, J. C. Lee, D. C. Leonard, M. Li, P. Lira, J. A. Nousek, J. C. Mauerhan, R. McGurk, I. M. McHardy, C. Montouri, L. Morelli, P. Uttley, A. Mosquera, D. Mudd, F. Muller-Sanchez, R. Musso, S. V. Nazarov, P. Ochner, D. N. Okhmat, B. Ou-Yang, A. Pancoast, I. Papadakis, J. Van Saders, J. R. Parks, T. G. Beatty, L. Pei, B. M. Peterson, A. Pizzella, R. Poleski, J.-U. Pott, S. E. Rafter, H.-W. Rix, J. Runnoe, M. Vestergaard, D. A. Saylor, J. S. Schimoia, M. C. Bentz, K. Schnülle, S. G. Sergeev, B. J. Shappee, I. Shivvers, M. Siegel, G. V. Simonian, A. Siviero, L. Vican, A. Skielboe, G. Somers, M. Spencer, A. Bigley, D. Starkey, D. J. Stevens, H.-I. Sung, J. Tayar, N. Tejos, C. S. Turner, S. Bisogni, S. Villanueva Jr., C. Villforth, Y. Weiss, J.-H. Woo, H. Yan, J. M. Gelbord, S. Young, H. Yuk, W. Zheng, W. Zhu, Y. Zu, G. A. Borman, T. A. Boroson, M. C. Bottorff, W. N. Brandt, A. A. Breeveld, V. Gorjian, J. E. Brown, J. S. Brown, E. M. Cackett, G. Canalizo, M. T. Carini, K. I. Clubb, J. M. Comerford, C. T. Coker, E. M. Corsini, D. M. Crenshaw, J. E. Greene, S. Croft, K. V. Croxall, E. Dalla Bontà, A. J. Deason, K. D. Denney, A. De Lorenzo-Cáceres
During the Space Telescope and Optical Reverberation Mapping Project (STORM) observations of NGC 5548, the continuum and emission-line variability became decorrelated during the second half of the 6-month long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as a part of the campaign. The Swift spectra show that excess flux (relative to a power-law continuum) in the soft X-ray band appears before the start of the anomalous emission-line behavior, peaks during the period of the anomaly, and then declines. This is a model-independent result suggesting that the soft excess is related to the anomaly. We divide the Swift data into on- and off-anomaly spectra to characterize the soft excess via spectral fitting. The cause of the spectral differences is likely due to a change in the intrinsic spectrum rather than being due to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with Swift data. Our preferred model of the soft excess is emission from an optically thick, warm Comptonizing corona, the effective optical depth of which increases during the anomaly. This model simultaneously explains all the three observations: the UV emission line flux decrease, the soft-excess increase, and the emission line anomaly.
We are grateful to Christine Done for discussions on the warm Comptonization model. Support for this work was provided by the National Aeronautics and Space Administration through Chandra Award Number G04-15114X to SM issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. Support for HST program number GO-13330 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. CSK is supported in part by NSF grants AST-1515876 and AST-1515927. KH acknowledges support from STFC grant ST/M001296/1. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester (see Evans et al. 2009). KLP and PAE ackowledge support from the UK Space Agency.
CitationAstrophysical Journal, 2017, 846:55
Author affiliation/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy
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