Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the ultraviolet anomaly in NGC 5548 with X-ray spectroscopy
Abstract
During the Space Telescope and Optical Reverberation Mapping Project (STORM) observations of NGC 5548, the continuum and emission-line variability became de-correlated 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.
Citation
Mathur , S , Gupta , A , Page , K , Pogge , R W , Krongold , Y , Goad , M R , Adams , S M , Anderson , M D , Arévalo , P , Barth , A J , Bazhaw , C , Beatty , T G , Bentz , M C , Bigley , A , Bisogni , S , Borman , G A , Boroson , T A , Bottorff , M C , Brandt , W N , Breeveld , A A , Brown , J E , Brown , J S , Cackett , E M , Canalizo , G , Carini , M T , Clubb , K I , Comerford , J M , Coker , C T , Corsini , E M , Crenshaw , D M , Croft , S , Croxall , K V , Bontà , E D , Deason , A J , Denney , K D , Lorenzo-Caceres , A D , Rosa , G D , Dietrich , M , Edelson , R , Ely , J , Eracleous , M , Evans , P A , Fausnaugh , M M , Ferland , G J , Filippenko , A V , Flatland , K , Fox , O D , Gates , E L , Gehrels , N , Geier , S , Gelbord , J M , Gorjian , V , Greene , J E , Grier , C J , Grupe , D , Hall , P B , Henderson , C B , Hicks , S , Holmbeck , E , Holoien , T W -S , Horenstein , D , Horne , K , Hutchison , T , Im , M , Jensen , J J , Johnson , C A , Joner , M D , Jones , J , Kaastra , J , Kaspi , S , Kelly , B C , Kelly , P L , Kennea , J A , Kim , M , Kim , S , Kim , S C , King , A , Klimanov , S A , Kochanek , C S , Korista , K T , Kriss , G A , Lau , M W , Lee , J C , Leonard , D C , Li , M , Lira , P , Ma , Z , MacInnis , F , Manne-Nicholas , E R , Malkan , M A , Mauerhan , J C , McGurk , R , McHardy , I M , Montouri , C , Morelli , L , Mosquera , A , Mudd , D , Muller-Sanchez , F , Musso , R , Nazarov , S V , Netzer , H , Nguyen , M L , Norris , R P , Nousek , J A , Ochner , P , Okhmat , D N , Ou-Yang , B , Pancoast , A , Papadakis , I , Parks , J R , Pei , L , Peterson , B M , Pizzella , A , Poleski , R , Pott , J -U , Rafter , S E , Rix , H -W , Runnoe , J , Saylor , D A , Schimoia , J S , Schnülle , K , Sergeev , S G , Shappee , B J , Shivvers , I , Siegel , M , Simonian , G V , Siviero , A , Skielboe , A , Somers , G , Spencer , M , Starkey , D , Stevens , D J , Sung , H -I , Tayar , J , Tejos , N , Turner , C S , Uttley , P , Saders , J V , Vestergaard , M , Vican , L , Villanueva Jr , S , Villforth , C , Weiss , Y , Woo , J -H , Yan , H , Young , S , Yuk , H , Zheng , W , Zhu , W & Zu , Y 2017 , ' Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the ultraviolet anomaly in NGC 5548 with X-ray spectroscopy ' , Astrophysical Journal , vol. 846 , no. 1 , 55 . https://doi.org/10.3847/1538-4357/aa832b
Publication
Astrophysical Journal
Status
Peer reviewed
ISSN
0004-637XType
Journal article
Rights
© 2017, American Astronomical Society. This work has been made available online in accordance with the publisher’s policies. This is the author created, accepted version manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at iopscience.iop.org / https://doi.org/10.3847/1538-4357/aa832b
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