Space telescope and optical reverberation mapping project. VII. Understanding the ultraviolet anomaly in NGC 5548 with X-Ray spectroscopy

During the Space Telescope and Optical Reverberation Mapping Project observations of NGC 5548, the continuum and emission-line variability became decorrelated during the second half of the six-month-long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as 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 to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with the 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 three observations: the UV emission-line flux decrease, the soft-excess increase, and the emission-line anomaly.National Aeronautics and Space Administration (NASA) through Chandra by the Chandra X-ray Observatory Center G04-15114X NASA NAS8-03060 NAS 5-26555 NASA through a grant from the Space Telescope Science Institute GO-13330 NSF AST-1515876 AST-1515927 AST-1211916 STFC ST/M001296/1 UK Space Agency TABASGO Foundation Christopher R. Redlich Fund CONACYT 168519 PAIIPIT IN10421