Abstract
Abstract The Space Telescope and Optical Reverberation Mapping Project (AGN STORM) on NGC 5548 in 2014 is one of the most intensive multiwavelength AGN monitoring campaigns ever. For most of the campaign, the emission-line variations followed changes in the continuum with a time lag, as expected. However, the lines varied independently of the observed UV-optical continuum during a 60–70 day “holiday,” suggesting that unobserved changes to the ionizing continuum were present. To understand this remarkable phenomenon and to obtain an independent assessment of the ionizing continuum variations, we study the intrinsic absorption lines present in NGC 5548. We identify a novel cycle that reproduces the absorption line variability and thus identify the physics that allows the holiday to occur. In this cycle, variations in this obscurer’s line-of-sight covering factor modify the soft X-ray continuum, changing the ionization of helium. Ionizing radiation produced by recombining helium then affects the level of ionization of some ions seen by the Hubble Space Telescope. In particular, high-ionization species are affected by changes in the obscurer covering factor, which does not affect the optical or UV continuum, and thus appear as uncorrelated changes, a “holiday.” It is likely that any other model that selectively changes the soft X-ray part of the continuum during the holiday can also explain the anomalous emission-line behavior observed.
Highlights
The fact that high-ionization absorption lines displayed the holiday while low-ionization absorption lines did not is an important clue to what is happening
We investigated the effect of a changing spectral energy distribution (SED) on Component 1 cloud producing the strong absorption lines
We identified a unique physical cycle in which changes in the LOS CF have a significant effect on the soft X-ray portion of the SED
Summary
In 2013, NGC 5548 was the subject of an intensive monitoring campaign based primarily on X-ray data from XMM-Newton and the Neil Gehrels Swift Observatory, supplemented with spectra from the Hubble Space Telescope (HST) Cosmic Origins Spectrograph (Kaastra et al 2014; Arav et al 2015; Di Gesu et al 2015; Mehdipour et al 2015, 2016; Ursini et al 2015; Whewell et al 2015; Cappi et al 2016; Ebrero et al 2016). Kriss et al 2019, in preparation), the Neil Gehrels Swift Observatory (Edelson et al 2015), ground-based telescopes for both imaging (Fausnaugh et al 2016) and spectroscopy (Pei et al 2017), and the Chandra X-Ray Observatory (Mathur et al 2017) This program yielded the first high-fidelity measurements of interband continuum lags in NGC 5548 (Edelson et al 2015; Fausnaugh et al 2016; Starkey et al 2017) and some very surprising emission-line results (Goad et al 2016; Pei et al 2017)—in particular, some 60 days into the campaign, the broad emission lines stopped responding strongly to continuum variations. We identify a novel physical process in which changes in the SED cause changes in the ionization of helium, which drives the absorption line changes observed by HST
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