Strong variability of the coronal line region in NGC 5548

Abstract

We present the second extensive study of the coronal line variability in an active galaxy. Our data set for the well-studied Seyfert galaxy NGC 5548 consists of five epochs of quasi-simultaneous optical and near-infrared spectroscopy spanning a period of about five years and three epochs of X-ray spectroscopy overlapping in time with it. Whereas the broad emission lines and hot dust emission varied only moderately, the coronal lines varied strongly. However, the observed high variability is mainly due to a flux decrease. Using the optical [Fe VII] and X-ray O VII emission lines we estimate that the coronal line gas has a relatively low density of n e ∼ 10 3 cm −3 and a relatively high ionisation parameter of log U ∼ 1. The resultant distance of the coronal line gas from the ionising source of about eight light years places this region well beyond the hot inner face of the dusty torus. These results imply that the coronal line region is an independent entity. We find again support for the X-ray heated wind scenario of Pier & Voit; the increased ionising radiation that heats the dusty torus also increases the cooling efficiency of the coronal line gas, most likely due to a stronger adiabatic expansion. The much stronger coronal line variability of NGC 5548 relative to that of NGC 4151 can also be explained within this picture. NGC 5548 has much stronger coronal lines relative to the low ionisation lines than NGC 4151 indicating a stronger wind, in which case a stronger adiabatic expansion of the gas and so fading of the line emission is expected.

The first spectroscopic dust reverberation programme on active galactic nuclei: the torus in NGC 5548

“…Since the growth and survival of dust particles requires low enough grain temperatures (well below the condensation limit), this result indicates that the hot dust consists mainly of carbon, with silicates having sublimated. This conclusion is also supported by previous near-IR spectra of NGC 5548, where blackbody dust temperatures as high as T ∼ 1700 K were found (Landt et al 2015). The fact that we have measured a dust reverberation signal in this campaign also points to the dust being well below sublimation so that further heating is possible without destroying the dust.…”

“…We have selected the type-1 AGN NGC 5548 as our science target for several reasons: (i) it has a short and variable dust response time (∼ 40 − 80 days; Koshida et al 2014), which means that in one year we can measure several dust lags and so can investigate why they vary; (ii) it has strong and variable near-IR coronal lines (Landt et al 2015); (iii) its broad emission line profiles have clearly discernible and therefore easily separable broad-and narrow-line components, and (iv) multiple optical reverberation campaigns have determined its black hole mass (MBH = 5×10 7 M⊙; Peterson et al 2004;Bentz et al 2009;Bentz & Katz 2015). Furthermore, two recent ambitious programs, namely, the X-ray spectroscopic monitoring campaign in summer 2013 for ∼ 1.5 months with XMM-Newton (Kaastra et al 2014;Mehdipour et al 2015) and the ultraviolet (UV) spectroscopic monitoring campaign with the Hubble Space Telescope (HST) during the first half of 2014 (De Rosa et al 2015;Edelson et al 2015;Fausnaugh et al 2016;Goad et al 2016;Pei et al 2017;Starkey et al 2017;Mathur et al 2017), both supported by extensive space-and ground-based multiwavelength observations, make it one of the best-documented AGN yet.…”

Anatomy of the AGN in NGC 5548

“…This scenario does not fully agree with the distance estimates of the X-ray NELR and warm absorber components in NGC 5548. Using the variability of the forbidden line of O VII, Detmers et al (2009) derived a distance of 1-15 pc for the X-ray NELR, consistent with the optical NELR distance estimate of 1-3 pc (Peterson et al 2013) and joint optical and X-ray NELR distance estimate of ∼2.4 pc (Landt et al 2015). According to the Cloudy based photoionization modeling, Whewell et al (2015) found that the X-ray narrow emission lines originate mainly from the illuminated face of the X-ray NELR with a distance of 14 pc.…”

“…Moreover, distance estimates (from previous studies) of the X-ray and optical NELR indicate that the two regions might be co-located, i.e. a few parsec away from the central engine (Detmers et al 2009;Peterson et al 2013;Landt et al 2015). These similarities further suggest that the X-ray and optical NELR might be the same multi-phase photoionized plasma that manifests its emission in different energy bands.…”

Photoionized emission and absorption features in the high-resolution X-ray spectra of NGC 3783

“…[O iii]) are historically thought to be constant in flux and used to calibrate spectroscopic monitoring data (Peterson et al 2013). However, there is also a growing number of studies (Detmers et al 2009;Peterson et al 2013;Denney et al 2014;Landt et al 2015) suggesting that the optical and/or X-ray narrow emission lines in active galactic nuclei (AGN, Peterson 1997;Krolik 1999) are variable over long timescales (at least a few years). This is not totally unexpected considering the photoionization origin of these emission lines, the variable ionizing source and the distance of the narrow-line region (at least a few parsecs, Bennert et al 2006a,b).…”