Anatomy of the AGN in NGC 5548

Abstract

Context. Our consortium performed an extensive multi-wavelength campaign of the nearby Seyfert 1 galaxy NGC 5548 in 2013−14. The source appeared unusually heavily absorbed in the soft X-rays, and signatures of outflowing absorption were also present in the UV. He-like triplets of neon, oxygen and nitrogen, and radiative recombination continuum (RRC) features were found to dominate the soft X-ray spectrum due to the low continuum flux. Aims. Here we focus on characterising these narrow emission features using data obtained from the XMM-Newton RGS (770 ks stacked spectrum).Methods. We use  for our initial analysis of these features. Self-consistent photoionisation models from C are then compared with the data to characterise the physical conditions of the emitting region. Results. Outflow velocity discrepancies within the O VII triplet lines can be explained if the X-ray narrow-line region (NLR) in NGC 5548 is absorbed by at least one of the six warm absorber components found by previous analyses. The RRCs allow us to directly calculate a temperature of the emitting gas of a few eV (∼10 4 K), favouring photoionised conditions. We fit the data with a C model of log ξ = 1.45 ± 0.05 erg cm s −1 , log N H = 22.9 ± 0.4 cm −2 and log v turb = 2.25 ± 0.5 km s −1 for the emitting gas; this is the first time the X-ray NLR gas in this source has been modelled so comprehensively. This allows us to estimate the distance from the central source to the illuminated face of the emitting clouds as 13.9 ± 0.6 pc, consistent with previous work.

Anatomy of the AGN in NGC 5548

“…The main consequence of the extraordinary obscuration observed during the 2013 campaign (Kaastra et al 2014) is the depression of the soft X-ray continuum, which makes the detection of WA features very challenging, although it gives us an excellent opportunity to study the X-ray emission of the narrow line region (NLR) of a Seyfert 1 galaxy with unprecedented detail (Whewell et al 2015). The depressed X-ray flux is not intrinsically low, since the hard X-ray continuum is comparable to the historical fluxes of NGC 5548 in that energy band, but it is caused by the presence of obscuring material crossing our line of sight.…”

“…With the exception of the forbidden lines of O vii and C v, these lines are difficult to detect in the L02 and H02 spectra. Therefore we fixed their normalization to the values obtained in the 2013 campaign (Whewell et al 2015), where the lines are clearly visible against the strongly obscured continuum. The narrow lines and RRCs are convolved with a velocity dispersion of σ v = 463 km s −1 (see the supplementary material in Kaastra et al 2014).…”

“…The narrow lines and RRCs are convolved with a velocity dispersion of σ v = 463 km s −1 (see the supplementary material in Kaastra et al 2014). For further information we point the reader to Whewell et al (2015), who present a detailed analysis of the narrow lines and RRCs in NGC 5548.…”

“…They found it to be compact (R NLR ∼ 1−3 pc) although its actual extent is possibly larger if it is elongated toward the line of sight. In this respect, the analysis of the narrow emission lines in the stacked RGS spectrum of NGC 5548 during the 2013−2014 campaign, renders a location for the NLR of r min = 13.9±0.6 pc from the nucleus (Whewell et al 2015). This analysis also shows the narrow emission lines to be absorbed by WA component B, located at 13 < R < 20 pc, a range consistent with the location of the NLR estimated by Whewell et al (2015), although no conclusions can be firmly drawn on the possible connection between the absorbers along our line of sight and the emitting clouds that extend over a much larger region.…”

Anatomy of the AGN in NGC 5548

“…The high energy properties of the source during, and before, the campaign are shown in Ursini et al (2015; hereafter Paper III), while the study of a short-timescale flaring event that happened in September 2013 and which triggered a Chandra LETG observation is addressed in Di Gesu et al (2015; hereafter Paper IV), together with the analysis of the WA short-term variability during this event. The time-averaged soft X-ray linedominated RGS spectrum is presented and modeled in detail in Whewell et al (2015;hereafter Paper V). For the longer-term timescales, the historical source behavior is presented in Ebrero et al (2016a,b;hereafter Paper VI), while the full Swift multiyear monitoring is presented in Mehdipour et al (2016;hereafter Paper VII).…”