Simultaneous X-ray and UV spectroscopy of the Seyfert galaxy NGC 5548

Abstract

Abstract. We present the results from a 500 ks Chandra observation of the Seyfert 1 galaxy NGC 5548. We detect broadened (full width half maximum = 8000 km s −1 ) emission lines of O  and C  in the spectra, similar to those observed in the optical and UV bands. The source was continuously variable, with a 30% increase in luminosity in the second half of the observation. The gradual increase in luminosity occurred over a timescale of ∼300 ks. No variability in the warm absorber was detected between the spectra from the first 170 ks and the second part of the observation. The longer wavelength range of the LETGS resulted in the detection of absorption lines from a broad range of ions, in particular of C, N, O, Ne, Mg, Si, S and Fe. The velocity structure of the X-ray absorber is consistent with the velocity structure measured simultaneously in the ultraviolet spectra. We find that the highest velocity outflow component, at −1040 km s −1 , becomes increasingly important for higher ionization parameters. This velocity component spans at least three orders of magnitude in ionization parameter, producing both highly ionized X-ray absorption lines (Mg , Si ) as well as UV absorption lines. A similar conclusion is very probable for the other four velocity components. Based upon our observations, we argue that the warm absorber probably does not manifest itself in the form of photoionized clumps in pressure equilibrium with a surrounding wind. Instead, a model with a continuous distribution of column density versus ionization parameter gives an excellent fit to our data. From the shape of this distribution and the assumption that the mass loss through the wind should be smaller than the accretion rate onto the black hole, we derive upper limits to the solid angle as small as 10 −4 sr. From this we argue that the outflow occurs in density-stratified streamers. The density stratification across the stream then produces the wide range of ionization parameter observed in this source. We determine an upper limit of 0.3 M yr −1 for the mass loss from the galaxy due to the observed outflows.

Anatomy of the AGN in NGC 5548

“…Unexpectedly, throughout the whole 2013-14 campaign, NGC 5548 appeared dramatically different than it appeared in the past (e.g., from the Chandra observation of 2002, Steenbrugge et al 2005): the object was ∼25 times less luminous in the soft X-rays. Moreover, NGC 5548 showed broad, asymmetric absorption troughs in the blue wings of the main UV broad emission lines (e.g., in Ly α, C , N ).…”

“…From a dynamical modeling of the BLR (Pancoast et al 2014), it is inferred that this source is observed at an inclination angle of ∼30 • and hosts a supermassive black hole (SMBH) of ∼3 × 10 7 M in its center. Previously, high resolution UV (Crenshaw & Kraemer 1999;Crenshaw et al 2003a) and X-ray (Kaastra et al , 2002Steenbrugge et al 2003Steenbrugge et al , 2005) spectra have revealed several deep NAL that can be ascribed to a moderate velocity (v out = 200−1200 km s −1 ) ionized outflow.…”

Strong variability of the coronal line region in NGC 5548

“…This hypothesis is attractive because it is hard to think of any other way to account for the unusual ratios found in our previous paper. But the problem with their explanation in the case of NGC 5548 is that the measured O VI column density (NOVI = 10 16 cm −2 ; Steenbrugge et al 2005) of the warm absorber, which potentially can be cospatial with the O VII line emitting region, is ∼ 1.5 dex smaller than that needed to produce the absorption assuming their atomic data. Then, using the line strengths from Whewell et al (2015, submitted), the G and R ratios become 3.40±0.52 and 3.70±0.27, respectively (filled circle in Fig.…”

“…Then, using the unabsorbed X-ray luminosity of L 2−10keV ∼ 4 × 10 43 erg s −1 of the highest-flux epoch as a proxy for the ionising luminosity producing O VII and the best-fit X-ray spectral slope of this epoch of Γ ∼ 1.88 (Steenbrugge et al 2005) to estimate the mean ionising photon energy, the calculated distance of the coronal line region in NGC 5548 from the central ionising source for the above gas density is τ lt ∼ 2850 light days ∼ 7.8 light years. As found previously for NGC 4151, this value places this region well beyond the hot inner face of the obscuring dusty torus, which in NGC 5548 is measured by dust reverberation campaigns to be ∼ 40 − 60 days (Koshida et al 2014).…”

Thermal instability in X-ray photoionized media in active galactic nuclei

“…Recent modeling of the warm absorbing medium in the central regions of active galactic nuclei (AGN) has shown that the cloud models in constant total (gas plus radiation) pressure (Różańska et al 2006;Gonçalves et al 2006;Chevallier et al 2007) are more sophisticated than those in constant density (Netzer 1993;Kaspi et al 2001;Kaastra et al 2002), or those invoking a continuous wind (Steenbrugge et al 2005;Chelouche & Netzer 2005). Constant pressure models, however, are not simple to deal with, as they require addressing the subject of thermal instability.…”