Anatomy of the AGN in NGC 5548

Abstract

In 2013, we conducted a large multi-wavelength campaign on the archetypical Seyfert 1 galaxy NGC 5548. Unexpectedly, this usually unobscured source appeared strongly absorbed in the soft X-rays during the entire campaign, and signatures of new and strong outflows were present in the almost simultaneous UV HST/COS data. Here we carry out a comprehensive spectral analysis of all available XMM-Newton observations of NGC 5548 (precisely 14 observations from our campaign plus three from the archive, for a total of ∼763 ks) in combination with three simultaneous NuSTAR observations. We obtain a best-fit underlying continuum model composed by i) a weakly varying flat (Γ ∼ 1.5-1.7) power-law component; ii) a constant, cold reflection (FeK + continuum) component; iii) a soft excess, possibly owing to thermal Comptonization; and iv) a constant, ionized scattered emission-line dominated component. Our main findings are that, during the 2013 campaign, the first three of these components appear to be partially covered by a heavy and variable obscurer that is located along the line of sight (LOS), which is consistent with a multilayer of cold and mildly ionized gas. We characterize in detail the short timescale (mostly ∼ks-to-days) spectral variability of this new obscurer, and find it is mostly due to a combination of column density and covering factor variations, on top of intrinsic power-law (flux and slope) variations. In addition, our best-fit spectrum is left with several (but marginal) absorption features at rest-frame energies ∼6.7−6.9 keV and ∼8 keV, as well as a weak broad emission line feature redwards of the 6.4 keV emission line. These could indicate a more complex underlying model, e.g. a P-Cygni-type emission profile if we allow for a large velocity and wide-angle outflow. These findings are consistent with a picture where the obscurer represents the manifestation along the LOS of a multilayer of gas, which is also in multiphase, and which is likely outflowing at high speed, and simultaneously producing heavy obscuration and scattering in the X-rays, as well as broad absorption features in the UV.

Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the Ultraviolet Anomaly in NGC 5548 with X-Ray Spectroscopy

“…This soft component was constant over the whole campaign, so it cannot be responsible for the variable soft excess we see here. Nonetheless, we added a similar component to our model (XSPEC model apec) with flux as in Cappi et al (2016) and found no improvement to the fit (c = n 1.16 2 for 1001 degrees of freedom). The best-fit spectra are shown in Figure 3 and the model parameters are given in Table 2 along with the flux in the soft X-ray excess after correcting for absorption.…”

“…The fit showed that the BB temperature is the same in the two spectra, so we tied the temperature and fitted the spectra again. The resulting fit was good (c = In their model of the entire XMM-Newton observing campaign, Cappi et al (2016) found an additional scattered soft X-ray component dominated by narrow emission lines, with 8% of the total soft X-ray flux. This soft component was constant over the whole campaign, so it cannot be responsible for the variable soft excess we see here.…”

“…In our STORM campaign, the absorption column density was over an order of magnitude lower (∼10 22 cm −2 ). A detailed spectral analysis of XMM-Newton observations of NGC5548 is presented by Cappi et al (2016), who modeled the spectra with six different components: a power-law continuum; a cold reflection component; a soft thermal Comptonization emission model; a scattered emissionline component; a warm absorber; and up to two high column density partial covering "obscurers." Given the S/N of our Swift observations, it is not possible to fit the spectra with such a complex model and deduce meaningful information.…”

“…In Section 4, we discuss how the appearance of a soft-X-ray excess a few days before the anomaly may clarify its origin. Although NGC 5548 was observed intensively with XMM-Newton in the years prior to the AGN STORM campaign (Kaastra et al 2014;Mehdipour et al 2015Mehdipour et al , 2016Cappi et al 2016), a detailed comparison with those data is beyond the scope of this paper. Here we focus on Swift and Chandra data obtained during our campaign.…”

X-ray spectroscopy of the z = 6.4 quasar SDSS J1148+5251

“…This scenario (hereafter called BLRscenario) nicely accounts for the X-ray emission variability of AGN 5 that occurs on very short time scales (from a fraction of a day up to years), and thus requires the absorber to be at sub-parsec distances from the X-ray source (e.g. Cappi et al 2016;Bianchi et al 2009;Risaliti et al 2005). It has been also proposed by Matt (2000) that while Compton-thick Seyfert 2 galaxies are observed through the torus, Compton-thin/intermediate Seyfert galaxies are obscured by dust lanes in the host galaxy.…”