Anatomy of the AGN in NGC 5548
Key Words.:X-rays: galaxies – galaxies: active – galaxies: Seyfert – galaxies: individual: NGC 5548 – techniques: spectroscopic
An extensive multi-satellite campaign on NGC 5548 has revealed this archetypal Seyfert-1 galaxy to be in an exceptional state of persistent heavy absorption. Our observations taken in 2013–2014 with XMM-Newton, Swift, NuSTAR, INTEGRAL, Chandra, HST and two ground-based observatories have together enabled us to establish that this unexpected phenomenon is caused by an outflowing stream of weakly ionised gas (called the obscurer), extending from the vicinity of the accretion disk to the broad-line region. In this work we present the details of our campaign and the data obtained by all the observatories. We determine the spectral energy distribution of NGC 5548 from near-infrared to hard X-rays by establishing the contribution of various emission and absorption processes taking place along our line of sight towards the central engine. We thus uncover the intrinsic emission and produce a broadband continuum model for both obscured (average summer 2013 data) and unobscured ( 2011) epochs of NGC 5548. Our results suggest that the intrinsic NIR/optical/UV continuum is a single Comptonised component with its higher energy tail creating the ‘soft X-ray excess’. This component is compatible with emission from a warm, optically-thick corona as part of the inner accretion disk. We then investigate the effects of the continuum on the ionisation balance and thermal stability of photoionised gas for unobscured and obscured epochs.
The supermassive black holes (SMBHs) at the heart of active galactic nuclei (AGN) grow through accretion of matter from their host galaxies. This accretion is accompanied by outflows (powerful relativistic jets of plasma and/or winds of ionised gas), which transport matter and energy away from the nucleus, thus linking the SMBHs to their host galaxies. Understanding the feedback mechanisms between SMBHs and their environments is important in AGN science, as well as in cosmology. They can have significant impacts and implications for the evolution of SMBHs and star formation in their host galaxies (e.g. Silk98; King10), chemical enrichment of their surrounding intergalactic medium (e.g. Oppen06), and the cooling flows at the core of galaxy clusters (e.g. Ciott01).
The ionised outflows are an essential component of the energy balance of AGN and are best studied through high-resolution X-ray and UV spectroscopy of their absorption line spectra. The Reflection Grating Spectrometer (RGS) of XMM-Newton and the Low-Energy & High-Energy Transmission Grating Spectrometers (LETGS and HETGS) of Chandra, together with the Cosmic Origins Spectrograph (COS) of the Hubble Space Telescope (HST) have vastly advanced our knowledge of these outflows in recent years (see e.g. the review by Costan10). With current instrumentation, the nearby and bright Seyfert type AGN are the best laboratories for studying these outflows. The majority of the absorbing gas is detectable in the X-ray band. The outflows detected in the soft X-rays (referred to as ‘warm absorbers’) are found to have column densities of cm and consist of multiple phases of photoionised gas with temperatures of K, travelling at velocities of up to a few (see e.g. Blu05). Understanding the origin and launching mechanism of the ionised outflows in AGN is an active area of research. It has been suggested that these outflows are produced by irradiation of the dusty gas torus structure, which surrounds the SMBH and accretion disk (e.g. Krol01), or that they originate as radiatively driven winds from the accretion disk (e.g. Bege83; Prog00), or are in the form of magnetohydrodynamic (MHD) winds from the accretion disk (e.g. Koni94; Bott00).
Many aspects and physical properties of these outflows are still poorly understood. For instance, the location of the ionised absorbers needs to be established to distinguish between the different outflow mechanisms and to determine their mass outflow rates and kinetic luminosities, which are essential parameters in assessing their impact on their surroundings and their contribution to AGN feedback (e.g. Hopk10). The distance of an X-ray or UV absorbing outflow to the ionising source can be determined from estimates of the density of the absorbing gas. The density can be measured from either density-sensitive UV lines (e.g. Gab05) or from the recombination timescale of the ionised absorber (Kaas12). In the X-ray band, the latter delivers more robust density measurements than the use of density-sensitive X-ray lines. In the recombination timescale method, as the intrinsic luminosity of the AGN varies over time, the ionisation state of the absorber changes with a time delay; by measuring this lag, the electron density and hence the distance of the absorber to the ionising source can be obtained. Such a study was first done as part of our 2009 multi-wavelength campaign on the Seyfert-1/QSO Mrk~509 (Kaas12). From the response of three absorber components with the highest ionisation in the soft X-rays, individual distances of 5–100 pc were derived, pointing to an origin in the narrow-line region (NLR) or the AGN torus region.
The archetypal Seyfert-1 galaxy NGC 5548 is one of the most widely studied nearby active galaxies. It was one of the 12 classified objects in the seminal work of Seyf43, and since the sixties it has been the target of various AGN studies. In more recent times it was the first object in which narrow X-ray absorption lines from warm absorbers were discovered by Kaa00 using a high-resolution Chandra LETGS spectrum. Later on, a detailed study of its warm absorber was carried out by Ste05. Furthermore, extensive optical/UV reverberation mapping studies of NGC 5548 (e.g. Pete02; Panc13) have provided one of the most detailed pictures available of the broad-line region (BLR) size and structure in this AGN. Prior to our recent campaign on NGC 5548, the X-ray properties and variability of NGC 5548 were known to be typical of standard Seyfert-1 AGN sharing common characteristics (e.g. Bian09; Pont12).
In 2013–2014 we carried out an ambitious multi-wavelength campaign on NGC 5548, which was similar but more extensive than our campaign on Mrk 509. This remarkable campaign has utilised eight observatories to take simultaneous and frequent observations of the AGN. It incorporates instruments onboard five X-ray observatories: XMM-Newton (Jans01), Swift (Gehr04), NuSTAR (Harr13), INTEGRAL (Wink03), Chandra’s LETGS (Brink00), as well as the HST COS (Green12), and two ground-based optical observatories: the Wise Observatory (WO) and the Observatorio Cerro Armazones (OCA). These observatories have collected over 2.4 Ms of X-ray and 800 ks of optical/UV observation time. As previously reported by Kaas14, NGC 5548 was discovered to be obscured in X-rays with mainly narrow emission features imprinted on a heavily absorbed continuum. This obscuration is thought to be caused by a stream of clumpy weakly-ionised gas located at distances of about 2–7 light days from the black hole and partially covering the X-ray source and the BLR. From its associated broad UV absorption lines detected in HST COS spectra, the obscurer is found to be outflowing with velocities of up to 5000 . The intense Swift monitoring on NGC 5548 shows the obscuration has been continuously present for a few years (at least since Feb 2012). As the ionising UV/X-ray radiation is being shielded by the obscurer, new weakly-ionised features of UV and X-ray absorber outflows have been detected. Compared to normal warm absorber outflows commonly seen in Seyfert-1s at pc scale distances, the remarkable obscurer in NGC 5548 is a new breed of weakly-ionised, higher-velocity outflowing gas, which is much closer to the black hole and extends to the BLR. As reported in Kaas14 the outflowing obscurer is likely to originate from the accretion disk. Based on the high outflow velocity of the obscurer, its short-timescale absorption variability, and its covering fractions of the continuum and the BLR, the obscurer is in close proximity to the central source, and its geometry extends from near the disk to outside the BLR.
In this work we present a broadband spectral analysis of the NGC 5548 data. The structure of the paper is as follows. Section 2 gives an overview of our multi-satellite campaign. In Sect. LABEL:lc_sect we present lightcurves of NGC 5548 constructed at various energies from near-infrared (NIR) to hard X-rays. In Sect. LABEL:data_correct_sect we explain the required steps in determining the spectral energy distribution (SED) of NGC 5548. In Sect. LABEL:soft_excess_sect we examine the soft X-ray excess in NGC 5548 and present an appropriate model for it. In Sec. LABEL:broadband_sect we describe the modelling of the broadband continuum in unobscured and obscured epochs and present our results. The thermal stability curves, corresponding to various ionising SEDs, are presented in Sect. LABEL:stability_sect. We discuss all our findings in Sect. LABEL:discussion and give concluding remarks in Sect. LABEL:conclusions. The processing of the data from all the instruments is described in Appendix LABEL:data_appendix.
The spectral analysis and modelling, presented in this work, were done using the SPEX 111http://www.sron.nl/spex package (Kaa96) version 2.05.02. We also made use of tools in NASA’s HEASOFT 222http://heasarc.nasa.gov/lheasoft v6.14 package. The spectra shown in this report are background-subtracted and are displayed in the observed frame, unless otherwise stated in the text. We use C-statistics (Cash79) for spectral fitting (unless otherwise stated) and give errors at (68%) confidence level. The redshift of NGC 5548 is set to 0.017175 (deVa91) as given in the NASA/IPAC Extragalactic Database (NED). The adopted cosmological parameters for luminosity computations in our modelling are , and .
2 Multi-wavelength campaign on NGC 5548
At the core of our campaign in summer 2013 (22 June to 1 August), there were 12 XMM-Newton observations, of which five were taken simultaneously with HST COS, four with INTEGRAL and two with NuSTAR observations. Throughout our campaign and beyond, Swift monitored NGC 5548 on a daily basis. There were also optical monitorings with WO and OCA. The summer XMM-Newton observations were followed by three Chandra LETGS observations taken in the first half of September 2013, one of which was taken simultaneously with NuSTAR. The September observations were triggered upon observing a large jump in the X-ray flux from our Swift monitoring. However, due to scheduling constraints by the time the triggered Chandra observations were made, the week-long peak of high X-ray flux was just missed and the tail end of the flare was caught. Nonetheless, the X-ray flux was still higher than during the XMM-Newton observations and improved LETGS spectra were obtained. During this autumn period (Sep-Nov 2013), NGC 5548 was not visible to XMM-Newton and thus no XMM-Newton observations could be triggered.
A few months later, two more XMM-Newton observations were taken, one in December 2013 and the other in February 2014. The former observation was simultaneous with HST COS and NuSTAR observations, and the latter close in time to an INTEGRAL observation. The timeline of all the observations in our campaign is displayed in Fig. 1 and the observation logs are provided in Table 2. In Appendix LABEL:data_appendix, we describe the observations made by the instruments of each observatory and give details on their data reduction and processing.
|Observatory||Obs.||ID||yyyy-mm-dd hh:mm||(ks)||Observatory||Obs.||ID||yyyy-mm-dd hh:mm||(ks)|
|XMM-Newton||1||0720110301||2013-06-22 03:53||50.5||HST COS||1||lc7001||2013-06-22 13:25||13.0|
|XMM-Newton||2||0720110401||2013-06-29 23:50||55.5||HST COS||2||lc7002||2013-07-12 02:23||14.2|
|XMM-Newton||3||0720110501||2013-07-07 23:28||50.9||HST COS||3||lc7003||2013-07-24 16:43||8.9|
|XMM-Newton||4||0720110601||2013-07-11 23:11||55.5||HST COS||4||lc7004||2013-07-30 15:15||16.0|
|XMM-Newton||5||0720110701||2013-07-15 22:56||55.5||HST COS||5||lc7005||2013-08-01 03:21||12.3|
|XMM-Newton||6||0720110801||2013-07-19 22:40||56.5||HST COS||6||lc7006||2013-12-20 02:49||13.0|
|XMM-Newton||7||0720110901||2013-07-21 22:32||55.5||NuSTAR||1||60002044002||2013-07-11 09:50||51.6|
|XMM-Newton||8||0720111001||2013-07-23 22:24||55.5||60002044003||2013-07-12 00:10||52.2|
|XMM-Newton||9||0720111101||2013-07-25 22:15||55.5||NuSTAR||2||60002044005||2013-07-23 14:25||97.2|
|XMM-Newton||10||0720111201||2013-07-27 22:06||55.5||NuSTAR||3||60002044006||2013-09-10 21:25||97.5|
|XMM-Newton||11||0720111301||2013-07-29 21:58||50.4||NuSTAR||4||60002044008||2013-12-20 08:30||98.1|
|XMM-Newton||12||0720111401||2013-07-31 21:49||55.5||Chandra||1||16369||2013-09-01 00:01||29.7|
|XMM-Newton||13||0720111501||2013-12-20 14:01||55.3||Chandra||2||16368||2013-09-02 10:33||67.5|
|XMM-Newton||14||0720111601||2014-02-04 09:33||55.5||Chandra||3||16314||2013-09-10 08:17||122.0|
|INTEGRAL||1||10700010001||2013-06-29 21:34||100.0||Swift (2013) 333The Swift monitoring in 2013 ended on 2013-12-31 18:19.||1–160||444The Swift target IDs of NGC 5548 in 2013: 30022, 80131, 91404, 91711, 91737, 91739, 91744, 91964.||2013-01-04 00:24||326.6|
|INTEGRAL||2||10700010002||2013-07-11 21:13||102.0||Swift (2014) 555The Swift monitoring up to 2014-07-01 00:00 is reported here. The Swift monitoring of NGC 5548 is currently ongoing in 2014–2015.||161–291||666The Swift target IDs of NGC 5548 in 2014: 30022, 33204, 91964.||2014-01-02 14:53||182.5|
|INTEGRAL||3||10700010003||2013-07-15 23:31||106.5||OCA 777The OCA monitoring ended on 2013-07-25 01:13. Observations taken in the B, V, R filters, with 150 s exposure in each filter.||1–27||-||2013-05-20 03:17|
|INTEGRAL||4||10700010004||2013-07-23 03:54||128.9||WO (2013) 888The WO monitoring ended on 2013-09-24 04:59. Observations taken in the B, V, R, I filters, with 300 s exposure in each filter.||1–93||-||2013-06-02 07:22|
|INTEGRAL||5||11200110003||2014-02-09 10:00||38.2||WO (2014) 999The WO monitoring ended on 2014-04-14 08:25. Observations taken in the B, V, R, I filters, with 300 s exposure in each filter.||94–150||-||2013-12-16 14:38|