A deep X-ray view of the bare AGN Ark 120

Abstract

Context. The physical characteristics of the material closest to supermassive black holes (SMBHs) are primarily studied through X-ray observations. However, the origins of the main X-ray components such as the soft X-ray excess, the Fe Kα line complex, and the hard X-ray excess are still hotly debated. This is particularly problematic for active galactic nuclei (AGN) showing a significant intrinsic absorption, either warm or neutral, which can severely distort the observed continuum. Therefore, AGN with no (or very weak) intrinsic absorption along the line of sight, so-called “bare AGN”, are the best targets to directly probe matter very close to the SMBH. Aims. We perform an X-ray spectral analysis of the brightest and cleanest bare AGN known so far, Ark 120, in order to determine the process(es) at work in the vicinity of the SMBH. Methods. We present spectral analyses of data from an extensive campaign observing Ark 120 in X-rays with XMM-Newton (4 × 120 ks, 2014 March 18–24), and NuSTAR (65.5 ks, 2014 March 22). Results. During this very deep X-ray campaign, the source was caught in a high-flux state similar to the earlier 2003 XMM-Newton observation, and about twice as bright as the lower-flux observation in 2013. The spectral analysis confirms the “softer when brighter” behavior of Ark 120. The four XMM-Newton/pn spectra are characterized by the presence of a prominent soft X-ray excess and a significant Fe Kα complex. The continuum is very similar above about 3 keV, while significant variability is present for the soft X-ray excess. We find that relativistic reflection from a constant-density, flat accretion disk cannot simultaneously produce the soft excess, broad Fe Kα complex, and hard X-ray excess. Instead, Comptonization reproduces the broadband (0.3–79 keV) continuum well, together with a contribution from a mildly relativistic disk reflection spectrum. Conclusions. During this 2014 observational campaign, the soft X-ray spectrum of Ark 120 below ~0.5 keV was found to be dominated by Comptonization of seed photons from the disk by a warm (kTe ~ 0.5 keV), optically-thick corona (τ ~ 9). Above this energy, the X-ray spectrum becomes dominated by Comptonization from electrons in a hot optically thin corona, while the broad Fe Kα line and the mild Compton hump result from reflection off the disk at several tens of gravitational radii.

The soft X-ray excess: NLS1s versus BLS1s

“…In this framework, the lack of variation of the soft excess strength with M BH may be explained by bulk motion Comptonization, or by thermal Comptonization, provided that the accretion power released in the disk and in the corona vary in concert in such a way to produce the proper physical conditions with optical depth τ > 1 and kT ∼ 1 keV (e.g., Done et al 2012;Różańska et al 2015;Petrucci et al 2018). This conclusion is supported and strengthened by recent findings obtained applying physically motivated warm Comptonization models to fit high-quality broad-band spectra of different AGN, obtained from simultaneous long observations carried out by XMM-Newton and NuSTAR (e.g., Porquet et al 2018;Middei et al 2018;Ursini et al 2018) We conclude by summarizing the main findings of our work. Starting from a flux-limited sample of 89 type 1 AGN (59 BLS1s and 30 NLS1s), with BLS1s and NLS1s matched in X-ray luminosities, and with NLS1s with lower M BH on average and considerably larger λ Edd values than BLS1s, we defined a clean sample of 68 objects (46 BLS1s and 22 NLS1s) after excluding 13 BLS1s and 8 NLS1s severely affected by warm absorbers that hamper the proper characterization of the soft excess.…”

“…The 13 BLS1s that required warm absorbers were Ark 120 (Nardini et al 2011;Porquet et al 2018), H 0557-385 (Longinotti 2009), PG 0844+349 (Pounds & Page 2004), NGC 3516 (Huerta 2014;Costantini et al 2000;Mehdipour et al 2010), PG 1114+445 (Ashton et al 2004), NGC 3783 (Blustin et al 2002), Ark 374, IC 4329A (Steenbrugge K. C. et al 2005), NGC 5548 (Mao et al 2018), Mrk 1383, Mrk 876 (Porquet et al 2004), Mrk 304 (Piconcelli et al 2004;Brinkmann et al 2004), and Fairall 1146.…”

X-ray, UV, and optical time delays in the bright Seyfert galaxy Ark 120 with co-ordinated Swift and ground-based observations

“…While signifi- cant variability is detected in both bands, we find that the soft band displays slightly stronger variability, with Fvar = 36.1 ± 2.1 per cent, compared to Fvar = 26.5 ± 1.3 per cent in the hard band. This is likely due to variations in a prominent steep component of the soft excess (see Lobban et al 2018;Porquet et al 2018). We searched for a correlation between the two bands by plotting the observed count rates against one another, as shown in Fig.…”

NuSTAR/XMM–Newton monitoring of the Seyfert 1 galaxy HE 1143-1810

“…Crummy et al 2006;Ponti et al 2006;Walton et al 2013;Jiang et al 2018;García et al 2019) and 'warm' Comptonization (see e.g. Magdziarz et al 1998;Mehdipour et al 2011;Done et al 2012;Petrucci et al 2013;Boissay et al 2014;Matt et al 2014;Middei et al 2018;Porquet et al 2018;Petrucci et al 2018;Ursini et al 2018;Middei et al 2019). In the latter hypothesis the optical-UV emission and soft X-ray excess could originate from the upper layer of the accretion disc, consisting of a warm (kT e ∼ 1 keV) optically thick (τ ∼ 10 − 20) slab-like corona (Różańska et al 2015;Petrucci et al 2018).…”