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

Abstract

Context. The spin of supermassive black holes (SMBH) in active galactic nuclei (AGN) can be determined from spectral signature(s) of relativistic reflection such as the X-ray iron Kα line profile, but this can be rather uncertain when the line of sight intersects the so-called warm absorber and/or other wind components as these distort the continuum shape. Therefore, AGN showing no (or very weak) intrinsic absorption along the line-of-sight such as Ark 120, a so-called bare AGN, are the ideal targets for SMBH spin measurements. However, in our previous work on Ark 120, we found that its 2014 X-ray spectrum is dominated by Comptonisation, while the relativistic reflection emission only originates at tens of gravitational radii from the SMBH. As a result, we could not constrain the SMBH spin from disc reflection alone. Aims. Our aim is to determine the SMBH spin in Ark 120 from an alternative technique based on the global energetics of the disc-corona system. Indeed, the mass accretion rate (Ṁ) through the outer disc can be measured from the optical-UV emission, while the bolometric luminosity (Lbol) can be fairly well constrained from the optical to hard X-rays spectral energy distribution, giving access to the accretion efficiency η = Lbol/(Ṁc2) which depends on the SMBH spin. Methods. The spectral analysis uses simultaneous XMM-Newton (OM and pn) and NuSTAR observations on 2014 March 22 and 2013 February 18. We applied the OPTXCONV model (based on OPTXAGNF) to self consistently reproduce the emission from the inner corona (warm and hot thermal Comptonisation) and the outer disc (colour temperature corrected black body), taking into account both the disc inclination angle and relativistic effects. For self-consistency, we modelled the mild relativistic reflection of the incident Comptonisation components using the XILCONV convolution model. Results. We infer a SMBH spin of 0.83+0.05−0.03, adopting the SMBH reverberation mass of 1.50 × 108 M⊙. In addition, we find that the coronal radius decreases with increasing flux (by about a factor of two), from 85+13−10Rg in 2013 to 14 ± 3 Rg in 2014. Conclusions. This is the first time that such a constraint is obtained for a SMBH spin from this technique, thanks to the bare properties of Ark 120, its well determined SMBH reverberation mass, and the presence of a mild relativistic reflection component in 2014 which allows us to constrain the disc inclination angle. We caution that these results depend on the detailed disc-corona structure, which is not yet fully established. However, the realistic parameter values (e.g. Lbol/LEdd, disc inclination angle) found suggest that this is a promising method to determine spin in moderate-Ṁ AGN.

Searching for ultra-fast outflows in AGN using variability spectra

“…Pinto et al 2018). Both of these factors could induce biases in finding the global fraction of AGN hosting UFOs, especially because Ark 120, for example, has an estimated black hole mass of around 1.5 × 10 8 M and is accreting at L bol /L Edd = 0.2, which should be sufficient to observe small broad absorption lines (Porquet et al 2019). Reeves et al (2016) actually point out that soft X-ray emission lines are seen in the high SNR XMM-Newton RGS data of Ark 120, meaning that substantial amounts of circumnuclear gas is present, but it is out of our line of sight, in line with the hypotheses outlined above.…”

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

“…UVW1) arises from regions much closer to the peak of the 'big blue bump' in the spectral energy distribution, where one may expect to observe a higher amplitude of intrinsic variability on relatively shorter timescales. The host-galaxy contribution in Ark 120 is estimated by Porquet et al (2019) following a flux-variation gradient method proposed by Choloniewski (1981) using XMM-Newton data (also see Winkler et al 1992;Winkler 1997;Glass 2004;Sakata et al 2010). The resultant estimates of the flux contribution in the central extraction aperture are: V (5 430 Å) = 56 ± 4 per cent, B (4 500 Å) = 17 ± 5 per cent, U (3 440 Å) = 15 ± 7 per cent.…”

“…Assuming that the lags are dominated by the light-crossing time, we can express these in units of light-days: X-rays-to-V: 4.1; X-rays-to-B: 3.0; X-rays-to-UVW1: 1.5 light-days. In contrast, we note that the viscous timescale for the disc in Ark 120 would be on a timescale of > 10 3 years (or at least ∼days-weeks if the disc is geometrically thick; also see Porquet et al 2019). The predicted distances are listed in Table 4 alongside the measured time delays from our correlation functions for comparison.…”

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

“…Ark 120 has been the target of a deep XMM-Newton (Jansen et al 2001) observation in 2014, starting on 2014 March 18 for a total elapsed time of ∼650 ks with the EPIC CCD cameras, the Pn (Strüder et al 2001) and the two MOS (Turner et al 2001), operated in small window and thin filter mode. In this work, we only focus on the third XMM orbit (observation 2014c in Porquet et al 2019) since it is the only one simultaneous with NuSTAR. We also analyse data from the 2013 February 18 pointing (total elapsed time of 130 ks), also simultaneous with NuS-TAR, applying the latest calibration files available on 2018 June.…”

“…Spectra were then binned in order not to over-sample the instrumental resolution more than a factor of three and to have no less than 30 counts in each background-subtracted spectral channel. Since no significant spectral variability is observed within each observation, we used time averaged spectra (we refer the reader to Matt et al 2014;Nardini et al 2016;Lobban et al 2018;Porquet et al 2019, for further details).…”