Abstract
Context. Soft and hard X-ray excesses, compared to the continuum power-law shape between ∼2−10 keV, are common features observed in the spectra of active galactic nuclei (AGN) and are associated with the accretion disc-corona system around the supermassive black hole. However, the dominant process at work is still highly debated and has been proposed to be either relativistic reflection or Comptonisation. Such an investigation can be problematic for AGN that have significant intrinsic absorption, either cold or warm, which can severely distort the observed continuum. Therefore, AGN with no (or very weak) intrinsic absorption along the line-of-sight, called bare AGN, are the best targets for directly probing disc-corona systems. Aims. We aim to characterise the main X-ray spectral physical components from the bright bare broad-line Seyfert 1 AGN Mrk 110, as well as the physical process(es) at work in its disc-corona system viewed almost face-on. Methods. We perform the X-ray broadband spectral analysis thanks to two simultaneous XMM-Newton and NuSTAR observations performed on November 16−17, 2019, and April 5−6, 2020. We also use a deep NuSTAR observation obtained in January 2017 for the spectral analysis above 3 keV. Results. The broadband X-ray spectra of Mrk 110 are characterised by the presence of a prominent and absorption-free smooth soft X-ray excess, moderately broad O VII and Fe Kα emission lines, and a lack of a strong Compton hump. The continuum above ∼3 keV is very similar at both epochs, while some variability (stronger when brighter) is present for the soft X-ray excess. A combination of soft and hard Comptonisation by a warm and hot corona, respectively, plus mildly relativistic disc reflection reproduce the broadband X-ray continuum very well. The inferred warm corona temperature, kTwarm ∼ 0.3 keV, is similar to the values found in other sub-Eddington AGN, whereas the hot corona temperature, kThot ∼ 21−31 keV (depending mainly on the assumed hot corona geometry), is found to be in the lower range of the values measured in AGN.
Highlights
In the standard picture, the emission of an active galactic nucleus (AGN) stems from an accretion disc around a supermassive black hole (SMBH) with mass spanning from a few million to billions of solar masses
Soft and hard X-ray excesses, compared to the continuum power-law shape between ∼2−10 keV, are common features observed in the spectra of active galactic nuclei (AGN) and are associated with the accretion disc-corona system around the supermassive black hole
This paper presents the X-ray spectral analysis of two first simultaneous XMM-Newton and NuSTAR observations of Mrk 110 performed within about a half-year interval, on November 16−17, 2019, and April 5−6, 2020
Summary
The emission of an active galactic nucleus (AGN) stems from an accretion disc around a supermassive black hole (SMBH) with mass spanning from a few million to billions of solar masses. Reeves et al (2021a) found that the XMM-Newton-RGS (Reflection Grating Spectrometer; den Herder et al 2001) spectra of Mrk 110 obtained between 2004 and 2020 exhibit no significant intrinsic X-ray warm absorption, with an upper limit for its column density of only 2.6 × 1020 cm−2 This indicates that Mrk 110 is a bare AGN whatever its flux level is. For the first time, Reeves et al (2021a) report that the O vii line flux varies significantly with the soft X-ray continuum flux level, being brightest when the continuum flux is highest, similar to the reported behaviour of the optical He ii line (Véron-Cetty et al 2007) This O vii line originates from the accretion disc at a distance of a few tens of gravitational radii.
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