The Nature of Soft Excess in ESO 362-G18 Revealed by XMM-Newton and NuSTAR Spectroscopy
We present a detailed spectral analysis of the joint XMM-Newton and NuSTAR observations of the active galactic nuclei (AGNs) in the Seyfert 1.5 Galaxy ESO 362-G18. The broadband (0.3–79 keV) spectrum shows the presence of a power-law continuum with a soft excess below 2 keV, iron Kα emission (~6.4 keV), and a Compton hump (peaking at ~20 keV). We find that the soft excess can be modeled by two different possible scenarios: a warm (kT_e ~ 0.2 keV) and optically thick (τ ~ 34) Comptonizing corona, or with a relativistically blurred reflection off a high-density (log [n_e/cm⁻³ > 18.3) inner disk. These two models cannot be easily distinguished solely from their fit statistics. However, the low temperature (kT_e ~ 20 keV) and the thick optical depth (τ ~ 5) of the hot corona required by the warm corona scenario are uncommon for AGNs. We also fit a "hybrid" model, which includes both disk reflection and a warm corona. Unsurprisingly, as this is the most complex of the models considered, this provides the best fit, and more reasonable coronal parameters. In this case, the majority of the soft excess flux arises in the warm corona component. However, based on recent simulations of warm coronae, it is not clear whether such a structure can really exist at the low accretion rates relevant for ESO 362-G18 (ṁ ~ 0.015). This may therefore argue in favor of a scenario in which the soft excess is instead dominated by the relativistic reflection. Based on this model, we find that the data would require a compact hot corona (h ~ 3 R_(Horizon)) around a rapidly spinning (a_★ > 0.927) black hole.
© 2021. The American Astronomical Society. Received 2020 August 10; revised 2021 March 24; accepted 2021 March 30; published 2021 May 19. We thank the referee for their constructive comments. J.A.G. acknowledges support from NASA grant NNX17AJ65G and from the Alexander von Humboldt Foundation. He is also a member of Teams 458 and 486 at the International Space Science Institute (ISSI), Bern, Switzerland, and acknowledges support from ISSI during the meetings in Bern. R.M.T.C. has been supported by NASA grant 80NSSC177K0515. D.J.W. acknowledges support from STFC through an Ernest Rutherford fellowship. This work was partially supported under NASA contract No. NNG08FD60C and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA. We thank the NuSTAR operations, software, and calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS), jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). Facilities: NuSTAR (Harrison et al. 2013), XMM-Newton (Jansen et al. 2001), Athena (Nandra et al. 2013), eXTP (Zhang et al. 2017), HEX-P (Madsen et al. 2018). Software: xillver (García & Kallman 2010; García et al. 2013), relxillD/relxilllpD (Dauser et al. 2014; García et al. 2014, 2016), borus12 (Baloković et al. 2019).
Accepted Version - 2103.17002.pdf
Published - Xu_2021_ApJ_913_13.pdf