Flares in the changing look AGN Mrk 590 – II. Deep X-ray observations reveal a Comptonizing inner accretion flow

ABSTRACT Mrk 590 is a changing look active galactic nucleus (AGN) currently in an unusual repeat X-ray and UV flaring state. Here, we report on deep X-ray observations with XMM–Newton, NuSTAR, and NICER, obtained at a range of X-ray flux levels. We detect a prominent soft excess below 2 keV; its flux is tightly correlated with that of both the X-ray and UV continuum, and it persists at the lowest flux levels captured. Our Bayesian model comparison strongly favours inverse Comptonization as the origin of this soft excess, instead of blurred reflection. We find only weak reflection features, with $R\lesssim 0.4$ assuming Compton-thick reflection. Most of this reprocessing occurs at least $\sim$800 gravitational radii (roughly three light-days) from the continuum source. Relativistically broadened emission is weak or absent, suggesting the lack of a standard ‘thin disc’ at small radii. We confirm that the predicted broad-band emission due to Comptonization is roughly consistent with the observed UV–optical photometry. This implies an optically thick, warm ($kT_e\sim 0.3$ keV) scattering region that extends to at least $\sim 10^3$ gravitational radii, reprocessing any UV thermal emission. The lack of a standard ‘thin disc’ may also explain the puzzling $\sim 3$-d X-ray to UV delay previously measured for Mrk 590. Overall, we find that the X-ray spectral changes in Mrk 590 are minimal, despite substantial luminosity changes. Other well-studied changing look AGN display more dramatic spectral evolution, e.g. disappearing continuum or soft excess. This suggests that a diversity of physical mechanisms in the inner accretion flow may produce a UV–optical changing-look event.