Transient obscuration event captured in NGC 3227 : I. continuum model for the broadband spectral energy distribution

Mehdipour, M. and Kriss, G.A. and Kaastra, J.S. and Wang, Y. and Mao, J. and Costantini, E. and Arav, N. and Behar, E. and Bianchi, S. and Branduardi-Raymont, G. and Brotherton, M. and Cappi, M. and De Marco, B. and Di Gesu, L. and Ebrero, J. and Grafton-Waters, S. and Kaspi, S. and Matt, G. and Paltani, S. and Petrucci, P.-O. and Pinto, C. and Ponti, G. and Ursini, F. and Walton, D.J. (2021) Transient obscuration event captured in NGC 3227 : I. continuum model for the broadband spectral energy distribution. Astronomy and Astrophysics, 652. A150. ISSN 0004-6361 (

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From Swift monitoring of a sample of active galactic nuclei (AGN) we found a transient X-ray obscuration event in Seyfert-1 galaxy NGC 3227, and thus triggered our joint XMM-Newton, NuSTAR, and Hubble Space Telescope (HST) observations to study this event. Here in the first paper of our series we present the broadband continuum modelling of the spectral energy distribution for NGC 3227, extending from near infrared (NIR) to hard X-rays. We use our new spectra taken with XMM-Newton, NuSTAR, and the HST Cosmic Origins Spectrograph in 2019, together with archival unobscured XMM-Newton, NuSTAR, and HST Space Telescope Imaging Spectrograph data, in order to disentangle various spectral components of NGC 3227 and recover the underlying continuum. We find the observed NIR-optical-UV continuum is explained well by an accretion disk blackbody component (Tmax=10 eV), which is internally reddened by E(B - V) = 0.45 with a Small Magellanic Cloud extinction law. We derive the inner radius (12 Rg) and the accretion rate (0.1 Myr-1) of the disk by modelling the thermal disk emission. The internal reddening in NGC 3227 is most likely associated with outflows from the dusty AGN torus. In addition, an unreddened continuum component is also evident, which likely arises from scattered radiation, associated with the extended narrow-line region of NGC 3227. The extreme ultraviolet continuum, and the 'soft X-ray excess', can be explained with a 'warm Comptonisation' component. The hard X-rays are consistent with a power-law and a neutral reflection component. The intrinsic bolometric luminosity of the AGN in NGC 3227 is about 2.2 × 1043 erg s-1 in 2019, corresponding to 3% Eddington luminosity. Our continuum modelling of the new triggered data of NGC 3227 requires the presence of a new obscuring gas with column density NH = 5 × 1022 cm-2, partially covering the X-ray source (Cf = 0.6).