The soft excess of the NLS1 galaxy Mrk 359 studied with an XMM-Newton-NuSTAR monitoring campaign

¹ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
e-mail: riccardo.middei@ssdc.asi.it
² Space Science Data Center, SSDC, ASI, Via del Politecnico snc, 00133 Roma, Italy
³ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
⁴ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵ INAF-Osservatorio di astrofisica e scienza dello spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
⁶ INAF/Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere, 00133 Roma, Italy
⁷ ASI – Unità di Ricerca Scientifica, Via del Politecnico snc, 00133 Roma, Italy
⁸ Núcleo di Astronomía de la Facultad dei Ingeniería, Universitad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile

Received: 7 April 2020
Accepted: 10 June 2020

Abstract

Context. Joint XMM-Newton and NuSTAR multiple exposures allow us to disentangle the different emission components of active galactic nuclei (AGNs) and to study the evolution of their different spectral features. In this work, we present the timing and spectral properties of five simultaneous XMM-NewtonandNuSTAR observations of the Narrow Line Seyfert 1 galaxy Mrk 359.

Aims. We aim to provide the first broadband spectral modeling of Mrk 359 describing its emission spectrum from the UV up to the hard X-rays.

Methods. We performed temporal and spectral data analysis, characterising the amplitude and spectral changes of the Mrk 359 time series and computing the 2–10 keV normalised excess variance. The spectral broadband modelling assumes the standard hot Comptonising corona and reflection component, while for the soft excess we tested two different models: a warm, optically thick Comptonising corona (the two-corona model) and a reflection model in which the soft-excess is the result of a blurred reflected continuum and line emission (the reflection model).

Results. High and low flux states were observed during the campaign. The former state has a softer spectral shape, while the latter shows a harder one. The photon index is in the 1.75–1.89 range, and only a lower limit to the hot-corona electron temperature can be found. A constant reflection component, likely associated with distant matter, is observed. Regarding the soft excess, we found that among the reflection models we tested, the one providing the better fit (reduced χ² = 1.14) is the high-density one. However, a significantly better fit (reduced χ² = 1.08) is found by modelling the soft excess with a warm Comptonisation model.

Conclusions. The present analysis suggests the two-corona model as the best scenario for the optical-UV to X-ray emission spectrum of Mrk 359.