Letter to the Editor

The optically elusive, changing-look active nucleus in NGC 4156

¹ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, FI, Italy
e-mail: giulia.tozzi@unifi.it
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ INFN – Sezione di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, FI, Italy
⁴ Fundación Galileo Galilei, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
⁵ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, RM, Italy
⁶ Institute of Theoretical Astrophysics, University of Oslo, Postboks 1029, Blindern, 0315 Oslo, Norway
⁷ Space Science Data Center (SSDC), ASI, Via del Politecnico snc, 00133 Roma, Italy
⁸ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁹ Dipartimento di Fisica, Università di Trieste, Via A. Valerio 2, 34127 Trieste, Italy
¹⁰ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹¹ Dipartimento di Matematica e Informatica “Ulisse Dini”, Università di Firenze, Viale G. B. Morgagni 67/a, 50134 Firenze, Italy
¹² Dipartimento di Fisica e Astronomia “Augusto Righi”, Università di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
¹³ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Pl. Jules Janssen, 92190 Meudon, France
¹⁴ Dipartimento di Fisica “E. Fermi”, Università di Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy

Received: 16 September 2022
Accepted: 13 October 2022

Abstract

We report on the changing-look nature of the active galactic nucleus (AGN) in the galaxy NGC 4156, which was serendipitously discovered thanks to data acquired in 2019 at the Telescopio Nazionale Galileo (TNG) during a student observing programme. Previous optical spectra had never shown any signatures of broad-line emission, and evidence of the AGN had come only from X-ray observations, given the optical narrow-line flux ratios made it unfeasible to unambiguously denote it as a Seyfert galaxy. Our 2019 TNG data unexpectedly revealed the appearance of broad-line components in both the Hα and Hβ profiles, along with a rise of the continuum, thus implying a changing-look AGN transitioning from a type 2 (no broad-line emission) towards a (nearly) type 1. The broad-line emission has then been confirmed by our 2022 follow-up observations, whereas the rising continuum is no longer been detected, which hints at a further evolution backwards to a (nearly) type 2. The presence of broad-line components also allowed us to obtain the first single-epoch estimate of the black hole mass (log(M_BH/M_⊙)∼8.1) in this source. The observed spectral variability might be the result of a change in the accretion activity of NGC 4156, although variable absorption cannot be completely excluded.