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

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(MBH/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.