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We report on the changing-look nature of the active galactic nucleus (AGN) inthe galaxy NGC 4156, as serendipitously discovered thanks to data acquired in2019 at the Telescopio Nazionale Galileo (TNG) during a students' observingprogramme. Previous optical spectra had never shown any signatures ofbroad-line emission, and evidence of the AGN had come only from X-rayobservations, being the optical narrow-line flux ratios unable to unambiguouslydenote this galaxy as a Seyfert. Our 2019 TNG data unexpectedly revealed theappearance of broad-line components in both the H$\alpha$ and H$\beta$profiles, along with a rise of the continuum, thus implying a changing-look AGNtransitioning 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-upobservations, whereas the rising continuum has no longer been detected, whichhints at a further evolution backwards to a (nearly) type 2. The presence ofbroad-line components also allowed us to obtain the first single-epoch estimateof the black hole mass (log(MBH/Msun) $\sim$ 8.1) in this source. The observedspectral variability might be the result of a change in the accretion activityof NGC 4156, although variable absorption cannot be completely excluded.

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