On the possible stellar origin of the optical variability of NGC 4151

It has been suggested that the activity seen in many active galactic nuclei (AGN) may be powered solely by young stars and compact supernova remnants in a nuclear burst of star formation. Owing to its proximity and historically well-sampled light curve, the type 1 Seyfert NGC 4151 is, among galaxies with AGN, the best candidate to test the origin of the observed long-term nuclear variability in low-luminosity AGN. We find that the energies and light curves of observed supernovae (SNe) and compact supernova remnants (cSNRs) are similar to those of the isolated variations of the nucleus of NGC 4151. Two strong predictions, valid for 10-to 60-Myr-old high-metallicity starbursts, are discussed. (i) The number of peaks of the light curve, or supernova rate, is uniquely related to the nuclear luminosity during the minimum of the light curve. This result is almost independent of the initial mass function, age and/or total mass of the cluster. For NGC 4151, the minimum luminosity M_B = – 19.5 mag indicates an SN rate ν_SN = 0.2–0.3 yr^–1. (ii) The time-averaged equivalent width of H$$\beta(\overline{W_{H\beta}})$$ is related to the total energy of the SN, almost independently of the initial mass function, age and/or total mass of the cluster and, furthermore, independently of the assumed cosmology. As a corollary, we conclude that the narrow range of observed values of $$(\overline{W_{H\beta}})$$ in AGN that show broad emission lines is a direct consequence of the universal value of the energy released in an SN explosion. For NGC 4151, $$(\overline{W_{H\beta}})$$ = 78 Å gives an energy per SN of 3 × 10⁵¹ erg. Monte Carlo simulations of the light curve are presented. Each of the three input parameters of the models, the SN rate, the total energy per SN and the density of the circumstellar medium, is well constrained by two independent sets of observations. The variability of the object fulfils the predictions of the model. The simulated light curves are remarkably similar to the observed one. The best fit of the simulated light curves to that of NGC 4151 is also produced for a supernova rate ν_SN ≈ 0.2–0.3 yr^–1, an energy per SN of about 3 × 10⁵¹ erg, and a cSNR circumstellar density of about 10⁷cm^–3.