Tidal disruption of a low-mass star in an active galactic nucleus as the origin of the PS16dtm outburst

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Context. The event PS16dtm, which occurred in the centre of the narrow-line Seyfert 1 (NLS1) galaxy SDSS J015804.75−005221.8(z = 0.080440), is one of the few candidates for a tidal disruption event in an already-accreting active galactic nucleus.Aims. We wish to shed light on the character of the tidal disruption event in this source since it exhibits unusual peculiarities, such asthe double-peak optical/UV light curve and a low blackbody temperature with a lack of X-ray emission.Methods. We performed a spectral analysis of the source before and during the event. We modelled the time evolution of the luminosityprofile using a numerical code that describes the viscous evolution of the flow.Results. Based on the combined spectral and timing studies, we interpret the event as the disruption of a ∼0.3 M main-sequencestar or as the gradual partial disruption of a low-mass giant star. The star is likely on a circular orbit, embedded in the accretion disc.We discuss that the evolution of the star rather suggests that the orbit is counter-rotating. We observe the system at a sufficientlylarge viewing angle that the actual disruption process is not directly observed. The disrupted star and inner disc are shielded from theobserver by a gaseous envelope. Further observations of the system as it returns to the previous NLS1 state, particularly in the X-rayband, are needed to confirm the proposed scenario and to place constraints on the return to a regular NLS1 state. The enclosed data are the luminosity time profile and the subsequent models used for the data fitting. The Table gives the parameters of ech fit.