Emission-line and continuum reverberation mapping of the NLS1 galaxy WPVS 48

Context. WPVS 48 is a nearby narrow-line Seyfert 1 galaxy without previous analysis of the broad-line region (BLR) by means of optical spectroscopic reverberation mapping. Aims. By studying the continuum and emission line variability of WPVS 48, our aim was to infer the BLR size as well as the mass of the central supermassive black hole (SMBH). Methods. We analysed data from a dedicated optical spectroscopic reverberation mapping campaign of WPVS 48 taken with the 10 m Southern African Large Telescope (SALT) at 24 epochs over a period of seven months between December 2013 and June 2014. Results. WPVS 48 shows variability throughout the campaign. We find a stratified BLR, where the variability amplitude of the integrated emission lines decreases with distance to the ionising continuum source. Specifically, the variable emission of H α , H β , H γ , and He I λ 5876 originates at distances of 16.0 +4.0 −2.0 , 15.0 +4.5 −1.9 , 12.5 +3.5 −2.5 , and 14.0 +2.5 −2.1 light-days, respectively, to the optical continuum at 5100 Å. The He II λ 4686 lag is ≲5 days. Based on the high S/N spectra, we identified variable emission of N III λ 4640 and C IV λ 4658 in the line complex with He II λ 4686. We derive interband continuum delays increasing with wavelength up to ∼8 days. These delays are consistent with an additional diffuse continuum originating at the same distance as the variable Balmer emission. We derive a central black hole mass of (1.3 +1.1 −0.6 ) × 10 7 M ⊙ based on the integrated line-widths and distances of the BLR and discuss corrections for the inclination angle. This gives an Eddington ratio L / L Edd ≈ 0.39 without correction for inclination.

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