Space Telescope and Optical Reverberation Mapping Project. IX. Velocity–Delay Maps for Broad Emission Lines in NGC 5548

2021 
Author(s): Horne, Keith; Rosa, G De; Peterson, BM; Barth, AJ; Ely, J; Fausnaugh, MM; Kriss, GA; Pei, L; Bentz, MC; Cackett, EM; Edelson, R; Eracleous, M; Goad, MR; Grier, CJ; Kaastra, J; Kochanek, CS; Krongold, Y; Mathur, S; Netzer, H; Proga, D; Tejos, N; Vestergaard, M; Villforth, C; Adams, SM; Anderson, MD; Arevalo, P; Beatty, TG; Bennert, VN; Bigley, A; Bisogni, S; Borman, GA; Boroson, TA; Bottorff, MC; Brandt, WN; Breeveld, AA; Brotherton, M; Brown, JE; Brown, JS; Canalizo, G; Carini, MT; Clubb, KI; Comerford, JM; Corsini, EM; Crenshaw, DM; Croft, S; Croxall, KV; Bonta, E Dalla; Deason, AJ; Dehghanian, M; Lorenzo-Caceres, A De; Denney, KD; Dietrich, M; Done, C; Efimova, NV; Evans, PA; Ferland, GJ; Filippenko, AV; Flatland, K; Fox, OD; Gardner, E; Gates, EL; Gehrels, N; Geier, S; Gelbord, JM; Gonzalez, L; Gorjian, V; Greene, JE; Grupe, D; Gupta, A; Hall, PB; Henderson, CB; Hicks, S; Holmbeck, E; Holoien, TW-S; Hutchison, T; Im, M; Jensen, JJ; Johnson, CA; Joner, MD; Jones, J; Kaspi, S; Kelly, PL; Kennea, JA; Kim, M; Kim, S | Abstract: We report velocity-delay maps for prominent broad emission lines, Ly_alpha, CIV, HeII and H_beta, in the spectrum of NGC5548. The emission-line responses inhabit the interior of a virial envelope. The velocity-delay maps reveal stratified ionization structure. The HeII response inside 5-10 light-days has a broad single-peaked velocity profile. The Ly_alpha, CIV, and H_beta responses peak inside 10 light-days, extend outside 20 light-days, and exhibit a velocity profile with two peaks separated by 5000 km/s in the 10 to 20 light-day delay range. The velocity-delay maps show that the M-shaped lag vs velocity structure found in previous cross-correlation analysis is the signature of a Keplerian disk with a well-defined outer edge at R=20 light-days. The outer wings of the M arise from the virial envelope, and the U-shaped interior of the M is the lower half of an ellipse in the velocity-delay plane. The far-side response is weaker than that from the near side, so that we see clearly the lower half, but only faintly the upper half, of the velocity--delay ellipse. The delay tau=(R/c)(1-sin(i))=5 light-days at line center is from the near edge of the inclined ring, giving the inclination i=45 deg. A black hole mass of M=7x10^7 Msun is consistent with the velocity-delay structure. A barber-pole pattern with stripes moving from red to blue across the CIV and possibly Ly_alpha line profiles suggests the presence of azimuthal structure rotating around the far side of the broad-line region and may be the signature of precession or orbital motion of structures in the inner disk. Further HST observations of NGC 5548 over a multi-year timespan but with a cadence of perhaps 10 days rather than 1 day could help to clarify the nature of this new AGN phenomenon.
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