The variability of the spectrum of Arakelian 120. II - Evidence for a small broad line emitting region

Abstract

The results of four years of spectroscopic and photometric observations of the Seyfert galaxy Akn 120 are reported. The observations were carried out using the image dissector scanner attached to the Perkins 1.8-meter reflector telescope at Lowell Observatory, as well as the photon-counting Reticon dstector attached to the Steward Observatory 2.3-meter telescope. The main conclusions of the observations were: (1) the cloud features of the broad-line emitting (BLR) region of Akn 120 were optically thick due to the variation of the Balmer emission with continuum; (2) no perceptible time delay was observed between the continuum variations and H-beta variations; (3) estimates of the ionizing flux of Akn 120 led to a BLR radiation which was two orders of magnitude too large, assuming a conventional electron density of 10 to the 9th per cubic cm; and (4) the broad Balmer lines in Akn 120 were displaced redward from the systemic velocity by about 400 km/s, implying a gravitational redshift. A mass of about 7 x 10 to the 8th solar mass was estimated for the central object of Akn 120, and the emission line widths of the entire system were found to be consistent with equipartition in the BLR. A complete list of the observational parameters is given in a table.