Arp 102B: variability patterns of the Hα line profile as evidence for gas rotation in the broad-line region

We present results of an optical spectral monitoring campaign of the Seyfert 1 galaxy Arp 102B for a period of about 4 years between 1992 July and 1996 October carried out with a CCD spectrograph at the 2.6-m Shajn Telescope of the Crimean Astrophysical Observatory. Our data set consists of 20 Hα and 1 Hβ region spectra. During the monitoring period, the Hα line and continuum fluxes have shown the root mean square (rms) variations as small as 2.7% and 5.7%, respectively. The Hα line lag greater than 66 days is ruled out at 99% confidence level. Strong rms variations at different bins of the Hα profile (5–10%), while its total flux is changed only a little can be explained by the rotational redistribution of matter emitted the broad lines. We consider two possibilities: a continuous disk with inhomogeneities in its surface brightness and a system of clouds rotating predominantly in the same plane. Different sections of the line profile generally show very poor correlation between themselves as well as with the continuum flux, while the correlation between the line integrated flux and continuum flux is much better. The line component that respond to the continuum changes is probably more prominent in the red central part of the line profile, where the correlation with the continuum fluxes is somewhat better. Some indications of the rotation are also seen from the evolution of the residuals between each normalized profile and the mean normalized profile. Typical timescale of rotation (∼ 1000 days) suggests a very large mass of the central body of about 3.5 × 108 M .