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For refcode 1994A&A...289...76W:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1994A&A...289...76W Echo mapping the Balmer-emission region in NGC 3516 I. Wanders and K. Horne Astronomical Observatory, Box 515,5-75120 Uppsala, Sweden Astronomical Institute, University of Utrecht, PO Box 80000, NL-3508 TA Utrecht, Netherlands Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Received 6 January 1994/Accepted 31 March 1994 Abstract. We use a maximum-entropy method to show that the transfer function (TF) of the broad-line region (BLR) in the Seyfert-1 galaxy NGC 3516 is time-varying. The TF decreased by nearly a factor of two over a time scale less than half a year during a monitoring campaign in 1990. We conclude that the observed time dependency is likely to be either due to variations of the spectral index of the ionizing continuum spectrum; due to a change in the number or covering fraction of broad-line clouds; due to nonlinear line response; or due to nonstationary anisotropy of the continuum source on time scales of several weeks to months. An extended continuum source could also explain the observed time-dependency. The symmetry of the time lag for variations in the red and blue wings of H{alpha} indicates that the BLR kinematics is not dominated by organized radial inflow or outflow. If we assume circular orbits, the observed time lag of 14 days at velocity v = 4000 km s^-1^ suggests a mass of the central object of ~2 x 10^7^ M_sun_. The H{alpha} TF peaks away from zero delay, indicating that the H{alpha} BLR is either non-spherical or inwardly emitting. Key words: methods: data analysis - galaxies: active - galaxies: NGC 3516 - galaxies: quasars: emission lines - galaxies: Seyfert
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