Galaxies and the Universe - Central Objects in Active Nuclei

Galaxies and the - Gala ie and he Uni e e Cen al Objec in Ac i e N clei Structure and Central Masses in Active Nuclei AGNs have been observed to

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
1/15/12 Gala[ieV and Whe UniYeuVe - CenWual ObjecWV in AcWiYe NXclei 1/5 ZZZ.aVWu.Xa.edX/keel/gala[ieV/agncoueV.hWml Structure and Central Masses in Active Nuclei AGNs have been observed to vary everywhere from the X-ray to radio regimes. This variability is associated with the continuum (core) sources and the BLR, with NLR variability mild (as would be inferred from its large size). Early work used broad-band photographic data (from archival plates, for example, giving spotty coverage over spans as large as 90 years) or photoelectric observations (accurate but time-consuming). These are sensitive mostly to the continuum (at low redshifts contamination by lines is usually of order 15%). The variations may be episodic (turning on and off, frustrating planning of major campaigns). They are of different kinds - flares, dips, with multiple timescales present. Timescales range from days to decades in the optical. Most QSOs are only slightly variable - 5% or so over a couple of years. Many of these results are summarized in Variabilit\ of Active Galactic Nuclei , ed. Miller and Wiita (Cambridge, 1991). In the continuum, one might look for the spectral shape of the variable component - if nothing else, there should be a nonvariable componant from the surrounding galaxy, and possible from any other large-size continuum components. In Seyferts and QSOs, the bluer wavelengths are generally most variable, and there is little or no variability in the IR. A few violently variable (OVV) QSOs and the Bl Lac objects often show "grey" variations, with no wavelength dependence. Note that these classes show polarization variations; intensity, polarization, and its direction may all vary independently. Some objects (especially in the radio) show components marching redward after a flare, as predicted by synchrotron aging theory. Much interpretation has been thwarted by the bewildering variety of forms it takes - there are no general patterns that could provide detailed physical clues beyond the light-time arguments on the general size of the emitting region. More useful is reYerberation mapping , in which the response of emission lines to continuum variations is used to study the radial distributions of line emissivity and radial velocity. Recombination is quite fast at these densities compared to the light-travel times, so that we can think of a volume of BLR gas as responding instantaneously to a change in ionizing intensity. In a first approximation, the delay time between our observation of a continuum pulse (assuming we measure a continuum that comes from the same place as the ionizing continuum) and the pulse of reprocessed line emission gives one measure of the radius of the BLR. In more detail, one determines a transfer function Ȍ such that where L is the line intensity and C is the continuum intensity. Various sizes and shapes of BLR give various forms for Ȍ. It is straightforward to show from geometrical definitions that the surface probed by any particular
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 01/15/2012 for the course AY 620 taught by Professor Williamkeel during the Fall '09 term at Alabama.

Page1 / 5

Galaxies and the - Gala ie and he Uni e e Cen al Objec in Ac i e N clei Structure and Central Masses in Active Nuclei AGNs have been observed to

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online