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From: allan@noao.UUCP
Newsgroups: net.physics,net.astro.expert
Subject: Re: quasars and the history of the universe
Message-ID: <344@noao.UUCP>
Date: Fri, 25-May-84 12:01:30 EDT
Article-I.D.: noao.344
Posted: Fri May 25 12:01:30 1984
Date-Received: Fri, 1-Jun-84 05:34:58 EDT
References: <7790@lanl-a.UUCP>
Organization: Natl. Optical Astronomy Obs.  Tucson AZ USA
Lines: 20

I certainly agree that quasars are far away, however, the problem with the
timescale for quasars variability is not quite as bad as you seemed to imply.
Certainly there is no way that a quasar can vary in brightest by a large 
amount on a time shorter than the light travel time across the source of the
energy. The favourite models have the energy being generated close to a black
hole, and so the minimum time is the light travel time across this distance.
Those quasars that have been found to be variable do have times of variation
that are longer than this minimum time, although there are some that are very
close to the limit.
There is a problem with some BL Lac objects though. Some of these (e.g. 3C446)
have been found to vary faster than the minimum time allowed. The usual way
to argue around this is to say that the source of the radiation is travelling
towards us in a beam so that any variations appear to be compressed in time
due to relativistic effects. There are problems with this model and it is not
universally accepted, but at least it does explain the time variations.


Peter (theories to go) Allan
Kitt Peak National Observatory
Tucson, Az