This is a small article by Gary Poyner regardig one specific AGN but with info that applies to all.
S5 0716+71 – A most active AGNActive Galactic Nuclei (AGN) are some of the most remarkable objects to be observed in the Universe, and a small number of these are favourite
targets for amateur Variable Star observers. Amongst the brightest and
most active of this group is S50716+71, a Blazar located 5.6 degrees east of the 5th magnitude star TYC 4345 2051 in a rather barren part
of the constellation Camelopardalis at RA 07h 21m 53’.5 +71 20 36 (2000.0). It’s classification was first noted in the fifth 5-GHz strong
source surveys (S5). This survey covers the area between declination 70
and 90 deg. [1] It’s redshift is assumed to be as >0.3 [2], although no precise measurement has yet been obtained.
S50716 is classified as a BL Lac object (or Blazar), a subgroup of the AGN which are radio and gamma ray loud, high energy objects, and display the largest amplitudes of all AGN. Blazars generally show featureless spectra, probably due to the high relativistic beaming found in these objects. It’s thought that this increased beaming, and associated ‘shock waves’ occurring in the jet are the cause of the high variability we see in these objects, and with S50716 in particular!
This high variability proves fascinating to observe both visually and with
CCD’s. S50716 is rarely inactive, and undergoes both ‘low states’ and
outbursts. During March 2004, an historical high magnitude of 12.0 was
reached (which began three months earlier in January), triggering many
satellite TOO’s, especially in the X-ray [3]. Quite apart from periods in
outburst, S50716 also shows easy to detect variations in the order of around one magnitude over the course of several days. This phenomenon has also been observed in BL Lac and W Com, but it would appear to be more
pronounced in S50716, presumably because of the higher relativistic beaming
phenomenon, which is consistent with the ‘shock wave jet’ model. Since the 2004 outburst, S50716 has proved to be more erratic than ever, with the light curve showing just how much this object has been varying from one night to the next [fig.1]. It would appear that the ‘jet’ is almost in a constant state of activity!
As with all Blazars, S50716 shows a high level of IDV (IntraDay Variations, first detected in 1986) in both optical and radio. These small amplitude,
rapid optical variations can be detected with amateur instruments (in V, R and I), particularly when the object is bright. Less common are reports of optical ‘flickering’, but this phenomenon has been detected by experienced visual observers [4].
S50716 is an object which will reveal a high degree of variation to both
visual and CCD astronomers. It’s circumpolar from the UK, and can
therefore be observed all year round. The extreme amplitude varies between 12.0-15.0, but more usually 13.0-14.0, and is therefore quite an easy object for 20-30cm telescopes. It’s variations will keep the observer very much on his/her toes, and is guaranteed to spring a surprise on anyone who observers it for even a short while.