A good basic introduction to active galaxies, Active Galactic Nuclei (AGN) and Blazars are the "Imagine the Universe" web pages from NASA:

Active Galaxies - level 1 (Seyferts, Quasars, Blazars)
Active Galaxies - level 2 (Seyferts, Quasars, Blazars)

A schematic view of the current picture of the diferent types of AGN is the icon on the top left of this page. View a larger version by clicking on it.

Detected AGN with COMPTEL :

Cen A   ,   PKS 0528+135   ,   3C 273   ,   3C 279   ,  

CTA 102   ,   3C 454.3   ,   GRO J0516-609   ,   PKS 0208-512   ,   PKS 1222+216   ,   PKS 1622-297    

      Centaurus A :

Centaurus A is the nearest AGN and it has been observed many times with the MPE-built COMPTEL instrument on board CGRO and other instruments. Quite a number of publications with MPE members as authors and a set of dedicated Web Pages is the result.

• The Dedicated Centaurus A Web pages
  H. Steinle

• Centaurus A: A Multifrequency Review
  Steinle H.
  Chin. J. Astron. Astrophys. 6, Suppl. 1, 106-113 (2006)
  ( Fig. 4 is not printed correctly! Use correct Fig. 4 instead!)

• BeppoSAX Observations of Centaurus A: The Hard Continuum and the Iron Line Feature
  Grandi P., Fiocchi M., Perola C.G., Urry M.C., Maraschi L., Massaro E., Matt G., Preite-Martinez A., Steinle H., and Collmar W.
  Astrophys. Journ. 593, 160-168 (2003)

• The Hard XRAY Spectrum of the Radio-Galaxy Centaurus A
  Fiocchi M., Grandi P., Perola G.C., Matt G., Massaro E., Maraschi L., Urry C.M., Giommi P., Steinle H., Collmar W.
  Proceedings of the international workshop
  "Blazar Astrophysics with BeppoSAX and Other Observatories"
  Frascati, Italy, December 10-11, 2001
  Eds.: Giommi P., Massaro E., Palumbo G.
  ESA-ESRIN 55-58 (2002)

• The spectral energy distribution of Centaurus A (NGC 5128)
  - A summary of all observations including all CGRO results -
  Steinle H.
  In: Proceedings Gamma 2001 - Gamma-Ray Astrophysics 2001, April 4-6, 2001,Baltimore, MD, USA,
  eds.: S. Ritz, N. Gehrels and Ch. Shrader
  AIP Conf. Proc. 587, 353-357 (2001)

• Monitoring NGC 5128 (Centaurus A) at X-ray and Gamma-ray Wavelenghts
  Steinle H.
  in: Probing the Physics of Active Galactic Nuclei by Multiwavelength Monitoring,
  eds.: Peterson B.M., Polidan R.S., and Pogge R.W.
  ASP Conference Series Vol 224, 183-191 (2001)
  ( in Fig. 5 the EGRET data point is plotted wrong. It should be at 11.94 [Jy Hz] instead of 11.04 [Jy Hz].)

• A bright X-ray transient towards NGC 5128 (Centaurus A)
  Steinle H., Dennerl K., and Englhauser J.
  Astron. Astrophys. 357, L57-L60 (2000)

• Centaurus A: Multiwavelength Observations of the nearest Active Galaxy from Radio to Gamma-Rays
  Steinle H., Bonnell J., Kinzer R.L., Tzioumis A., Bignall H., Schönfelder V., Bennett K., Hermsen W., Ryan J.
  in: Proceedings of the 32nd COSPAR Scientific Assembly, Nagoya, Japan, 1998;
  Adv. Space Res. Vol. 23 No. 5/6, pp. 911-914 (1999)

• COMPTEL observations of Centaurus A at MeV energies in the years 1991 to 1995
  Steinle H., Bennett K., Bloemen H., Collmar W., Diehl R., Hermsen W., Lichti G.G., Morris D., Schönfelder V., Strong A.W., and Williams O.R.
  Astron. & Astrophys. 330, 97-107 (1998)

• Compton Gamma-Ray Observatory Observations of the Nearest Active Galaxy Centaurus A
  Steinle H., Bennett K., Bloemen H., Collmar W., Diehl R., Hermsen W., Lichti G.G., Morris D., Schönfelder V., Strong A.W., Williams O.R.
  in: Proceedings of the Fourth Compton Symposium, Williamsburg, VA (USA) 1997,
  part II: Papers and Presentations;
  eds. C.D. Dermer M.S. Strickman, and J.D. Kurfess, (New York: AIP)
  AIP Conf. Proc. 410, 1298-1302 (1997)

• Seyferts and Radio Galaxies
  Johnson W.N., Zdziarski A.A., Madejski G.M., Paciesas W.S., Steinle H., Lin Y-C.
  in: Proceedings of the Fourth Compton Symposium, Williamsburg, VA (USA) 1997,
  part I: The Compton Observatory in Review;
  eds. C.D. Dermer M.S. Strickman, and J.D. Kurfess, (New York: AIP)
  AIP Conf. Proc. 410, 283-305 (1997)

• CGRO-COMPTEL observations of the Centaurus A Region
  Steinle H., Bennett K., Bloemen H., Collmar W., Diehl R., Hermsen W., Lichti G., McConnell M., Morris D., Much R., Ryan J., Schönfelder V., Strong A.W., Varendorff M., and Winkler C.
  XXX COSPAR, Hamburg 1994
  Adv. Space Res. Vol. 15, No. 5, pp. (5)37-(5)40 (1995)

• COMPTEL Observations Of AGNs
  Steinle H., Bloemen H., Collmar W., Diehl R., Hermsen W., Lichti G., McConnell M., Ryan J., Schoenfelder V., Stacy G., Strong A.W., Swanenburg B.N., Varendorff M. and Williams O.R.
  Adv. Space Res. Vol. 13, No. 12, pp. 731-734 (1993)

PKS 0528+135 :

PKS 0528+135 is a difficult object to observe with COMPTEL, as it is only few degrees offset from the strongest MeV source in the sky - the Crab.

• Spectral variability in PKS 0528+134 at gamma-ray energies.
  Böttcher M., W. Collmar
  Astron. Astrophys. Lett., 329, L57 - L60 (1998).

• COMPTEL observations of the quasar PKS 0528+134 during the first 3.5 years of the CGRO mission
  Collmar W., Bennett K., Bloemen H., Blom J.J.,, Hermsen W., Lichti G.G., Pohl M., Ryan J., Schönfelder V., Stacy J.G., Steinle H., and Williams O.R.
  Astron. Astrophysics 328, 33-42 (1997)

3C 273 :

• Simultaneous Observations of the Continuum Emission of the Quasar 3C 273 from Radio to Gamma-ray Energies
  Lichti G.G., von Montigny C., Balonek T., Courvoisier T.J.-L., Johnson N., McConnell M., Paciesas W., Robson E.I., Sadun A., Schalinski C., Steppe H., Smith A.G., Staubert R., Swanenburg B.N., Turner M.J.L., Ulrich M.-H., and Williams O.R.
  In: Proc. of 2nd Compton Symposium, University of Maryland, Washington, 1993,
  ed. Fichtel C.E., Gehrels N. and Norris J.P.
  AIP Conf. proc. 304, pp. 611-615 (1994)
  Electronically avaiable: scanned article (pdf)
  

• A Large Gamma-ray Flare from the Blazar 3C 273
  W. Collmar, O. Reimer, K. Bennett, H. Bloemen, W. Hermsen, G. Lichti, J. Ryan, V. Schönfelder, H. Steinle, O.R. Williams
  3rd Integral Workshop, Taormina, Italy (1998)
  Electronically available: poster compressed to A4 format (ps)

• Multifrequency Observations of the Virgo Blazars 3C 273 and 3C 279 in the CGRO Cycle 8
  Collmar W., Benlloch S., Grove J.E., Hartman R.C., Heindl W.A., Kraus A., Teräsranta H., Villata M., Bennett K., Bloemen H., Johnson W.N., Krichbaum T., Raiteri C.M., Ryan J., Sobrito G., Schönfelder V., Williams O.R., Wilms J.
  Proceedings of the 5th Compton Symposium, Sep 15 - 17, 1999, Portsmouth, NH, USA
  AIP, .... (2000).
  Electronically avaiable: not yet

3C 279 :

• Evidence for Gamma-Ray Flares in 3C 279 and PKS 1622-297 at ~10 MeV .
  Collmar W., V. Schönfelder, H. Bloemen, J.J. Blom, W. Hermsen, M. McConnell, J.G. Stacy, K. Bennett, and O.R. Williams
  In: Proceedings of the Fourth Compton Symposium",
  Eds. C.D. Dermer, M.S. Strickman & J.D. Kurfess (New York: AIP),
  AIP Conf. Proc., 410, 1341 - 1345 (1997).
  Electronically avaiable: ADS entry and submitted article (ps)

• Multifrequency Observations of the Virgo Blazars 3C 273 and 3C 279 in the CGRO Cycle 8
  Collmar W., Benlloch S., Grove J.E., Hartman R.C., Heindl W.A., Kraus A., Teräsranta H., Villata M., Bennett K., Bloemen H., Johnson W.N., Krichbaum T., Raiteri C.M., Ryan J., Sobrito G., Schönfelder V., Williams O.R., Wilms J.
  Proceedings of the 5th Compton Symposium, Sep 15 - 17, 1999, Portsmouth, NH, USA
  AIP, .... (2000).
  Electronically avaiable: not yet

Cen A     PKS 0528+135     3C 273     3C 279