Requirements:

• Effective area at least 5 times that of EGRET, for high counting rate; substantially increased FOV for total pulse statistics.
• Moderate spectral resolution from 10 MeV to 100 GeV; excellent calibration at low energies to allow comparison  with Compton/Scintillator detectors.
• Source locations one arcmin or better to allow deep X-ray and radio searches for pulsars.
• All-sky monitoring of hard (10-200 keV) X-rays with sensitivity at least two orders of magnitude better than  HEAO-1.

2.3 EXTRAGALACTIC ASTROPHYSICS

2.3.1 SEYFERTS

Prior to the launch of the CGRO, only four AGN had been detected significantly at energies above 100 keV; these are NGC 4151, 3C 273, Centaurus A, and MCG 8-11-11. The CGRO instruments clearly separate the high-energy emission of AGN into a gamma-ray blazar class discovered by EGRET and characterized by strong emission above 30 MeV and a Seyfert class seen by OSSE below a few hundred keV with softer spectra which cut off below the EGRET band. The blazar AGN are generally identified with core-dominant radio sources such as flat-spectrum quasars and BL Lac objects. The distinction between the blazars and Seyfert classes of AGN is thought to be the importance of beamed, relativistic jet emission which presumably dominates in blazars relative to emissions associated with an accretion disk which dominate in Seyfert AGN. The X-ray emission of Seyferts can be described by both thermal and nonthermal mechanisms. Key measurements in hard X-rays and gamma rays are needed to resolve the thermal/nonthermal nature of the energy source.

In nonthermal scenarios, the creation of pairs through g-g -> e+-e- interactions significantly alters the emerging high-energy spectrum. Under certain source density-luminosity conditions, broadened pair annihilation features would be observable. In addition, the continuum spectral shape above100 keV should be different for nonthermal vs. thermal emission mechanisms. OSSE has detected significant emission above 50 keV from 14 Seyferts. The average spectrum above 50 keV is adequately described by either a power law with energy spectra index a = 2.4 or by a simple exponential with an e-folding energy of 45 keV. Correlated observations with Ginga and/or ROSAT on specific bright sources provides more physical interpretation of the emission. In the case of NGC 4151, the brightest Seyfert detected by OSSE, the spectrum is clearly cutoff. IC 4329A is a more representative Seyfert 1 AGN but is significantly weaker than NGC 4151 in the OSSE band. Combined analysis of IC 4329A data from Ginga, ROSAT, and OSSE require no strong cutoff as in NGC 4151 with the cutoff limit constrained-only to be above 250 keV; consequently both thermal and nonthermal models are acceptable. A simple model of Comptonization in a relativistic, optically-thin, thermal corona above the surface of an accretion disk provides a good description of the observed spectrum.