Considerations are presented which could serve as nourishment for a “devil's advocate” with regard to the concept of a very massive (~ 106 M⊙) black hole at the center of the Galaxy. Constraints on the BH mass given by various processes are summarized. Most attention is paid to a novel probe of the black hole by means of a “wind diagnostic,” i.e. by accounting for interaction of the BH with the wind responsible for the broad line region at the Galactic Center. All available data taken together do not require a very high mass for the BH, but a moderately massive black hole currently seems to present the prime candidacy from several alternatives.

Observations of the galactic centre region in the photon energy range 2–500 keV are reviewed. Point sources, transients, bursts and a patch of apparently diffuse emission ~1° in extent have all been observed. The relatively detailed information obtained with the Einstein observatory just above the bottom edge of the x-ray window is starting to be supplemented by observations at higher photon energies. Although there is known to be a strong, variable, source of high energy x-rays somewhere in the region there is little reason to associate it with Sgr A West, which is detectable, but relatively weak, in the energy range below 30 keV where detailed measurements have been possible.

The Caltech imaging γ-ray telescope was launched by balloon from Alice Springs, NT, Australia and performed observations of the galactic center during the period 12.62 to 13.00 April 1988 UT. The first coded-aperture images of the galactic center region at energies above 30 keV show a single strong γ-ray source which is located 0.7±0.1° from the galactic nucleus and is tentatively identified as 1E1740.7-2942. If the source is at the distance of the galactic center, it is one of the most luminous objects in the galaxy at energies from 35 to 200 keV.

Observations of the time-dependent, electron-positron annihilation line radiation and gamma-ray continuum emission from the region of the Galactic Center show that there are two components to the annihilation line emission: a variable, compact source at or near the Galactic Center, and a steady, diffuse interstellar distribution. We suggest that the annihilating positrons in the compact source, observed from 1977 through 1979, result from photon-photon pair production, most likely around an accreting black hole, and that the annihilating, interstellar positrons result from the decay of radionuclei produced by thermonuclear burning in supernovae.

Several attempts were made to detect the possible radio recombination lines of positronium near the galactic center. An absorption feature seen at λ6cm, in the D-configuration of the VLA was not confirmed by subsequent observations at λ6cm and λ20cm using the B and C configurations of the VLA. An observation at λ3mm using the IRAM 30m telescope also did not detect any line. On the basis of one recombination line photon for every positron (McClintock 1984), our non-detections imply an upper limit to the positron production rate of < 3.1 × 1043 s−1, within about 2″ of the galactic center.

MPE Compton Telescope observations of MeV radiation from the direction of the Galactic Center result in constraints on the central gamma-ray source in the Galaxy: The extent of 1.8 MeV line emission from 26Al suggests an 26Al production process with pronounced concentration towards the Galactic Center. The absence of other gamma-ray lines constrains nucleosynthesis and cosmic ray excitation parameters in the Galaxy. Calculations demonstrate that time variable Galactic Center annihilation radiation may be related to a central 26Al source.

The hard X-ray and low energy γ-ray emission from the galactic center region (GCR) has four components: a power-law continuum between 20/50 keV and 200/300 keV with a power-law photon index β in the range ~ 2.5 to ~ 3.1; a harder spectrum with β ~ 1.–1.5 between 200/300 keV and ~ 511 keV; a narrow electron-positron annihilation line at 511 keV, reported to disappear in less than < 1/2 yr, although the temporal variation is controversial; and an equally variable continuum emission between 511 keV and several MeV (“MeV bump”). All four have luminosities 1037–1038 erg s−1, if they are located 10 kpc away. We propose non-thermal processes in low mass X-ray binaries (LMXB's) concentrated in the galactic bulge as the direct source of the three continuum components of the emission, as well as of an escaping electron-positron e± wind whose positron annihilation relatively far from the star could be the source of the 511 keV line. We consider a model for energetic emission from LMXB's that reproduces the softer power-law component of the GCR continuum through synchrotron emission of relativistic electrons in the strongly non-uniform (dipolar) magnetic field of the neutron star. We also explain, with less confidence, the variable MeV bump as the result of interaction of harder γ-rays with the power-law photons. The harder power law might be due to Compton scattering of relativistic electrons or photons.

The GRASP mission (Gamma-ray Astronomy with Spectroscopy and Positioning) is currently under study as an ESA space astronomy mission to be launched in the mid 90's. GRASP is designed as a high quality spectral imager (E/ΔE ≈ 500 at 1 Mev) with positioning to the arc minute level within a large field of view (≈7°) which operates over a wide spectral range (30 Kev-100 Mev) with a 3σ sensitivity of typically 10 mcrab or better over the entire operational range within an observational period of ≈105 seconds. In this paper, we will mainly discuss the capability of the instrument with respect to the study of both point source and diffuse source measurements of the galactic center region.

It is generally accepted that massive black holes are the most likely source for the energy radiated from active galactic nuclei, and may explain the enormous amount of energy emitted by quasars, radio galaxies, Seyfert galaxies, and BL Lacertid objects. Although the detailed mechanisms of the black hole formation in galactic nuclei are not clear at present, it seems to be quite possible that the formation of massive black holes is a general outcome of the evolution of galactic nuclei.

Well, you all look a little tired, and it's getting late; perhaps now is a good time to offer you some oversimplifications…….… – I've been sitting here this week listening to a large number of presentations of a very high caliber. There can be no doubt that this has been a remarkable conference. We've all learned a vast amount; I can't possibly make a coherent explanation of what I've seen and heard, any more than any one of you can. But I want to emphasise a few topics that struck me very forcibly.

A conference devoted to the center of our Galaxy inevitably draws together astronomers from many different tribes speaking in tongues about what they each know to be truly important. We have heard from stellar dynamicists and users of magnificent infra-red arrays, (who are apparently only here by courtesy of a hole in the dust distribution), from molecular radio astronomers and experts on solar MHD, cosmic ray physicists and those who misguidedly believe that understanding the Galactic center is much easier than solving the mysteries of quasars. The Galactic center can lay claim to being the melting pot of astronomy and so it is quite appropriate that this conference should be held here in Southern California.