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The COMPTEL experiment aboard CGRO, exploring the previously unknown sky at MeV-energies, has so far detected 10 Active Galactic Nuclei (AGN): 9 blazars and the radio galaxy Centaurus A. No Seyfert galaxy has been found yet. With these results COMPTEL has opened the field of extragalactic Gamma-ray astronomy in the MeV-band., Comment: 4 pages, 2 figures including 1 color plot, to appear in the Proceedings of the 3rd INTEGRAL Workshop "The Extreme Universe", held in Taormina, Italy, 14-18 September 1998

The Core Programme of the INTEGRAL mission is defined as the portion of the scientific programme covering the guaranteed time observations for the INTEGRAL Science Working Team. This paper describes the current status of the Core Programme preparations and summarizes the key elements of the observing programme., Comment: Contributed paper, 3rd INTEGRAL Workshop, Taormina/Sicily, Sep 1998, to be published in Astrophys. Letters & Communications, 1999

We report the COMPTEL detection of pulsed $\gamma$-emission from PSR B1509-58 up to at least 10 MeV using data collected over more than 6 years. The 0.75-10 MeV lightcurve is broad and reaches its maximum near radio-phase 0.38, slightly beyond the maximum found at hard X-rays/ soft $\gamma$-rays. In the 10-30 MeV energy range a strong source is present in the skymap positionally consistent with the pulsar, but we do not detect significant pulsed emission. However, the lightcurve is consistent with the pulse shape changing from a single broad pulse into a double-peak morphology. Our results significantly constrain pulsar modelling., Comment: 4 Pages, 2 Figures including one color plot, to appear in Proceedings 3rd INTEGRAL Workshop : The extreme Universe, Taormina, Italy, 14-18 sept. 1998

Large-scale skymapping with COMPTEL using the full survey database presents challenging problems on account of the complex response and time-variable background. A new approach which attempts to address some of these problems is described, in which the information about each observation is preserved throughout the analysis. In this method, a maximum-entropy algorithm is used to determine image and background simultaneously. Because of the extreme computing requirements, the method has been implemented on a parallel computer, which brings large gains since the response computation is fully parallelizable. The zero level is left undetermined in this method. Results using data from 7 years of COMPTEL data are presented., Comment: 4 pages including 12 figures (9 in color), latex2e, uses epsfig.sty. A paper presented at the 3rd INTEGRAL Workshop "The Extreme Universe" (14-18 Sep. 1998, Taormina, Italy), paper #15, in press

We use recent data obtained by three (OSSE, BATSE, and COMPTEL) of four instruments on board the Compton Gamma Ray Observatory, to construct a model of Cyg X-1 which describes its emission in a broad energy range from soft X-rays to MeV gamma-rays self-consistently. The gamma-ray emission is interpreted to be the result of Comptonization, bremsstrahlung, and positron annihilation in a hot optically thin and spatially extended region surrounding the whole accretion disk. For the X-ray emission a standard corona-disk model is applied. We show that the Cyg X-1 spectrum accumulated by the CGRO instruments during a ~4 year time period between 1991 and 1995, as well as the HEAO-3 gamma1 and gamma2 spectra can be well represented by our model. The derived parameters match the observational results obtained from X-ray measurements., Comment: 11 pages including 6 ps-figures and 2 tables, latex2e, uses emulateapj.sty (ver. of 18 Sep 96, enclosed), epsfig.sty, times.sty. To appear in July 20, 1998 issue of ApJ (v.502)

The COMPTEL experiment aboard the Compton Gamma-Ray Observatory (CGRO) has observed the gamma-ray blazar 3C 279 several times between April 1991 and September 1995. This paper reports on a consistent analysis of these observations using the most recent COMPTEL data analysis tools. Detections and non-detections of 3C 279 along the CGRO-mission indicate a time-variable MeV-flux. Spectral variability is indicated as well, however can not be significantly proven by spectral fitting. The average MeV-spectrum of 3C 279, as measured by COMPTEL over the four-year period, is consistent with a photon power-law slope of about -1.9. This spectrum smoothly connects to the simultaneous 30 MeV to 10 GeV spectrum obtained from an analysis of the combined EGRET observations. No spectral break is required to fit the time-averaged MeV- to GeV-spectrum of 3C 279., Comment: 4 pages, 5 postscript figures, latex, appeared in: 'Proc. of 25th ICRC', Vol. 3, 101 (1997); eds. M.S. Potgieter, B.C. Raubenheimer, D.J. van der Walt, Durban, South Africa

The COMPTEL observations of the blazar-type quasar PKS 0528+134 in the energy range 0.75 MeV to 30 MeV carried out between April 1991 and September 1994 have been analyzed. During the first two years PKS 0528+134 was most significantly detected at energies above 3 MeV. During the last year there is only evidence for the quasar at energies below 3 MeV indicating a spectral change. The time-averaged COMPTEL energy spectrum between 0.75 MeV and 30 MeV is well represented by a power-law shape. Spectra collected from different observational periods reveal different power-law shapes: a hard state during flaring observations reported by EGRET, and a soft state otherwise. The combined simultaneous EGRET and COMPTEL spectra indicate these two spectral states as well. During low intensisty gamma-ray phases no spectral break is obvious from the combined COMPTEL and EGRET measurements. For the gamma-ray flaring phases however, the combined COMPTEL and EGRET data require a spectral bending at MeV-energies. By fitting broken power-law functions the best-fit values for the break in photon index range between 0.6 and 1.7, and for the break energy between ~5 MeV and ~20 MeV. Because the flux values measured by COMPTEL below 3 MeV in both states are roughly equal, the observations would be consistent with an additional spectral component showing up during gamma-ray flaring phases of PKS 0528+134. Such a component could be introduced by e.g. a high-energy electron-positron population with a low-energy cutoff in their bulk Lorentz factor distribution. The multiwavelength spectrum of PKS 0528+134 for gamma-ray flaring phases shows that the major energy release across the entire electro-magnetic spectrum is measured at MeV-energies., Comment: 10 pages, 8 postscript figures, latex, to appear in: A&A 328, 33 (1997)

The EGRET experiment aboard the Compton Gamma-Ray Observatory (CGRO) has observed at energies above 100 MeV strong gamma-ray flares with short-term time variability from the gamma-ray blazars 3C 279 and PKS 1622-297. During these flaring periods both blazars have been detected by the COMPTEL experiment aboard CGRO at photon energies of about 10 MeV, revealing simultaneous gamma-ray activity down to these energies. For both cases the derived fluxes exceed those measured in previous observations, and 3C 279 shows an indication for time variability within the observational period. Both sources show evidence for `hard' MeV spectra. In general the behaviour of both sources at gamma-ray energies is found to be quite similar supporting the conclusion that the underlying physical mechanism for both gamma-ray flares might be the same., Comment: 5 pages, 4 postscript figures, latex, aipproc.sty, aipproc.cls, to apear in: "Proceedings of the Fourth Compton Symposium," 1997, eds. C. D. Dermer, M. S. Strickmann, and J. D. Kurfess, AIP Conf. Proc. 410 (New York: AIP)

We construct a model of Cyg X-1 which describes self-consistently its emission from soft X-rays to MeV gamma rays. Instead of a compact pair-dominated gamma-ray emitting region, we consider a hot optically thin and spatially extended proton-dominated cloud surrounding the whole accretion disc. The gamma-ray emission is due to the bremsstrahlung, Comptonization, and positron annihilation, while the corona-disc model is retained for the X-ray emission. We show that the Cyg X-1 spectrum accumulated by OSSE, BATSE, and COMPTEL in 1991--95, as well as the HEAO-3 gamma1 and gamma2 spectra can be well fitted by our model. The derived parameters are in qualitative agreement with the picture in which the spectral changes are governed by the mass flow rate in the accretion disc. In this context, the hot outer corona could be treated as the advection-dominated flow co-existing with a standard thin accretion disc., Comment: 5 pages including 2 figures, latex, aipproc.sty, aipproc.cls, epsfig.sty. To be published in Proc. 4th Compton Symp., 1997 (27-30 April, Williamsburg, Virginia)

The COMPTEL instrument on the Compton Gamma Ray Observatory is used to measure the locations of gamma-ray bursts through direct imaging of MeV photons. In a comprehensive search, we have detected and localized 29 bursts observed between 1991 April 19 and 1995 May 31. The average location accuracy of these events is 1.25$\arcdeg$ (1$\sigma$), including a systematic error of $\sim0.5\arcdeg$, which is verified through comparison with Interplanetary Network (IPN) timing annuli. The combination of COMPTEL and IPN measurements results in locations for 26 of the bursts with an average ``error box'' area of only $\sim$0.3 deg$^2$ (1$\sigma$). We find that the angular distribution of COMPTEL burst locations is consistent with large-scale isotropy and that there is no statistically significant evidence of small-angle auto-correlations. We conclude that there is no compelling evidence for burst repetition since no more than two of the events (or $\sim$7% of the 29 bursts) could possibly have come from the same source. We also find that there is no significant correlation between the burst locations and either Abell clusters of galaxies or radio-quiet quasars. Agreement between individual COMPTEL locations and IPN annuli places a lower limit of $\sim$100~AU (95% confidence) on the distance to the stronger bursts., Comment: Accepted for publication in the Astrophysical Journal, 1998 Jan. 1, Vol. 492. 33 pages, 9 figures, 5 tables

We estimate the 1.8 MeV luminosity of the Sco-Cen association due to radioactive decay of 26Al to (4-15) 10e-5 ph cm**-2 s**-1. We propose a low surface brightness, limb brightened bubble for the 1.8 MeV intensity distribution. The detectibility of this distribution with existing gamma-ray telescopes is discussed., Comment: 4 pages, LaTeX, lamuphys macro, to be published in "Lecture Notes in Physics"

An extensive program for the calculation of galactic cosmic-ray propagation has been developed. Primary and secondary nucleons, primary and secondary electrons, and secondary positrons are included. The basic spatial propagation mechanisms are (momentum-dependent) diffusion, convection, while in momentum space energy loss and diffusive reacceleration are treated. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields. Predictions of the diffuse Galactic gamma ray and synchrotron emission are made. Our new estimate of the cosmic-ray positron spectrum is compared with the latest experimental data., Comment: 4 pages including 12 figures, latex2e, epsfig.sty, wrapfig.sty, times.sty, mathptm.sty. Proc. 25th International Cosmic Ray Conference (Durban, South Africa), 1997, v. 4, p. 257-260. Details can be found at http://www.gamma.mpe-garching.mpg.de/~aws/aws.html N.B. An error in Fig.6 (a scale of the axis Intensity) is now corrected

We present an analysis of COMPTEL observations made between November 1991 and May 1994 of 2CG 135+01, a bright gamma-ray source located near the Galactic plane. At energies above 1 MeV, an excess consistent with the position of 2CG 135+01 is detected in the sum of the observations, at flux levels which are a factor of 10-100 below those published in the past. The detection significance of this excess, when the possible presence of underlying Galactic diffuse emission is neglected, is 6.6 sigma for 3 degrees of freedom. The differential photon spectrum in the 1-30 MeV energy range can be described by a power law with a spectral index of $1.95^{+0.2}_{-0.3}$. Due to the uncertainties involved in modelling the Galactic-disk diffuse emission underneath the source, the absolute flux levels must be considered uncertain by a factor of two. They are consistent with the extrapolation of the time-averaged spectrum of 2CG 135+01 measured with EGRET, thereby strengthening the identification. No significant temporal correlation between the gamma-ray emission and the monitored radio emission of the possible counterpart radio source GT 0236+610 (showing a 26.5 day modulation) is found., Comment: 9 pages, uses l-aa.sty, five postscript figures included. Also obtainable as postscript file at ftp://estsa2.estec.esa.nl/pub/sciproj/rvdijk_etal_AaA_23_04_96.ps.gz Accepted for publication in A&A

COMPTEL imaging analysis revealed a patchy, asymmetric distribution of diffuse 1.8 MeV emission along the Galactic plane, which is attributed to the decay of radioactive 26Al in the ISM. If massive stars were the major source of Galactic 26Al, the 1.8 MeV emission should be asymmetric and trace the spiral arms of the Galaxy, presumed site of massive star formation. Using model fits, we indeed find weak evidence in the COMPTEL data that the observed 1.8 MeV emission is at least partly confined to spiral arms. We derive a total Galactic 26Al mass of 2.5 Msol from which at least 0.7 Msol can be attributed to massive stars., Comment: For further preprints see http://www.gamma.mpe-garching.mpg.de/~jrk/jrk.html

Significant 1.8 MeV emission from the Carina region has been detected by COMPTEL. The emission is concentrated within 6 degrees or less near the Carina nebula NGC 3372, one of the brightest HII regions known in our Galaxy. This region contains a wealth of extreme young open clusters whose massive stars possibly contributed to an enrichment of 26Al in the ISM within the last few million years. The relation of these clusters and the peculiar object Eta Carinae with the observed emission is discussed. The 26Al yield of the clusters is estimated using current theoretical nucleosynthesis models., Comment: 4 pages, accepted by A&AS, for further preprints see http://www.gamma.mpe-garching.mpg.de/~jrk/jrk.html

We have analyzed the Ulysses, BATSE, and COMPTEL spectral data from the $\gamma$-ray burst of June 22, 1992 (GB 920622). COMPTEL data reveal a hard to soft evolution within the first pulse of the burst, while the mean hardness ratios of the three pulses are the same. Unlike the single instrument spectra, the composite spectrum of GB 920622 averaged over the total burst duration ranging from 20 keV up to 10 MeV cannot be fit by a single power law. Instead, the spectrum shows continuous curvature across the full energy range. COMPTEL imaging and BATSE/Ulysses triangulation constrain the source location of GB 920622 to a ring sector 1.1 arcmin wide and 2 degrees long. This area has been searched for quiescent X-ray sources using \ros survey data collected about two years before the burst. After the optical identification of the X-ray sources in and near the GRB location we conclude that no quiescent X-ray counterpart candidate for GB 920622 has been found., Comment: 12 pages uuencoded compressed postscript with figures Accepted for publ. in A&A