R. Much

The first COMPTEL source catalogue

The imaging Compton telescope COMPTEL aboard NASAs Compton Gamma-Ray Observatory has opened the MeV gamma-ray band as a new window to astronomy. COMPTEL provided the first complete all-sky survey in the energy range 0.75 to 30xa0MeV. The catalogue, presented here, is largely restricted to published results. It contains firm as well as marginal detections of continuum and line emitting sources and presents upper limits for various types of objects. The numbers of the most significant detections are 32 for steady sources and 31 for gamma-ray bursters. Among the continuum sources, detected so far, are spin-down pulsars, stellar black-hole candidates, supernova remnants, interstellar clouds, nuclei of active galaxies, gamma-ray bursters, and the Sun during solar flares. Line detections have been made in the light of the 1.809xa0MeV 26 Al line, the 1.157xa0MeV 44 Ti line, the 847 and 1238 keV 56 Co lines, and the neutron capture line at 2.223xa0MeV. For the identification of galactic sources, a modelling of the diffuse galactic emission is essential. Such a modelling at this time does not yet exist at the required degree of accuracy. Therefore, a second COMPTEL source catalogue will be produced after a detailed and accurate modelling of the diffuse interstellar emission has become possible.

CGRO-Comptel observations of the Centaurus A region

Abstract The sky region containing the active radio-galaxy Centaurus A has repeatedly been observed with the COMPTEL instrument onboard the Compton Gamma Ray Observatory (CGRO). The nine observation periods during the CGRO phases I and II in which Cen A was in the field of view of COMPTEL are spread over 18 months in the years 1991 to 1993. The energy range 0.75 to 30 MeV is covered. Clear evidence for a source with emission up to several MeV is seen from a region coinciding with the position of Cen A. The spectra change significantly over ∼ 6 months between the two observation phases. A possible source confusion with the nearby gamma-ray source MS1312.1-4221 is discussed.

The MeV spectrum of Cygnus X-1 as observed with COMPTEL

The COMPTEL experiment on the Compton Gamma-Ray Observatory (CGRO) has observed the Cygnus region on several occasions since launch. These data represent the most sensitive observations to date of Cygnus X-1 in the 0.75–30 MeV range. The spectrum shows significant evidence for emission extending out to several MeV. These data alone suggest a need to modify the thermal Comptonization models or to incorporate some type of non-thermal emission mechanism. Here we report on the results of an analysis of selected COMPTEL data collected during the first three years of the CGRO mission. These data are then compared with contemporaneous data from both BATSE-EBOP and OSSE. Given a lack of consistency between the OSSE and BATSE-EBOP spectra, it is difficult to draw firm conclusions regarding the exact shape of the spectrum near 1 MeV. A few general conclusions can, however, be drawn from these data.

COMPTEL gamma-ray study of the Crab nebula

We re-examine the Crab nebula, using five years of COMPTEL observations. A 30 bin 0.78–30 MeV spectrum is presented. It shows a spectral break at the edge of the COMPTEL energy range, and connects well to the EGRET spectrum, probably reflecting electron energy losses in the synchrotron emission scenario. Such a smooth continuum model alone may not be sufficient to explain the observations. A weak bump in the spectrum at 1–2 MeV may be present. No evidence for distinct line emission is seen.

5 years of Crab Pulsar observations with COMPTEL

Using the COMPTEL data of the first 5 years of the CGRO mission we have derived the average pulsed spectrum of the Crab Pulsar, as well as phase-resolved spectra of the pulsed emission of the Crab Pulsar. The spectra in the COMPTEL energy range (0.75 to 30 MeV) are compared to those in the neighboring energy bands. The pulsed flux has been examined for its stability. Pulsed lightcurves have been derived for different energy intervals. Preliminary results of this analysis are presented.

Statistical analysis of COMPTEL maximum likelihood-ratio distributions: evidence for a signal from previously undetected AGN

The maximum likelihood-ratio method is frequently used in COMPTEL analysis to determine the significance of a point source at a given location. In this paper we do not consider whether the likelihood-ratio at a particular location indicates a detection, but rather whether distributions of likelihood-ratios derived from many locations depart from that expected for source free data. We have constructed distributions of likelihood-ratios by reading values from standard COMPTEL maximum-likelihood ratio maps at positions corresponding to the locations of different categories of AGN. Distributions derived from the locations of Seyfert galaxies are indistinguishable, according to a Kolmogorov-Smirnov test, from those obtained from random locations, but differ slightly from those obtained from the locations of flat spectrum radio loud quasars, OVVs, and BL Lac objects. This difference is not due to known COMPTEL sources, since regions near these sources are excluded from the analysis. We suggest that it might arise from a number of sources with fluxes below the COMPTEL detection threshold.

Further COMPTEL observations of the region around GRO J1753+57: are there several MeV sources present?

GRO J1753+57 was first reported by COMPTEL as a very bright source in the range 1–3 MeV during an observation in November 1992. It was not detected in three other observations of the same region carried out in June 1991, December 1992, and March 1993 and it was not possible to identify a likely candidate at other wavebands which lay within the 3σ location contour. Six further observations of the region have been made and after co-adding all the available data evidence has emerged for a more complex structure than previously indicated. In the analysis presented here, we show that the observed signal can be successfully modelled by emission from a combination of two or more point sources which lie outside the original 3σ location contour of GRO J1753+57, at the positions of the blazar 4C 56.27, the EGRET quasar QSO 1739+522, and the unidentified EGRET source GRO J1837+59. All these sources can reasonably be expected to be γ-ray emitters. However, uncertainties over the origin of the observed emission persis...

Evidence for emission in the MeV band from GRO J1837+59 and QSO 1739+522

Emission in the MeV band from the region containing the bright unidentified EGRET source GROu2009J1837+59 and the steep-spectrum EGRET blazar QSOu20091739+522 was first reported by COMPTEL following an observation in November 1992. During this observation the emission was consistent with a single point source, designated GROu2009J1753+57. However, the location of GROu2009J1753+57 was not consistent with either GROu2009J1837+59 or QSOu20091739+522 and identifying its counterpart in other wavelengths proved difficult. Moreover, subsequent observations suggested that the source could not in fact be described as a single point source and explanations involving extended or multiple sources were invoked. We present an analysis of recent observations of this region, which confirm that the emission cannot arise from a single source, but can be modelled as a combination of emission from both GROu2009J1837+59 and QSOu20091739+522. The spectrum and time variability derived for these two sources are discussed and compared to observations in neighbo...

Latest COMPTEL results on Galactic black hole candidates

Abstract Since its launch in April of 1991, the COMPTEL experiment on the Compton Gamma-Ray Observatory has surveyed the entire sky in the energy range of 0.75–30.0 MeV. Here we survey the latest results obtained from the COMPTEL data with respect to galactic black hole candidates. The most prominent such object, Cygnus X-1, is also one of the brightest sources observed by COMPTEL . In addition, we report on the progress of a full survey of the sky, searching for evidence of MeV emission from other black hole candidates.

The Crab nebula and pulsar in the MeV energy range

Abstract The imaging Compton Telescope COMPTEL is sensitive in the energy range 0.75 to 30 MeV. COMPTEL observed the Crab several times during the CGRO sky survey and CGRO Phase II. Both the Crab pulsar and nebula are detected over the entire COMPTEL enrgy range. The phase-averaged energy spectra of the Crab Pulsar and Nebula are presented. The combined observations provide sufficient statistics for a phase-resolved analysis of the Crab pulsar spectrum.