Improved COMPTEL maps of the milky way

In the course of the mission we have gradually developed an analysis method that separates in an iterative manner the celestial emission and the (a priori unknown) instrumental background. It has become our standard analysis tool for point sources. We illustrate here that this method is widely applicable now. It provides mutually consistent sets of model-fitting parameters (spectra) and sky maps, both for continuum and line studies. Because of the wide applicability, it has been possible to make various cross-checks while building up confidence in this procedure

Recent results from COMPTEL observations of Cygnus X‐1

The COMPTEL experiment on the Compton Gamma‐Ray Observatory (CGRO) has now observed Cyg X‐1 on four separate occasions during phase 1 and phase 2 of its orbital mission (April, 1991 to August, 1993). Here we report on the results of the latest analysis of these data, which provide a spectrum extending to energies greater than 2 MeV. A spectral analysis of these data, in the context of a classical Comptonization model, indicates an electron temperature much higher than previous hard X‐ray measurements would suggest (200 keV vs 60–80 keV). This implies either some limitations in the standard Comptonization model and/or the need to incorporate a reflected component in the hard X‐ray spectrum. Although significant variability near 1 MeV has been observed, there is no evidence for any ‘MeV excess.

Diffuse Galactic continuum emission: Recent studies using COMPTEL data

COMPTEL full sky maximum entropy maps using 5 years of data have been produced using background estimates based on high-latitude observations. The Galactic diffuse emission can be studied using latitude and longitude profiles from these maps. Direct comparison of profiles with theoretical models is illustrated for the 10–30 MeV range. We demonstrate the presence of a broad latitude component, consistent with results from model-fitting studies. The method is also used to obtain model-independent broad-band spectra of the Galactic emission

Comptel observations of the quasar PKS 0528+134

During Phase I and Phase II of the CGRO‐mission, the quasar PKS 0528+134 was in the field of view of the COMPTEL instrument during several viewing periods. The quasar was detected by COMPTEL mainly at energies above 10 MeV. Below 10 MeV there is evidence for the source during some CGRO viewing periods, while below 3 MeV no signal is detected. The detections and non‐detections during different viewing periods follow the trend seen by EGRET, thereby indicating a time‐variable MEV‐flux of the quasar. The COMPTEL spectral results together with the simultaneously measured EGRET spectrum, indicate a spectral break in the upper part of the COMPTEL energy range at energies between 10 MeV and 30 MeV

Spectral properties of gamma‐ray bursts observed by COMPTEL

During the first year of operation, the COMPTEL instrument on board the Compton Gamma Ray Observatory detected 22 γ‐ray bursts within its field of view. Spectra and time histories for the strongest 7 of these bursts have been obtained from both the main instrument (0.75–30 MeV) and the burst modules (0.1–10 MeV). The deconvolved photon spectra for the majority of bursts are fit by a single power law model with spectral index between −1.6 and −2.8. One strong burst, GRB 910814, exhibited significant curvature and could not be fit by a single power law model. A broken power law model with a break in slope at ∼2 MeV is a good fit to the time averaged spectrum of this burst. There is evidence, at the 2.8σ level, for a change in the break energy of GRB 910814, from above 2 MeV to below 1 MeV during the first 9 s of the burst

COMPTEL’s solar flare catalog

COMPTEL, the imaging gamma‐ray telescope, capable of detecting gamma rays in the range of 0.1–30 MeV, is one of four instruments aboard NASA’s Compton Gamma‐Ray Observatory. The Comptel burst detectors (single Defector Mode) have a field of view of ∼2.5 π sr. These detectors of COMPTEL permit measurements of energy spectra and time histories of solar flare gamma‐ray emission. A search through the Single Detector Mode’s data is being conducted. We summarize the preliminary results of this search

Search for gamma‐ray emission from AGN with COMPTEL

The COMPTEL data (∼0.7–30 MeV) were searched for emission from AGN. Four sources have been detected so far: the quasars 3C 273, 3C 279, PKS 0528+134, and the radio galaxy Centaurus A. 3C 273 and 3C 279 were detected in CGRO observation period 3 with quite different spectral shapes. There is also evidence for 3C 273 at a weak flux level in observation period 11. The quasar PKS 0528+134 was detected above 3 MeV as part of a search for AGN already observed by EGRET. Cen A was seen up to 3 MeV by combining data from different observation periods

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 all-sky imaging at 2.2 MeV

It is now generally accepted that accretion of matter onto a compact object (white dwarf, neutron star or black hole) is one of the most efficient processes in the universe for producing high energy radiations. Measurements of the γ-ray emission will provide a potentially valuable means for furthering our understanding of the accretion process. Here we focus on neutroncapture processes, which can be expected in any situation where energetic neutrons may be produced and where the liberated neutrons will interact with matter before they decay (where they have a chance of undergoing some type of neutron capture). Line emission at 2.2 MeV, resulting from neutron capture on hydrogen, is believed to be the most important neutroncapture emission. Observations of this line in particular would provide a probe of neutronproduction processes (i.e., the energetic particle interactions) within the accretion flow. Here we report on the results of our effort to image the full sky at 2.2 MeV using data from the COMPTELexperiment on the Compton Gamma-Ray Observatory (CGRO)

Observations of Cygnus X-1 COMPTEL during 1991

The Compton Telescope (COMPTEL) experiment on the Compton Gamma-Ray Observatory (CGRO) has observed the Cygnus region on two occasions during the first (sky survey) phase of its mission. These data represent the most sensitive observations to date of Cygnus X-1 in the 0.75-30 MeV range. The observations in 1991 June and August both showed evidence for emission in the 0.75-2 MeV energy range. The flux level was larger by about a factor of 2 during the August observation. The spectral data were analyzed in the context of a Wien spectral model (the high-energy limit of the Sunyaev-Titarchuk Comptonization spectrum). Fits to this model gave electron plasma temperature (kT) values of 192 +/- 27 keV and 204 +/- 21 keV for the June and August data, respectively. These values are much higher than those typically derived from hard X-ray data, suggesting that an alternative to the single-temperature Comptonization model may be required to explain the entire spectrum. The present data may be explained by resolving the inadequacies of the standard Comptonization model which is used to fit the X-ray data and/or incorporating a spectral component which represents the reflection of hard X-rays from an optically thick accretion disk. Finally, there is no evidence in these data for any substantial hardening of the spectrum above 1 MeV (the so-called \u27MeV excess\u27), such as that suggested in the past by several balloon and satellite observations