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

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 persist, in part due to the difficulty in unambiguously modelling COMPTEL data in the possible presence of several point sources. Moreover, it has previously been suggested that a diffuse galactic source related to high-velocity clouds may contribute to the observed emission in this region. This possibility would provide an alternative explanation for part of the observed signal and is not excluded by the analysis presented here, since extended emission is particularly difficult to image with COMPTEL.