1999ApJ...521L.137B

COMPTEL observations of the Orion/Monoceros region have shown distinct evidence for excessive 3-7 MeV emission that was attributed to nuclear de-excitation lines from accelerated ¹²C and ¹⁶O nuclei. Unfortunately, we must conclude now that this appears to be a spurious result. This conclusion follows from a better understanding of the instrumental background, from a better exposure of the region, and from an improved analysis method. We show here how the impact of each of these gradually reduces the signal to a less than 3 σ result. The prime underlying cause seems to be ²⁴Na activation in and around the upper COMPTEL detectors. Combining all available data, we now set a 2 σ flux upper limit on the 3-7 MeV emission of Orion of 3x10^–5 γ.cm^–2.s^–1, to be compared with the previously derived flux of ∼10^–4 γ.cm^–2.s^–1.