Is there a common origin for the cosmic γ-ray lines at 0.51 and 1.81 MeV near the galactic centre?

Abstract

The discovery of the 1.81-MeV ²⁶Al decay line in the present interstellar medium¹ provided direct experimental proof that intermediate mass nuclei are being synthesized continuously in the Galaxy. The instruments making this and a subsequent observation² had broad angular responses, thus, although both observations were consistent with a source near the centre of the Galaxy, interpretations for the origin of this radiation have included objects distributed throughout the Galaxy capable of synthesizing ²⁶Al, such as type II supernova, Wolf Rayet stars, novae and red giants. New observations³ have localized this source to the galactic centre region consistent with a point source at the centre within the resolution function of the telescope (∼10° FWHM, full width at half maximum). Observations of the galactic 0.51-MeV positron annihilation line have also localized this line to the galactic centre with an angular resolution ∼15° FWHM⁴. Time variability requires that it come from a region ∼1 pc in size, hence it is probably produced near a compact object at or near the galactic centre⁵. Here we discuss the possibility that the 0.51- and 1.81-MeV lines have a common origin: the initial decay of ²⁶Al to ²⁶Mg with the most probable emission of a positron which eventually annihilates producing the 0.51-MeV line, followed by the decay of ²⁶Mg to the ground state with the emission of the 1.81-MeV line. For this process to be solely responsible for both lines the time-integrated intensities of the lines must be compatible, and we show that this is not contradicted by the existing data. The measurement of the properties of the two lines individually and taken together place strong constraints on the source region of this radiation.