VLA observations of 1E 1740.7-2942 - A search for radio recombination lines of positronium

Abstract

We have searched for the Ps87-alpha recombination line of positronium at the position of the Einstein X-ray source 1E 1740.7-2942 which has been identified as a strong source of time-variable annihilation line radiation. No line was detected with a 3 sigma upper limit of 330 microJy for a line width of 23.5 MHz, the expected width if the temperature of the annihilation region is about 7 x 10 exp 4 K. A continuum image of the field made from the same data shows, within the 90 percent confidence error circle of the X-ray source, a weak radio source with a core and lobe structure similar to extragalactic double radio sources. Comparison with other observations shows that the radio flux density of the core at 6 cm had increased by a factor of about 2.6 between 1991 October 7 and November 16 and has been decreasing steadily since then. We have explored some possible relations between the radio source and the X-ray source. We find that the radio and the X-ray sources could well be associated with each other. The nature of this source is uncertain - it could be either Galactic or extragalactic. In either case this source is a unique object.