Limit to Pulses of Radiofrequency Emission from the Galactic Centre

THE nucleus of our galaxy, at the position of the Sgr A radio source, has been searched for pulses of radio emission in the frequency range 1,650 to 1,720 MHz. Pulses of short duration might be emitted in association with pulses of gravitational radiation or with other shock wave, collapse, or flare phenomena^1,2. Previous attempts to detect radio pulses have been made^3-7: one of these⁷ suggested a possible source of pulses near the galactic centre. The work described here was designed to be capable of detecting the relative dispersion delay that the interstellar medium would impose at different frequencies on any signal from the galactic centre; interstellar electrons delay a radio pulse at a frequency v as v^-2 ∫N_e, dl, where ∫N_e dl is the integrated electron density or dispersion measure to the source. To accomplish such detection, continuous simultaneous observations at two frequencies were made; the demand for delayed pulses at the lower frequency with respect to the higher provided a discrimination against man-made or terrestrial pulses, which lack dispersion effects.