The radio sky is relatively unexplored for transient signals¹, although the potential of radio-transient searches is high. This was demonstrated recently by the discovery of a previously unknown type of source^{2, 3}, varying on timescales of minutes to hours. Here we report a search for radio sources that vary on much shorter timescales. We found eleven objects characterized by single, dispersed bursts having durations between 2 and 30 ms. The average time intervals between bursts range from 4 min to 3 h with radio emission typically detectable for <1 s per day. From an analysis of the burst arrival times, we have identified periodicities in the range 0.4–7 s for ten of the eleven sources, suggesting origins in rotating neutron stars. Despite the small number of sources detected at present, their ephemeral nature implies a total Galactic population significantly exceeding that of the regularly pulsing radio pulsars. Five of the ten sources have periods >4 s, and the rate of change of the pulse period has been measured for three of them; for one source, we have inferred a high magnetic field strength of 5 10¹³ G. This suggests that the new population is related to other classes of isolated neutron stars observed at X-ray and -ray wavelengths⁴.

1. Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire SK11 9DL, UK
2. Australia Telescope National Facility – CSIRO, PO Box 76, Epping, New South Wales 1710, Australia
3. Astronomy Department and NAIC, Cornell University, Ithaca, New York 14853, USA
4. Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, New York 10027, USA
5. INAF - Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada 54, 09012 Capoterra, Italy
6. Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, British Columbia V6T 1Z1, Canada
7. Dipartimento di Fisica, Universita di Cagliari, Strada Provinciale Monserrato-Sestu, km 0.700, 09042 Monserrato, Italy

Correspondence to: M. A. McLaughlin¹ Correspondence and requests for materials should be addressed to M.A.McL. (Email: Maura.McLaughlin@manchester.ac.uk).

Received 25 August 2005; Accepted 15 November 2005

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