Stewart, A. J., Fender, R. P., Broderick, J. W., Hassall, T. E., Muñoz-Darias, T., Rowlinson, A., Swinbank, J. D., Staley, T. D., Molenaar, G. J., Scheers, B., Grobler, T. L., Pietka, M., Heald, G., McKean, J. P., Bell, M. E., Bonafede, A., Breton, R. P., Carbone, D., Cendes, Y., Clarke, A. O., Corbel, S., de Gasperin, F., Eislöffel, J., Falcke, H., Ferrari, C. et al. 2016. "LOFAR MSSS: detection of a low-frequency radio transient in 400 h of monitoring of the North Celestial Pole." Monthly Notices of the Royal Astronomical Society, 456 2321–2342. https://doi.org/10.1093/mnras/stv2797.
We present the results of a four-month campaign searching for low-frequency radio transients near the North Celestial Pole with the Low-Frequency Array (LOFAR), as part of the Multifrequency Snapshot Sky Survey (MSSS). The data were recorded between 2011 December and 2012 April and comprised 2149 11-min snapshots, each covering 175 deg2. We have found one convincing candidate astrophysical transient, with a duration of a few minutes and a flux density at 60 MHz of 15-25 Jy. The transient does not repeat and has no obvious optical or high-energy counterpart, as a result of which its nature is unclear. The detection of this event implies a transient rate at 60 MHz of 3.9^{ 14.7}_{-3.7}× 10^{-4} d-1 deg-2, and a transient surface density of 1.5 × 10-5 deg-2, at a 7.9-Jy limiting flux density and ˜10-min time-scale. The campaign data were also searched for transients at a range of other time-scales, from 0.5 to 297 min, which allowed us to place a range of limits on transient rates at 60 MHz as a function of observation duration.