LOFAR MSSS: Detection of a low-frequency radio transient in 400 hrs of monitoring of the North Celestial Pole

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., Gasperin, F. de, Eislöffel, J., Falcke, H., Ferrari, C., Grießmeier, J. -M., Hardcastle, M.J., Heesen, V., Hessels, J. W. T., Horneffer, A., Iacobelli, M., Jonker, P., Karastergiou, A., Kokotanekov, G., Kondratiev, V. I., Kuniyoshi, M., Law, C. J., Leeuwen, J. van, Markoff, S., Miller-Jones, J. C. A., Mulcahy, D., Orru, E., Pandey-Pommier, M., Pratley, L., Rol, E., Röttgering, H. J. A., Scaife, A. M. M., Shulevski, A., Sobey, C. A., Stappers, B. W., Tasse, C., van der Horst, A.J., Velzen, S. van, Weeren, R. J. van, Wijers, R.A.M.J., Wijnands, R., Wise, M., Zarka, P., Alexov, A., Anderson, J., Asgekar, A., Avruch, I. M., Bentum, M. J., Bernardi, G., Best, P., Breitling, F., Brüggen, M., Butcher, H. R., Ciardi, B., Conway, J. E., Corstanje, A., Geus, E. de, Deller, A., Duscha, S., Frieswijk, W., Garrett, M.A., Gunst, A. W., Haarlem, M. P. van, Hoeft, M., Hörandel, J., Juette, E., Kuper, G., Loose, M., Maat, P., McFadden, R., McKay-Bukowski, D., Moldon, J., Munk, H., Norden, M. J., Paas, H., Polatidis, A. G., Schwarz, D., Sluman, J., Smirnov, O., Steinmetz, M., Thoudam, S., Toribio, M. C., Vermeulen, R., Vocks, C., Wijnholds, S. J., Wucknitz, O. and Yatawatta, S. (2016) LOFAR MSSS: Detection of a low-frequency radio transient in 400 hrs of monitoring of the North Celestial Pole. Monthly Notices of the Royal Astronomical Society (MNRAS), 456 (3). pp. 2321-2342. ISSN 0035-8711

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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-minute snapshots, each covering 175 deg^2. 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) x 10^-4 day^-1 deg^-2, and a transient surface density of 1.5 x 10^-5 deg^-2, at a 7.9-Jy limiting flux density and ~10-minute 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.