New methods to constrain the radio transient rate: results from a survey of four fields with LOFAR.
We report on the results of a search for radio transients between 115 and 190 MHz with the LOw-Frequency ARray (LOFAR). Four fields have been monitored with cadences between 15 min and several months. A total of 151 images were obtained, giving a total survey area of 2275 deg2. We analysed our data using standard LOFAR tools and searched for radio transients using the LOFAR Transients Pipeline. No credible radio transient candidate has been detected; however, we are able to set upper limits on the surface density of radio transient sources at low radio frequencies. We also show that low-frequency radio surveys are more sensitive to steep-spectrum coherent transient sources than GHz radio surveys. We used two new statistical methods to determine the upper limits on the transient surface density. One is free of assumptions on the flux distribution of the sources, while the other assumes a power-law distribution in flux and sets more stringent constraints on the transient surface density. Both of these methods provide better constraints than the approach used in previous works. The best value for the upper limit we can set for the transient surface density, using the method assuming a power-law flux distribution, is 1.3 × 10-3deg-2for transients brighter than 0.3 Jy with a time-scale of 15 min, at a frequency of 150 MHz. We also calculated for the first time upper limits for the transient surface density for transients of different time-scales. We find that the results can differ by orders of magnitude from previously reported, simplified estimates.
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands.
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D
Carbone
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands; Department of Physics, The George Washington University, 725 21 Street NW, Washington, DC 20052, USA.
A J
AJ
van der Horst
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands.
R A M J
RA
Wijers
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands; Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA.
J D
JD
Swinbank
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands; CSIRO Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia; ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands.
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A
Rowlinson
ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands; Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK; Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK.
J W
JW
Broderick
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands.
Y N
YN
Cendes
Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK.
A J
AJ
Stewart
CSIRO Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia; ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), The University of Sydney, NSW 2006, Australia.
M E
ME
Bell
Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK.
R P
RP
Breton
Laboratoire AIM (CEA/IRFU - CNRS/INSU - Université Paris Diderot), CEA DSM/IRFU/SAp, F-91191 Gif-sur-Yvette, France; Station de Radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, USR 704 - Univ. Orléans, OSUC, F-18330 Nançay, France.
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S
Corbel
Thüringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg, Germany.
J
J
Eislöffel
Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK.
R P
RP
Fender
Station de Radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, USR 704 - Univ. Orléans, OSUC, F-18330 Nançay, France; Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, LPC2E UMR 7328 CNRS, F-45071 Orléans, France.
J-M
JM
Grießmeier
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands; ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands.
J W T
JW
Hessels
SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, NL-3584-CA Utrecht, the Netherlands; Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, NL-6500 GL Nijmegen, the Netherlands.
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P
Jonker
Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK; Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.
M
M
Kramer
Department of Astronomy and Radio Astronomy Lab, University of California, Berkeley, CA, USA.
C J
CJ
Law
International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia.
J C A
JC
Miller-Jones
Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK.
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M
Pietka
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands; Centrum Wiskunde & Informatica, PO Box 94079, NL-1090 GB Amsterdam, the Netherlands.
L H A
LH
Scheers
Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK.
B W
BW
Stappers
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands; ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands.
J
J
van Leeuwen
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands.
R
R
Wijnands
Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, NL-1090 GE Amsterdam, the Netherlands; ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, NL-7990 AA Dwingeloo, the Netherlands.
M
M
Wise
LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, F-92195 Meudon, France.
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P
Zarka
instrumentation: interferometers; methods: data analysis; methods: statistical; radio continuum: general; techniques: image processing