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Astron. Astrophys. 321, 229-235 (1997)

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Two variable radio sources near the position of GRB 940301

T.J. Galama 1, A.G. de Bruyn 2, 3, J. van Paradijs 1, 4, L. Hanlon 5, P.J. Groot 1, M. van der Klis 1, R. Strom 1, 2, T. Spoelstra 2, K. Bennett 6, G. Fishman 7 and K. Hurley 8

1 Astronomical Institute 'Anton Pannekoek'/ CHEAF, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
2 NFRA, Radio Observatory, Postbus 2, 7990 AA Dwingeloo, The Netherlands
3 Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
4 Physics Department, UAH, Huntsville, AL 35899, USA
5 Physics Department, University College Dublin, Belfield, Dublin 4, Northern Ireland
6 Astrophysics Division, ESTEC, Noordwijk, The Netherlands
7 NASA-Marshall Space Flight Center, USA
8 UC Berkeley Space Sciences Laboratory, USA

Received 19 July 1996 / Accepted 7 October 1996

Abstract

We report on the results of a search for a radio counterpart to the strong gamma-ray burst GRB 940301. Observations with the Westerbork Synthesis Radio Telescope of the Compton Telescope error box region of GRB 940301 began on March 4, 1994, at 21 cm and April 2, 1994, at 92 cm. No flux density variations were detected at 92 cm above S = 10 mJy (5 [FORMULA]) within a period of 1 to 4 months after the burst. However, when we compared the field with Westerbork Northern Sky Survey data, taken two years prior to GRB 940301, we found two radio sources with significantly increased flux densities. These sources, only [FORMULA] apart, are located at the 2.3 and 2.6 [FORMULA] Compton Telescope confidence contours. Their separation from the Inter Planetary Network annulus virtually excludes association with GRB 940301. Further observations in January 1996 reveal that the sources continued to change in flux density. The relatively large flux density variations at 92 cm, compared to those at higher frequencies, and the inverted spectra in the frequency range from 325-380 MHz make the sources somewhat unusual. Because the sources were already detected at 5 GHz in 1986 most, if not all, of the radio emission is probably associated with activity in Active Galactic Nuclei in distant galaxies.

Key words: gamma rays: bursts – techniques: interferometric

Send offprint requests to: T.J. Galama

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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