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Possible Radio Afterglow of a 1989 Gamma-Ray ... - Williams College

Possible Radio Afterglow of a 1989 Gamma-Ray ... - Williams College

Possible Radio Afterglow of a 1989 Gamma-Ray ... - Williams

Publications of the Astronomical Society of the Pacific, 113:6–9, 2001 January 2001. The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A. Research Note Possible Radio Afterglow of a 1989 Gamma-Ray Burst Daniel B. Seaton Hopkins Observatory and Department of Astronomy, Williams College, Williamstown, MA 01267; Daniel.B.Seaton@williams.edu and R. B. Partridge Haverford College, Department of Astronomy, Haverford, PA 19041; bpartrid@haverford.edu Received 2000 September 21; accepted 2000 October 13 ABSTRACT. We report on 3.6 cm observations which we interpret as the radio afterglow of an undetected gamma-ray burst which occurred in 1989 December. 1. INTRODUCTION Radio wavelength afterglows of gamma-ray bursts (GRBs) were discovered 3 years ago by Frail et al. (1997). The radio observations provided precise locations for the source of the GRBs, enabling further study at other wavelengths and strengthening the case for the cosmological origin of GRBs. Assuming that the rapid fluctuations in the observed radio signals were due to interstellar scintillation, limits could be placed on the physical size of the emitter (Goodman 1997; Frail et al. 1997). In this Research Note, we report observations made at l p 3.6 cm of a source which we suggest may be the radio afterglow of an otherwise undetected GRB in late December of 1989. The observations are described briefly in § 2; in § 3 we consider alternative explanations of the radio wavelength measurements and conclude that the most likely explanation is that we serendipitously detected the radio afterglow of a GRB. In the final section, we draw some conclusions, including estimates of surface density of GRBs based on these and similar deep radio observations. 2. OBSERVATIONS AND DETERMINATION OF FLUX DENSITIES The observations that revealed a possible GRB afterglow were made in 1989 December and 1990 January with the aim of improving radio source counts at 3.6 cm and setting upper limits on fluctuations in the cosmic microwave background on angular scales between 10 and 80 (Fomalont et al. 1993; Windhorst et al. 1993). The observations were conducted at NRAO’s Very Large Array (VLA) 1 over the course of eight nights scattered between 1989 December 29 and 1990 January 1 The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. 12. The VLA was in its D configuration, providing a synthesized beamwidth (resolution) of 10. The half-power full width of the primary beam (that is, the diffraction maximum of the individual antennas of the array) was 5.2. Sources lying within 4.6 of the center of the image were included in the complete catalog of Windhorst et al. (1993). Sources out to ≈6 could be detected, however, and we noticed at the time that one source appeared to change in flux density over the 2 weeks of the observations. It is source 16V38 in the catalog of Windhorst et al. (1993), located at (B1950) a p 08 h 42 m 05 ṣ 08(0 ṣ 4), d p 444335.3(0.4). It lay 4.75 from the center of the image. Its flux varied from ≈500 to 1800 mJy and back to 500 mJy during the course of our observations. We erroneously assumed at the time that it was a flare star. 2.1. Reimaging the 1989–1990 Data The GRB afterglow light curves (Frail et al. 1997) reminded one of us (R. B. P.) of the source 16V38. In the summer of 1999 the other of us (D. B. S.) reimaged all the 1989–1990 VLA data day by day. Standard imaging programs in AIPS were used to construct 1024 # 1024 images with 2 pixels and to correct for the primary-beam response of the VLA antennas (see also Fomalont et al. 1993 for details). The resulting values for the corrected flux density of 16V38 are shown in Figure 1. The error bars include uncertainty in the correction for primary-beam response of the VLA. Because 16V38 was 4.75 from the center of the images, the primary-beam response was only ≈8%. We were concerned, therefore, that small changes in the pointing of the VLA telescopes could cause variations in the calculated flux density of the source. So we measured the flux of another source of roughly comparable flux density and also ≈4.75 from the image center to check for variability—its corrected flux density as a function of time is the lower curve in Figure 1. No significant variation in the flux density of this comparison source is apparent. The data were not of 6

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