TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  170
SUBJECT: GRB 981220, radio observations
DATE:    98/12/27 17:50:52 GMT
FROM:    Dale A. Frail at NRAO  <dfrail@nrao.edu>

D. A. Frail (NRAO), and S. R. Kulkarni (Caltech) report on behalf of a
larger NRAO/Caltech collaboration:

"We have observed the error box of GRB 981220 (GCN #159, GCN #160)
with the Very Large Array (VLA) for 12 minutes each at 4.86 GHz and
8.46 GHz on Dec 23.14 UT, 1998. On this day we detected a source at RA
= 3h42m28.94s +- 0.01s, Dec = +17d09'14.6'' +- 0.2'' with a 4.86 GHz
flux density of 200 +- 45 microJy and a 8.46 GHz flux density of 370
+- 43 microJy. This is the same radio source reported by Galama et
al. (GCN #168). Noting its unusual rising spectrum we carried out
another set of observations on Dec 27.21. We observed the field for 18
minutes at each frequency and measured a 4.86 GHz flux density of 540
+- 40 microJy and a 8.46 GHz flux density of 460 +- 36 microJy. These
wide swings of flux and spectral index resemble the behavior of radio
afterglows at early times. Other sources in the field have remained
constant during this time.  We suggest that J034228.94+170914.6 is the
radio afterglow of GRB 981220. Our position is accurate to 0.2 arcsec
(1-sigma) and should enable deep optical studies.  Further radio
observations are in progress."

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