2 VLBI observations of radio afterglows

G970508: A first epoch VLBA observation was obtained just 8 days after the gamma-ray burst, and within 48 hours of the VLA discovery of a radio counterpart (Frail et al. 1997a). An additional 8 epochs were obtained until $\sim 120$ days after the burst when the radio afterglow faded below 0.3 mJy, the nominal sensitivity limit. From this data we derived a position accurate to better than 0.1 mas, placed a 3$\sigma$ limit on the proper motion of less than 5 mas y^-1 and placed a limit on the parallax of less than 0.3 mas corresponding to a distance limit of > 3 kpc (Taylor et al. 1997; Taylor et al. 1998). For all epochs the limit on the angular size was less than 0.5 mas (<4 pc).

G980329: We observed, but did not detect, G980329, because variability drove it below 0.2 mJy on the day we observed. The radio afterglow from this source peaked below 0.5 mJy so we did not attempt further observations.

G980425: Associated with SN1998bw in the galaxy ESO 184-G82 at z=0.0083, G980425 was studied with the ATCA during the first 60 days of its outburst (Kulkarni et al. 1998) and both scintillation and brightness temperature arguments predict a size of 0.1 mas on day 16. This time is near the peak in the radio light curve of 50 mJy at 8.4 GHz. Unfortunately, only limited VLBI observations were feasible for this far southern declination ($-53^\circ$) source. On May 10 we observed G980425 at 22 GHz with the VLBA antenna at Mauna Kea and with the 70 meter antenna of the Deep Space Network at Tidbinbilla. The radio source was not detected above a 3$\sigma$ limit of 30 mJy. On August 19 we observed with the same two telescopes but at 8.4 GHz and employing phase-referencing to increase the coherence time. No source was detected above a 3$\sigma$ limit of 2.5 mJy. An 8.4 GHz observation by a southern hemisphere network of radio telescopes on May 9th is still being analyzed.

G980519: No VLBI observation was attempted of this very faint radio afterglow.

G980703: The radio afterglow from G980703 reached 1 mJy in the first two weeks after the burst. We detected G980703 at a level of 0.5 mJy 30 days after the burst, when it was already in decline (Fig. 1). We place a limit on the angular size of G980703 of <0.3 mas, and derive a position of $\alpha$(J2000) = 23$^{\rm h}$59$^{\rm m}$06.$^ {\rm s}$6661, $\delta$(J2000) = 8$^\circ$35$^{\prime}$07.$^{\prime\prime}$0939 with an uncertainty of 0.0007 arcsec in each coordinate.

  

Figure 1: VLBA observation of G980703 at 8.4 GHz for 5 hours. The synthesized beam is drawn in the lower left-hand corner of the plot and has dimensions: $2.2 \times 0.92$ mas in position angle $-10^\circ$.Contours are drawn at -0.1, 0.1, 0.2, and 0.3 mJy/beam