Imaging the Radio Afterglow from GRB 030329

On March 29, 2003, an exceptionally bright Gamma-Ray Burst (GRB) was discovered by the HETE-2 satellite (Vanderspek et al. 2003, GCN, 1997, 1).  A bright optical afterglow was found by Peterson & Price (2003, GCN, 1985,1), and studied by a host of optical telescopes around the world.  With a redshift of z = 0.1685 (Greiner et al. 2003, GCN, 2020, 1), GRB 030329 is the nearest cosmological burst detected to date, and its extreme brightness can be attributed to its proximity. At radio wavelengths the afterglow is equally impressive, reaching a peak flux density of 55 mJy at 43 GHz one week after the burst, more than 50 times brighter than any previously studied event (Berger et al. 2003, Nature, in press).

A VLBI campaign by Taylor et al. began two days after the burst with VLBA observations at 5 and 8 GHz designed primarily to look at the scintillation.  After 4 days the standard fireball models predict a size for the afterglow of just 10e17 cm or 0.01 mas, but still large enough to begin quenching the scintillation as confirmed in the second VLBI epoch 7 days after the burst.  A third VLBI epoch including Effelsberg and going to 22 GHz to maximize resolution measured the size of the afterglow to be 0.08 mas 24 days after the burst.  In the fourth epoch, 51 days after the burst, GRB 030329 appeared resolved at 15 GHz with a component appearing to the north east at 0.28 +/- 0.05 mas from the main component (Fig. a). This component was tentatively identified as a fast jet component.  To reach its position starting from the initial outburst would require an average velocity of 19c.  Such a jet component is somewhat surprising since it is not predicted from the standard models.

a) The afterglow from GRB 030329 at 15 GHz taken with the VLBA, GBT, and Effelsberg telescopes 51 days after the burst.
Contours are drawn starting at 0.5 mJy/beam and increase by factors of 2.
b) GRB 030329 at 8.4 GHz made with the VLBA, Effelsberg, Arecibo, phased-VLA, and phased-WSRT telescopes taken 83
days after the burst. Contours are drawn starting at 0.4 mJy/beam and increase by factors of 2.

The fifth VLBI epoch took place on June 19, 83 days after the burst. To cope with the fading of the afterglow, the observing frequency was reduced to 8.4 GHz and time was requested from a large array of telescopes.  Fortunately, GRB030329 can be seen by Arecibo for nearly 2 hr, and just as it transits there is good mutual visibility between the U.S. and Europe, so that the maximum resolution can be achieved. It was hoped that this epoch would confirm the existence of a jet component to the north east. However, as shown in Fig. b, there is only a slight indication of extended emission to the north east.

Although GRB 030329 has faded considerably, it is still detectable with VLBI techniques.  The next epoch, with an even more impressive array consisting of the VLBA, phased-VLA, GBT, Effelsberg, phased-WSRT, Noto, Medecina, and Arecibo took place on November 1st, 216 days after the burst.  The observing frequency was again 8.4 GHz, and the size predicted by the standard fireball model was 0.2 mas. However, the peak flux density was predicted to be only 0.5 mJy.  Measuring the size of the afterglow places independent constraints on the energetics and environment of GRB 030329.  Constraints on the proper motion can also constrain theoretical models.