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.
