HST STIS Images of GRB 020305


The images show the STIS/Clear-filter images of GRB 020305 taken on April 12 (left) and June 16 (right). The object consistent with the circle has clearly faded, so we conclude that it is the optical afterglow of GRB 020305. The position of the circle has been derived subtracting the two images and determining the centroid of the residuals. The blue circles are 0.1" in radius.




The two panels show the STIS/Long-Pass filter images of GRB 020305 taken on April 14 (left) and June 16 (right). As in the upper figures the circles indicate the afterglow position. The afterglow position is consistent with a faint galaxy, the likely host galaxy.


We have observed the position of the optical afterglow (GCN 1267) of GRB 020305 (GCN 1262) with HST/STIS. The GRB field was visited four times; on April 12.7-12.9 UT (Clear filter), April 14.1-14.4 UT (Long-Pass), June 16.4-16.5 UT (Clear filter), and June 16.6-16.8 UT (Long-Pass).

Subtraction of the two epoch HST/STIS images taken for each filter reveals a fading point source. The centroids of the Long-Pass and Clear filter residuals are consistent with the afterglow position independently derived registering an afterglow NOT image (taken on March 16) on our HST/STIS frames.

The magnitudes of the afterglow (AB system) are: 24.693+/-0.008 (Clear, April 12.7-12.9 UT), 24.452+/-0.013 (Long-Pass, April 14.1-14.4 UT), 26.197+/-0.027 (Clear, June 16.4-16.5 UT) and 25.951+/-0.047 (Long-Pass, June 16.6-16.8 UT). The decay indexes associated to the Clear and Long-Pass filters are, respectively; Alpha_Clear = 1.40+/-0.03, Alpha_Long-Pass = 1.45+/-0.05. So, the optical decay is consistent with being achromatic.

The afterglow position is consistent with an extended (~1.2"x0.5") faint galaxy which shows a complex morphology. A preliminary aperture photometry (AB system, considering a 0.675" aperture radius and the corresponding aperture correction) of the whole host galaxy system yields 25.771+/-0.054 (Clear filter) and 25.951+/-0.047 (Long-Pass filter), respectively.

The optical measurements reported to date (our HST/STIS data points and the ones given in the GCNs; 1264, 1265, 1267, 1270, 1271, 1275, 1279, 1283) might suggest the presence of a break ~10 days after the burst. However, a final confirmation would require a more careful analysis.


The figure shows the multicolor light curve of the GRB 020305 afterglow. The data points are based on the GCNs reported to date and our HST/STIS measurements. A first glance at the photometric points might suggest the presence of a break ~10 days after the burst. However, a final confirmation would require a more careful analysis.