HST ACS Images of the Afterglow of GRB 020813

The optical transient of GRB 020813 viewed with ACS/WFC in three filters approximately 100 hours after the burst. The afterglow is the bright, point like object at the center of the frame. Each image is approximately 10" across.

The field containing the optical counterpart of GRB 020813 was imaged by HST using ACS/HRC, ACS/WFC and NICMOS on 17th August beginning at 04:00 UT approximately 98 hours after the burst.

The optical counterpart is clearly visible in our images. Using the zeropoints for the filters on ACS/WFC as given below we find the magnitude of the counterpart to be:

Filter	Zeropoint (AB)	OT Mag (AB)
F455W	25.47	23.20
F606W	26.35	22.71
F814W	25.86	22.33

This is consistent with a spectral slope of beta =-0.8.

Comparing to early (3 hour) I Band observations (GCN 1513) gives an approximate slope of ~t^-1.07. Comparing to observations taken on 23 hours after burst indicates a somewhat faster (broken) slope of t^-1.35.

The OT appears largely point like, however a PSF subtraction reveals some evidence for a possible 25th magnitude host galaxy underlying the OT.

Analysis is continuing and further observations are planned.

This result was reported as GCN 1514 .

A wide field taken with ACS/WFC in F606W. The field is approximately 70" across.