2 Data

Using the STIS-instrument onboard the HST the GRB-systems were observed using a four-point dithering pattern with shifts of 2.5 pixels ($\simeq$0 $\hbox{$.\!\!^{\prime\prime}$ }$127) between exposures. The data was pre-processed using the standard STIS pipeline and combined using the DITHER (v2.0) software (Fruchter & Hook 2002) as implemented in IRAF (v2.11.3) and STSDAS (v2.3). The STIS images were drizzled using "pixfrac  =  0.6'' and "scale  =  0.5'' (giving a pixel size of $0\hbox{$.\!\!^{\prime\prime}$ }0254$). Note that drizzling introduces correlated noise between neighbouring pixels. All GRB-systems were observed using the STIS 50CCD (hereafter CL) passband with pivotal wavelength PivW  =  5851.5 Å. GRB 980329 was also observed using the STIS F28X50LP (hereafter LP) passband, PivW  =  7228.5 Å. The HST images presented here are, as opposed to the images discussed in our previous GCN circulars, all oriented such that East is to the right and North towards the top. Whenever referring to pixel coordinates, increasing x corresponds to decreasing RA, while increasing y corresponds to increasing Dec.

The photometry was performed in circular or elliptical apertures, as appropriate, according to the morphology of the host. The size of the apertures was selected so as to measure the total flux, by first choosing a plausible shape and then increasing the size until no gain in flux could be achieved. The sky was measured in an annular aperture with a shape corresponding to that of object aperture, with inner and outer aperture size 1.5 and 4 times that of the object aperture. For the STIS zero-points, we adopted the values found by Gardner et al. (2000) for the HDF-south. The zero-points used were therefore $ZP_{\rm CL}=26.387$ and $ZP_{\rm LP}=25.291$. Foreground (Galaxy) extinction estimates were computed using the on-line NED extinction calculator based on the dust maps provided by Schlegel et al. (1998).

Signal-to-noise (S/N) estimates of the host detections were computed as the ratio between the measured counts in a circular aperture centered on the object and the sky variance as measured from circular apertures at ten (10) random positions (on the sky). The aperture diameter used was 19, 19, and 9 pixels for GRB 980329, GRB 980519 and GRB 990308, respectively. Note that the errors on the photometry is computed as in IRAF DAOPHOT/APPHOT and does not necessarily correspond to the S/N estimates.

The host pixel positions were determined by using the IRAF/APPHOT CENTER task and the "centroid'' algorithm therein. The "centroid'' algorithm computes the intensity weighted means of the marginal profiles in x and y. The results are given in Table 1.

Throughout this paper we use the following cosmological parameters; $\Omega_M=0.3$, $\Omega_\Lambda=0.7$, H₀ = 70 km s^-1 Mpc^-1.