GCN Circular 777

HST/STIS observations of the chaotic environment of GRB 980613

Stephen Holland, Bjarne Thomsen (University of Aarhus),
Jens Hjorth, Johan Fynbo (University of Copenhagen),
Michael Andersen (University of Oulu),
Gunnlaugur Bjornsson (University of Iceland),
Andreas Jaunsen (ESO),
Priya Natarajan (University of Cambridge, & Yale), and
Nial Tanvir (University of Hertfordshire)

     We have used the Space Telescope Imaging Spectrograph (STIS) on
the Hubble Space Telescope to image the environment where GRB 980613
occurred.  We obtained 5851 seconds of STIS/CCD images with the 50CCD
(clear) aperture and 5936 seconds of images with the F28X50LP (long
pass) aperture.  This data was taken as part of the Survey of the Host
Galaxies of Gamma-Ray Bursts (Holland et al., GCN 698
Not found
 
 
) approximately
799 days after the burst.  Combined images are now available at
"http://www.ifa.au.dk/~hst/grb_hosts/data/index.html".

     A comparison with ground-based R-band images of the optical
afterglow associated with GRB 980613, taken with from the Nordic
Optical Telescope (NOT) (Hjorth et al., GCN 109
Not found
 
 
) suggests that the GRB
occurred at X = 1023.95 +/- 1.52, Y = 1068.73 +/- 0.96 on the drizzled
50CCD image.  The quoted uncertainties are the uncertainties in the
transformation between the NOT and STIS images.  They do not reflect
the uncertainty in the position of the optical afterglow in the NOT
images due to the GRB being superimposed on a fragmented, extended
structure (object A in Djorgovski et al., astro-ph/0008029).  This
systematic uncertainty means that the position of the optical
afterglow may be consistent with the position of a blue, compact
source at the southeast end of the A complex.  This source has a
full-width at half-maximum of 0.13 arcsec (the resolution of the
drizzled STIS images is 0.089 arcsec), and AB magnitudes of CL = 26.3
+/- 0.1 and LP = 26.2 +/- 0.1 in an aperture of radius 0.25 arcsec.

     An image of the complex structure where GRB 980613 occurred is
available at "http://www.ifa.au.dk/~hst/grb_hosts/data/grb980613cd.gif".
There are several extended objects within a few arcseconds of the GRB
(Djorgovski et al., astro-ph/0008029).  These objects have a wide
range of colours, significant substructure, and some of them appear to
have extended tails that may be due to tidal interactions.  The total
diameter of the chaotic environment around GRB 980613 is approximately
eight arcseconds.  If we assume that all of the structure lies at the
same redshift (z = 1.0969, Djorgovski et al., astro-ph/008029), and
adopt a cosmology with H0 = 70, Omega_matter = 0.3, and Omega_lambda =
0.7, then this corresponds to a diameter of approximately 65 proper
kpc.  The total light in this region is CL = 23.0 +/- 0.1, LP = 22.9
+/- 0.2.  A detailed analysis of this data is in progress (Hjorth et
al., in preparation).