HST/STIS Images of GRB 000418

The HST STIS 50CCD (clear aperture) image of GRB 000418 taken on June 4, 2000, or 47 days after outburst. The blue circle shows the 1-sigma error of the estimated location of the transient. The likely host galaxy has a bright core extended close to the north/south. The image above is about 5 arcseconds on a side.

On this page the HST GRB Collaboration presents the Hubble Space Telescope images of GRB 000418. The Gamma-Ray Burst of April 18, 2000 was first located by Ulysses, NEAR, and KONUS-WIND via the IPN (Hurley et al., GCN #642). The optical transient was first discovered in near-infrared images by S. Klose and collaborators (GCN #643). A redshift of z=1.12 was determined by Bloom et al. using the Keck II telescope and ESI (GCN #661). A complete list of GCN circulars for this burst can be found at the UCO Lick REACT web pages.

The field of GRB 000418 was observed by the Hubble Space Telescope using the STIS camera in open (50CCD) mode on UT 2000 June 4.17. The position of the transient with respect to the STIS image was made by performing relative astrometry of early infrared images of the transient from the Calar Alto 3.5m (GCN 645) and optical images on Apr 20.9 UT from the Telescopio Nazionale Galileo 3.5m with deep late-time images from Keck (GCN 669). The transient position on the Keck images was then transformed to the STIS image.

Near the position of the transient, a compact galaxy with half-light radius of 0.13 arcsec is detected with R = 23.9 +- 0.2 mag (where much of the photometric uncertainty is due to the uncertain spectral energy distribution of the galaxy). The best fit position of the OT is approx 0".08 +- 0".15 arcsec east of the center of this galaxy, which we infer is the likely host. The magnitude measured is within a few tenths of a magnitude of the May 6 Keck measurement (GCN 669), and thus the OT does not likely contribute significant flux to the host in the STIS image.

The next nearest galaxies to the transient are approx 2 arcsec southeast of the host and are detected in both the STIS and Keck images at total R magnitude of 26.2 +- 0.3 mag. These galaxies show no significant extension toward the host galaxy in either the STIS or Keck images.

A Re-observation of GRB 000418

In order to determine whether the optical transient associated with GRB 000418 contributed significantly to the HST image taken 47 days after outburst, the field was re-observed by by the Hubble Space Telescope starting on UT 2001 Feb 11.64 using the STIS camera in open (50CCD) mode. The individual exposures were combined using Drizzle for a total exposure time of 5120 s.

When this second HST image is subtracted from that obtained on 2000 June 4.17 a small residual of magnitude R~28 is found at the position of the compact galaxy believed to be the host galaxy (GCN 669); however, the residual is sufficiently faint to be consistent with no detection at about the 2 sigma level. Taking into account the varying brightness of across the host galaxy, we find an upper limit to the brightness of the OT on 2000 June 4.2 of R > 27.5.

This upper limit on the R-band flux implies that the decay of the optical transient steepened to at least t ^ -1.75 to -2 at late times, in agreement with the result obtainted primarily from radio data by Berger et al (20001, astro-ph/0102278).

The upper image shows the HST STIS 50CCD (clear aperture) image of GRB 000418 taken on February 11 2001. The circle shows the 1-sigma error of the estimated location of the transient. On the lower image we show a subtraction of this image from the first HST image. No significant residual is found, placing a limit of R > 27.4 on the magnitude of the OT 47 days after outburst.