HST/STIS Images of GRB 990510

The HST images of GRB 990510. The images were obtained on 8.1 June 1999 (UT), 17.9 June 1999 and 29 April 2000. The first image was taken in a single orbit; the later images used two orbits, and this accounts for the differing background noise levels. North is up; east is to the left. The bright stars to the north-west and south-east of the optical transient (OT) are separated by 5."7. The OT lies close to the line connecting the two bright stars.
The OT fell in brightness by about a factor of two between the first two observations. Although no emission can be seen in the final image at the position of the OT in the image at native resolution, convolution with a kernel about the size of a distant faint galaxy at the limit of the image (V=28) reveals the possible presence of a host galaxy, as is shown later on this page.

On this page we present the Hubble Space Telescope images of GRB 990510. The Gamma-Ray Burst of May 10, 1999 was first located by the Beppo-Sax satellite; the optical transient was found by Vreeswijk et al. and has since been followed by multiple groups, most of whom have posted results through the GCN archive.

The field of GRB 990510 was imaged by HST on 8.1 June 1999 (UT) 17.9 June 1999 (UT) and 29 April 2000 with the STIS/CCD camera in Open Filter (50CCD) mode. The OT of GRB 990510 was detected in both cases. Assuming the color of the OT did not change from the first few days when it could be modelled as f(nu) = nu^{-0.6} passing through a Galactic extinction of E(B-V)=0.2 (Stanek et al. 1999, Harrison et al. 1999), we find magnitudes of V = 27.0 +/- 0.2 on 8.1 June and 27.8 +/- 0.3 on 17.9 June. The images of the OT and a small region of the field about the GRB are shown in the figure above. The OT is not detected in the 29 April 2000 image.

A zoom of the region about the OT on the 17.9 June 1999 (the second image in the cycling gif above). Eight individual images with a total exposure time of 5200s have been ``drizzled'' onto a final image with pixels 0."025 across (the entire image is about 1."25 across). An extension to the point-spread function (PSF), with a formal significance of 2-3 sigma, is visible primarily to the north (above) and east (left) of the PSF. This was the first indication of a possible host. However, the unknown color of the OT (which affects the size and shape of the PSF), and the strong variability of the STIS PSF did not let exclude instrumental effects (or, indeed at this level, random statistical fluctuation) as the cause (Fruchter et al., GCN 386).

The fits of Stanek et al. 1999 and Harrison et al. 1999 to the early light curve curve predict, at both epochs, an OT fainter than observed by at least several tenths of a magnitude. However, the excess counts above predicted are between a factor of three (first epoch) to seven (second epoch) less than would exist were a supernova Type Ic of the luminosity of SN1998bw at the probable redshift of the GRB (z=1.6, Vreewijk et al.) superposed on the decaying power-law light curve. Below we show the light curve of GRB 990510. We have used data compiled from Stanek et al. 1999 and Harrison et al. 1999, as well as from the GCN archive. The fits were done using the functional form described in Stanek et al. 1999.

A fit to the light curve of GRB 990510. Note that the HST/STIS points lie several tenths of a magnitude above the level predicted by a naive extrapolation of the functional form fit to the earlier data (although the STIS points lie on the R light curve, the photometry has been performed to produce a V magnitude). As noted in the text above, the differences between the observed and predicted counts are, however, much less than would be expected were a supernova as bright as SN1998bw underlying the burst.

The Possible Host of GRB 990510

The suggestion of a host in the June 1999 image, led us to re-image the field in April 2000. To attempt to locate a host galaxy, we have convolved the image with a gaussian kernel with a 0."3 FWHM. This size was based the observed sizes of galaxies in the Hubble Deep Fields with magnitudes comparable to the detection limit of our image for extended sources (V~28).

The 29 April 2000 image of the field of GRB 990510. At left at the native resolution of the drizzled image, at right, the image convolved with a gaussian kernel. The two images are shown at the same scale, with a field of view from East (left) to West (right) of about 6."5. The probable host galaxy is the central oblong feature (see image below for an arrow pointing to the host). The position of the OT (which is known to ~0."003 accuracy from the previous images) lies ~0."075 (three drizzle pixels) west of of the extended feature seen in the convolved image. The formal significance of the extended feature is above 3 sigma. The reality of the other possible galaxy slightly above the probable host is significantly more uncertain.

A wider view of the field of the convolved image, with the arrow pointing towards the probable host galaxy. North is again up, East to the left. The image is 13."4 wide. The high background near the probable host and the irregular shape of the scattered light from the nearby stars makes estimating the true significance of the detection difficult. However, we believe this is most probably the host of GRB 990510.