HST STIS Images of the Host of XRF/GRB 011030

Above: The field of XRF/GRB 011030. This image shows the majority of the field of view of the STIS 50CCD (open) image. The field is 35" across. The probable host of the XRF/GRB is the amorphous galaxy at the center of the field.

The probably host of XRF/GRB 011030. This image shows a region 3."2 across about the probable host. The scattered light from the nearby bright star seen in the image above has been largely subtracted. The blue circle shows the X-ray error circle for the position of the XRF/GRB, the magenta circle the radio position error circle. The X-ray error circle represents a one sigma error radius. The radio error circle is described as "conservative" by Taylor et al (GCN 1136).

A field containing the reported X-ray (GCN 1143) and radio (GCN 1136) positions of XRF/GRB 011030 (GCN 1118) was observed with HST on 12 December 2001 using the STIS CCD camera both in open mode (50CCD) and with a red-pass filter (LP). Due to the low-galactic latitute (b=20.6) of this field, the estimated foreground extintion is Av=1.2 (Schlegel, Finkbeiner, Davis 1999). Nevertheless, a 26th magnitude galaxy is found to be coincident with both the X-ray and radio positions (within the reported 1" errors of either).

With the public release of the second epoch of Chandra observations (taken on 29 November 2001), we have been able to determine that the X-ray source fell between the two Chandra epochs approximately as t^{-2} (using the time of the XRF/GRB as the zero-point), thus confirming the association of this source with the XRF/GRB.

We have therefore attempted to refine the Chandra error circle. We aligned the first, deeper (9-November) Chandra observation with wide-field R band images obtained on the 1 and 3 November on the WIYN telescope using six point sources which are bright in both images. We find an r.m.s. positional offset between sources in the two images of ~0."18. As the XRF/GRB is brighter in the Chandra image than any source used in the fit (though not visible in the WIYN image) we estimate that the error in transforming its position to the optical field should be no larger than 0."18, one sigma. We have also been able to place the WIYN image on the J2000 coordinate system to an accuracy of ~0."1, using both Naval Observatory and Tycho standards.

We find that the 0."18 Chandra error circle overlaps the bluer, eastern end of the small irregular galaxy noted above. The galaxy is approximately 0."5 across, and has a total magnitude of V~26.2, or approximately V=25, after removal of the estimated foreground extinction. This is comparable to the peak of the distribution of GRB host magnitudes (Hogg and Fruchter 1999). The color of the galaxy (again after removal of the foreground extinction) is quite blue (consistent with a power-law of nu^{-0.5}), suggesting both a relatively young stellar population and that the galaxy has a redshift, z < 3. XRF/GRB 011030 thus appears to be similar to GRBs in its probable redshift and in its association with a faint, blue galaxy; this in turn suggests that XRFs are not a radically different class of object, but are very soft GRBs.

This result has appeared as a GCN. Please see the GCN web page for more information on these circulars.