HST WFPC2 Images of GRB 010222

The WFPC2 F606W image of GRB010222 and its host galaxy. On the left we show the image as obtained on the 4 and 5 May 2001 in a 6000s exposure. On the right we have subtracted a point source from the position of the optical transient (OT), assuming flux derived from the fit which is described below.

We have reduced the now public archival HST images of GRB 010222, taken on 28 February, 17 and 18 March, 5 and 6 April, and 4 and 5 May of this year using the F606W (V/R) and F814W (I) filters on WFPC2. Using the most recent data, we find that OT of GRB 010222 is clearly superposed on a host galaxy.

In order to accurately determine the decay of the OT and the magnitude of the host, we have drizzled the images onto an output grid that allows interlacing of these 2-point dithered images. We have then fit for the decay of the OT and the host galaxy flux using a small (~0."2) aperture, which provides the best estimate of the OT as a function of time, as well as using a larger aperture (~0."5), which is more appropriate for determining the host galaxy magnitude.

We find that the late-time decay of the OT is significantly steeper than previously reported. We find a power-law decay between 28 February (day 6) and 5 May (day 71) of -1.7 +/- 0.05. Formally, we find a slightly steeper decay (-1.73 +/- 0.02) in F606W than in F814W (-1.64 +/- 0.04), but this discrepancy may indicate the level of systematic error in the slope measurement rather than true color evolution of the OT.

We find that the host galaxy is dominated by a compact core (FWHM ~ 0."15) located directly under the OT. The magnitude of the host is F606W(AB) = 26.0 +/- 0.1, F814W(AB) = 25.8 +/- 0.15.

A fit to the HST observations of the decay of GRB 010222. HST data points are shown as filled circles; ground based data taken from the compilation of Stanek et al. (2001) are shown as open squares. Our best estimate of the host galaxy flux has been removed from both the HST and ground-based magnitudes. The lines show the slopes described above. F606W has been transformed to Vega R, and F814W to Vega I, using an assumed spectral slope for the optical transient of -0.9 ( Lee et al. 2001 ), which agrees well with the HST photometry.

This result has appeared as GCN1087. Please see the GCN web page for more information on these circulars.