HST STIS Images of the Afterglow of GRB 020410

The proposed optical transient and possible host galaxy of GRB 020410. The left image shows the field in first epoch, 28 days after burst, the right hand image shows the same region in the second epoch, 65 days after burst. The fading behavior of the proposed transient is clear. The images are approximately 13" across.

We observed the field of GRB 020410 (GCN 1349) with HST using the STIS CCD in open (50CCD) mode on the 8 May and 14 June 2002. The 50" field of STIS was centered on the refined NFI Beppo-Sax postion of Nicastro et al. (GCN 1374), which had a 90% confidence error-circle radius of 20".

We find a variable source located at RA=22h 06m 31.87s, DEC=-83d 49m 28.3" where the astrometry has been derived from the headers of the two HST images (which agree to approximately 0."5). This position is offset from the center of the NFI error circle by approximately 6".

Using a zeropoint of V=26.3 for the wide-band STIS 50CCD image, we find the source had a magnitude of V=25.35 on 8 May and 26.90 on 14 June, where the uncertainty in the photometry is dominated by the color correction (which one might expect to be of order 0.1 mags). The source is largely point-like, though it lies near a 25th magnitude galaxy, and shows possible evidence of a faint underlying object.

The decline of ~1.55 magnitudes between the two observation dates is equivalent to an effective power-law decay of -1.65. While this value is completely consistent with the late-time decay of the optical afterglow of a GRB, we cannot rule out the possibility that a supernova, unrelated to the GRB, occurred in our search region. Given the rates of discovery by HST of supernovae in deep images of random fields (c.f. Gilliland, Nugent and Phillips 1999) we conservatively estimate a ~5% chance that this could be such a chance superposition. Further observations, which we expect will give us both a late-time magnitude and color, should allow us to distinguish between these possibilities.

Were the source to have faded consistently as a power law of exponent -1.65 from early after the burst, then its magnitude would have been ~17 at 6 hours after the burst when observations by Castro-Tirado et al. (GCN 1355), which reached the DSS2 limit, failed to detect a counterpart. BeppoSAX observations (GCN 1358, 1366) taken 20 and 54 hours after burst indicated that the x-ray afterglow associated with the GRB fell by only a factor of two, consistent with an effective power law decline of -0.9. Therefore a break may be the most likely explanation for the non-detection of this transient in early optical observations.

If, as expected, this is indeed the OT of GRB 020410, this would be the first time that HST (or any orbiting observatory) has discovered the OT of a GRB.

This result was reported as GCN 1353 .

The difference image between the two epochs of observations of GRB 020410, taken 28 and 65 days post burst, the fading counterpart is clearly visible