TITLE: GCN CIRCULAR NUMBER: 6457 SUBJECT: GRB070521: Second Epoch Gemini Imaging DATE: 07/05/23 07:21:00 GMT FROM: S. Bradley Cenko at Caltech S. B. Cenko, E. O. Ofek (Caltech) and P. A. Price (IfA) report on behalf of a larger collaboration: We have re-imaged the field of GRB070521 (Guidorzi et al., GCN 6431) with the Gemini Multi-Imaging Spectrograph mounted on the Gemini North telescope. We obtained 5 x 180 s in the Sloan i' filter at a mean epoch of 10:23:38 UT 22 May. The host candidate (S1) identified by Hattori et al. (GCN 6444; see also Perley et al., GCN 6451) is well detected in our second epoch. In comparison with our first epoch of i' imaging (GCN 6450), we find marginal evidence for fading of this source. Specifically, we measure the following magnitudes, with calibration performed relative to SDSS field stars (Cool et al., GCN 6432): t-tb (hr) Magnitude ---------------------------------------------------------------- 1.21 24.07 +\- 0.17 27.54 24.30 +\- 0.09 However, PSF-matched image subtraction using the ISIS software package (Alard & Lupton, ApJ, 503, 1998) reveals no variability at the location of this source. We do not find any new sources inside the revised XRT error circle (Guidorzi et al., GCN 6452) in either epoch of imaging, to limits of i' > 24.0 (epoch 1) and i' > 25.0 (epoch 2). The source S3 is marginally detected in our second epoch near the detection limit. This would represent a decline from measurements reported previously (I = 24.39; Perley et al., GCN 6451). However, we caution our photometry at this location is affected by a variable background from the nearby bright galaxy and is therefore quite uncertain. We encourage further observations to establish the nature of this object.