TITLE: GCN CIRCULAR NUMBER: 6664 SUBJECT: GRB 070724: P60 and Gemini Observations DATE: 07/07/25 07:28:40 GMT FROM: S. Bradley Cenko at Caltech S. B. Cenko, A. Rau (Caltech), E. Berger (Carnegie/Princeton), P. A. Price (IfA), and A. Cucchiara (Penn State) report on behalf of a larger collaboration: We have imaged the field of GRB070724 (Ziaeepour et al., GCN 6654) on the night of 2007 July 24 UT with the automated Palomar 60-inch telescope (beginning approximately 2.5 minutes after the burst) and the Gemini Multi-Object Spectrograph (GMOS) and Near-Infrared Imager (NIRI) mounted on the 10-m Gemini North Telescope (beginning approximately 2 hours after the burst). The nearby galaxy first mentioned by Bloom (GCN 6658) shows no sign of variability, either from our P60 to our GMOS imaging (remaining at a constant magnitude of i' ~ 20.3), or within our Gemini imaging (verified with psf-matched digital image subtraction). In addition the aforementioned galaxy, we find a faint source on the outskirts of the revised XRT error circle (Page and Ziaeepour, GCN 6659) at coordinates (J2000.0): RA: 01:51:14.05 Dec: -18:35:42.1 with an estimated astrometric uncertainty of approximately 0.3" in each coordinate. The object is only visible in our Gemini/GMOS i' imaging of the field, with a magnitude of i' ~ 24.6 (using GMOS zeropoints from the Gemini webpage). This object is not present in our earlier P60 imaging of the field; however it is well below our detection threshold (see below). As our only two Gemini/GMOS i' images were taken back-to-back, we have no information on the variability of this source at the current time. No other object is detected in the revised XRT error circle in any other filter. Our limiting magnitudes, calculated with respect to the USNO-B catalog (P60) or using zeropoints from the Gemini website (GMOS and NIRI), are quoted below: Instrument Filter UT (July 24) dt (hr) Exposure (s) Magnitude --------------------------------------------------------------------------- P60 i' 11:13 0.32 3x60 + 3x120 > 21.5 GMOS i' 13:17 2.38 2x180 > 24.5 NIRI Ks 13:44 2.83 15x60 > 20.0 NIRI J 14:02 3.13 15x60 > 22.0 NIRI H 14:19 3.42 15x60 > 21.0 NIRI Ks 14:36 3.70 15x60 > 20.0