GCN Circular 11827
S. B. Cenko (UC Berkeley), P. E. Nugent (LBNL / UC Berkeley), Derek B. Fox (Penn State), E. O. Ofek and M. M. Kasliwal (Caltech) report on behalf of a larger collaboration: As part of the Palomar Transient Factory, we have obtained pre-outburst optical (R-band) imaging of the field of the high-energy transient source GRB 110328A / Swift J164449.3+573451 (GCNs 11823, 11824) with the Palomar 48-inch Oschin Schmidt telescope over the time period from 2009 May to 2010 October. In a stacked frame of all available data, we find a faint, unresolved source at location (J2000.0): RA: 16:44:49.97 Dec: +57:34:59.7 The astrometric uncertainty associated with this position is ~ 150 mas in each coordinate (based on the USNO-B1 catalog). This is consistent with the enhanced XRT position (GCN 11826), and is therefore likely to be associated with the high-energy transient. The detection of such a relatively bright optical counterpart strongly disfavors a cosmological long-duration GRB, and instead suggests that Swift J164449.3+573451 is more likely a new Galactic transient source (GCN 11824, ATEL 3242). In attempting to estimate the brightness of this object, we find that nearby calibration stars from the USNO-B catalog are likely to be inaccurate (resulting in limits significantly deeper than our system can achieve). Based on past observations of other fields, we estimate the brightness of the counterpart to be R ~ 22, although we caution that this estimate may suffer from significant uncertainty.