TITLE: GCN CIRCULAR NUMBER: 14604 SUBJECT: GRB 130505B: Further Swift observations DATE: 13/05/08 15:39:28 GMT FROM: Andrea Melandri at INAF-OAB A. Melandri, P. D'Avanzo, (INAF-OAB), M. De Pasquale, S. Oates (MSSL/ UCL), D. Kocevski (NASA/GSFC/ORAU), F. E. Marshall (NASA/GSFC), P. A. Evans (U. Leicester) on behalf of the Swift team The optical source identified by Swift/UVOT in the field of MAXI GRB130505B (Kocevski et al., GCN circ. 14594) and coincident with SDSS J091721.89+014203.2 has dimmed by more than 3 magnitudes between 58.6 and 86.7 ks after the trigger (Serino et al., GCN circ. 14586), and it is not detected in the last exposure, centered at 153.9 ks, down to a 3 sigma upper limit of u=22.1. This source is seen to be fading also by Swift/XRT between 58.6 and 163.0 ks with a decay index ~2.6. The spectrum can be fitted with an absorbed power-law with a spectral photon index of 2.19 (+2.09, -0.79). The best fitting absorption column is 1.6 (+4.1, -1.6) × 10^21 cm-2, consistent with the Galactic value of 2.4 x 10^20 cm-2 (Kalberla et al. 2005). Assuming this spectrum, the last XRT detection correspond to a 0.3-10 keV flux of (4.5 ± 2.2) x 10^-14 erg cm-2 s-1. The very fast optical decay is unprecedented in GRB afterglows, and we cannot exclude a Galactic transient nature for this transient.