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.