{
  "bibcode": "2013GCN.14604....1M",
  "body": "A. Melandri, P. D'Avanzo, (INAF-OAB), M. De Pasquale, S. Oates (MSSL/ \nUCL), D. Kocevski (NASA/GSFC/ORAU), F. E. Marshall (NASA/GSFC), P. A.  \nEvans (U. Leicester) on behalf of the Swift team\n\nThe optical source identified by Swift/UVOT in the field of MAXI  \nGRB130505B (Kocevski et al., GCN circ. 14594) and coincident with SDSS  \nJ091721.89+014203.2 has dimmed by more than 3 magnitudes between 58.6  \nand 86.7 ks after the trigger (Serino et al., GCN circ. 14586), and it  \nis not detected in the last exposure, centered at 153.9 ks, down to a  \n3 sigma upper limit of u=22.1.\n\nThis source is seen to be fading also by Swift/XRT between 58.6 and  \n163.0 ks with a decay index ~2.6. The spectrum can be fitted with an  \nabsorbed power-law with a spectral photon index of 2.19 (+2.09,  \n-0.79). The best fitting absorption column is 1.6 (+4.1, -1.6) � 10^21  \ncm-2, consistent with the Galactic value of 2.4 x 10^20 cm-2 (Kalberla  \net al. 2005). Assuming this spectrum, the last XRT detection  \ncorrespond to a 0.3-10 keV flux of (4.5 � 2.2) x 10^-14 erg cm-2 s-1.\n\nThe very fast optical decay is unprecedented in GRB afterglows, and we  \ncannot exclude a Galactic transient nature for this transient.",
  "circularId": 14604,
  "createdOn": 1368027568000,
  "email": "andrea.melandri@brera.inaf.it",
  "subject": "GRB 130505B: Further Swift observations",
  "submitter": "Andrea Melandri at INAF-OAB  <andrea.melandri@brera.inaf.it>",
  "eventId": "GRB 130505B"
}