[vsnet-grb-info 13267] GRB 130505B: Further Swift observations

[GCN Circulars]

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  <andrea.melandri at brera.inaf.it>

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.