Further Swift observations of GRB130505B - a possible Galactic transient?
ATel #5055; M. De Pasquale, S. Oates (MSSL/UCL), D. Kocevski (NASA/GSFC/ORAU), P. D'Avanzo, A. Melandri (INAF-OAB), P. Evans (U. Leicester), F. E. Marshall (NASA/GSFC) on behalf of the Swift team
on 8 May 2013; 12:59 UT
Credential Certification: Pat Romano (romano@ifc.inaf.it)
Subjects: Optical, X-ray, Transient
The optical source identified by Swift/UVOT in the field of MAXI GRB130505A (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 preliminary 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)x10e+21 cm-2,consistent with the Galactic value of 2.4e20 (Kalberla et al. 2005). Assuming this spectrum, the last XRT detection correspond to a 0.3-10 keV flux of (4.5+/- 2.2) e-14 erg cm-2 s-1.
The very fast optical decay at late time is unprecedented in GRB afterglows, and we cannot exclude a Galactic transient nature for this transient.
We thank D.A. Kann a P. Romano for useful discussion.
