GRB 130505B

M. Serino (RIKEN), H. Negoro (Nihon U.), 
S. Ueno, H. Tomida, S. Nakahira, M. Kimura, M. Ishikawa (JAXA), 
T. Mihara, M. Sugizaki, M. Morii, T. Yamamoto, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN), 
N. Kawai, R. Usui, K. Ishikawa, T. Yoshii (Tokyo Tech), 
A. Yoshida, T. Sakamoto, Y. Nakano, Y. Kawakubo (AGU), 
H. Tsunemi, M. Sasaki (Osaka U.), 
M. Nakajima, T. Onodera, K. Fukushima, K. Suzuki (Nihon U.), 
Y. Ueda, M. Shidatsu, T. Kawamuro (Kyoto U.), 
Y. Tsuboi, M. Higa (Chuo U.), 
M. Yamauchi, K. Yoshidome, Y. Ogawa, H. Yamada (Miyazaki U.), 
K. Yamaoka (Nagoya U.) report on behalf of the MAXI team

At 2013-05-05T23:54 UT, the MAXI/GSC detected an uncatalogued X-ray
transient source. Assuming a constant source flux over the scan transit,
we obtain the source position at 

 (R.A., Dec.) = (139.500 deg 1.500 deg) = (09 18 00, +01 30 00)(J2000)

with a 90% C.L. statistical error of 0.3 deg and an additional systematic 
uncertainty of 0.1 deg (90% containment radius). 

The averaged X-ray flux was 160 +/- 40  mCrab (4-10 keV). 
There were no significant excess fluxes at the previous
and followed scan transits at 2013/05/05 22:20 UT and 2013/05/06 01:26 UT, respectively, 
with an upper limit of 20 mCrab for each.

D. Kocevski (NASA/GSFC/ORAU), F. E. Marshall (NASA/GSFC), 
M. De Pasquale (MSSL-UCL), P. D'Avanzo (INAF-OAB), 
and A. Melandri (INAF-OAB) report on behalf of the Swift team:

The Swift began tiling observations of the field of GRB 130505B
16 hours after the MAXI trigger (Serino et al., GCN Circ. 14586
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 ).
A bright source not in the DSS is seen in the UVOT observation
at a preliminary position of 
   RA(J2000)  =    09:17:22.18 = 139.34124
   DEC(J2000) =   +01:42:03.1  =  +1.70085
with a 90%-confidence error radius of about 0.6 arc sec.
It is not possible to determine whether the source is varying
with the currently available data.

The position is consistent with that of SDSS J091721.89+014203.2,
but more than 5 magnitudes brighter in the u band.
The SDSS source could be the host galaxy for the burst,
or could indicate that the MAXI source is a galactic transient.

The preliminary detection using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the exposure is:

Filter         T_start(s)   T_stop(s)      Exp(s)        Mag
u                58617        59042         425        18.0 +/- 0.1

The magnitude is not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.024 in the direction of the burst
(Schlegel et al. 1998).

We have also analysed 426 s of Swift-XRT data from 58.6 ks 
to 59.0 ks after the MAXI trigger. The data are all in 
Photon Counting (PC) mode. A preliminary
analysis reveals the presence of a source at the following position 
RA, Dec = 139.34180, +1.70215 which is equivalent to:

RA (J2000): 09 17 22.03
Dec(J2000): +01 42 07.7

with an uncertainty of 4.5 arcsec (radius, 90% confidence). 
This position is consistent with the UVOT one. The observed count rate 
is (3.0 +/- 1.0) x 10^-2 cts/s.  Given the low significance of 
the detection, at the present stage, it is not possible to determine 
whether the source is variable.

Automated analysis of the XRT data can be found online at
http://www.swift.ac.uk/xrt_products/TILED_GRB00015/

Details of the XRT automated analysis methods are detailed in 
Evans et al. (2007, A&A, 469, 379; and 2009, MNRAS, 397, 1177).

Additional observations with Swift are scheduled. Observations
with other observatories of this interesting object are encouraged.

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
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 ) 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
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 ), 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.