[vsnet-grb-info 8798] Swift Trigger 381434 is not a GRB

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10297
SUBJECT: Swift Trigger 381434 is not a GRB
DATE:    09/12/29 19:21:53 GMT
FROM:    Vanessa Mangano at INAF-IASFPA  <vanessa at ifc.inaf.it>

S. D. Barthelmy (GSFC), C. B. Markwardt (GSFC/UMD), 
V. Mangano, B. Sbarufatti (INAF-IASF PA), A. Breeveld (MSSL/UCL)
report on behalf the Swift team:

Using the full data set from telemetry downlinks, we report
that Swift Trigger 381434 is due to a cosmic ray shower event
in the BAT instrument.  It is not due to a GRB nor any other
astrophysical source.  There were two showers ~8 msec apart
which caused the 64msec trigger to fire.

The XRT observed the field of the target starting 68 s after
the trigger. In the initial 7 ks in PC mode no source is
detected within the 3 arcmin BAT error circle down to a limiting
flux of 8 x 10^-14 erg cm^-2 s^-1. Three X-ray sources are
detected with more than 3 sigma significance in the whole XRT 
field of view, the brightest one likely associated to 
1RXS J050001.1-550121, and the other two unidentified. 
All of these sources are further than 4 arcmin from the target position.

The UVOT began settled observations of the field of the target 
65 s after the BAT trigger. No new source is detected. 
Preliminary 3-sigma upper limits using the UVOT photometric system 
(Poole et al. 2008, MNRAS, 383, 627) for the first finding chart 
(FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC            65          215          147         >21.1
u_FC               277          526          246         >20.4
white               65         1699          411         >21.6
v                  606         1748          136         >19.4
b                  532         1834          123         >20.3
u                  277         1822          363         >20.6
w1                 656         1798          136         >19.9
m2                 631         1773          136         >19.8
w2                 582         1551          117         >20.1

The values quoted above are not corrected for the Galactic extinction 
due to the reddening of E(B-V) = 0.02 in the direction of the burst 
(Schlegel et al. 1998).

The lack of a bright X-ray/optical counterpart of Trigger 381434 in 
prompt observations strengthens the spurious trigger interpretation.