[vsnet-grb-info 13365] Swift Trigger 556533 is not an astrophysical event

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  14701
SUBJECT: Swift Trigger 556533 is not an astrophysical event
DATE:    13/05/24 14:04:50 GMT
FROM:    Hans Krimm at NASA-GSFC  <hans.krimm at nasa.gov>

S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester),
H. A. Krimm (CRESST/GSFC/USRA), and M. H. Siegel (PSU)
report on behalf of the Swift Team:

We conclude that subthreshold trigger #556533 (Beardmore, et al.,
GCN Circ. 14699) was caused by a cosmic ray shower and chance
coincidence and is not due to a GRB or other astrophysical phenomenon.

Using the data set from T-240 to T+963 sec from the recent telemetry
downlink, we report that the full Swift/BAT event data shows no
significant features in the light curve.

The Swift/XRT began settled observations of the field of trigger #556533
155 s after the BAT trigger. There is nothing visible in the XRT
data at the position of the BAT onboard error circle, or in the entire image.
The XRT upper limit is 0.01 c/s in 614 s of PC mode data.

The Swift/UVOT began settled observations of the field of trigger #556533
155 s after the BAT trigger. No optical afterglow consistent with the BAT
position is detected in the initial UVOT exposures. Preliminary 3-sigma
upper limits using the UVOT photometric system (Breeveld et al. 2011,
AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and
subsequent exposures are:

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

u_FC               155          405          246         >19.7
v                  460          629           39         >18.3
b                  410          724           55         >19.1
u                  155          703          285         >19.7
w1                 510          678           39         >18.2
w2                 584          604           19         >17.9

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.85 in the direction of the burst
(Schlegel et al. 1998).