GCN Circular 36612

A. P. Beardmore (U Leicester), A. A. Breeveld (UCL-MSSL), A. Y. Lien (U
Tampa), D. M. Palmer (LANL) and T. M. Parsotan (GSFC) report on behalf of
the Neil Gehrels Swift Observatory Team:

Using the full data set from recent downlinks, we report further ground
data analysis for Swift trigger 1232755 (initially called GRB240603B;
Beardmore  et al. GCN Circ. 36604
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 ).

The BAT analysis uses data from T-239 s to T+963 s. There are several very
noisy detectors during this time interval. Further ground analysis after
removing the contributions from these noisy detectors shows an image
significance of 7.5 sigma and a small bump in the BAT mask-weighted light
curve from ~T0 to ~T+0.5 s. However, since there are many active noisy
detectors around this time, we also see higher chances of getting
detections above 7 sigma. Thus, the signal is likely to be consistent with
the larger noise fluctuation.

Swift performed a follow-up observation of the trigger, with XRT collecting
1.1 ks of Photon Counting (PC) mode data between T0+3.3 ks and T0+4.6 ks.
No X-ray sources were detected within 296 arcsec of the initially reported
BAT position. The 3-sigma upper limit in the field ranges from ~0.005 to
~0.007 ct s^-1, corresponding to a 0.3-10 keV observed flux of 2.1e-13 to
2.7e-13 erg cm^-2 s^-1 (assuming a typical GRB spectrum).

Additionally, 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 initial exposures are:

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

white          4758       4958       197      >20.2
b              4553       4753       197      >19.6
u              4347       4547       197      >19.2
v              3733       3932       197      >18.7
uvw1           4143       4342       197      >19.2
uvw2           3528       5102       332      >19.6
uvm2           3939       4137       197      >19.2

The increased detector noise visible in BAT and the absence of any X-ray
and optical afterglow detection, suggest this trigger did not have an
astrophysical origin and is consistent with noise fluctuations.