TITLE: GCN CIRCULAR NUMBER: 36612 SUBJECT: Swift Trigger 1232755: likely consistent with noise and not astrophysical DATE: 24/06/04 13:18:33 GMT FROM: Amy yarleen@gmail.com
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).
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.
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