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GCN Circular 22010

BAT trigger 778435 is not a GRB.
2017-10-13T16:03:27Z (7 years ago)
Boris Sbarufatti at PSU <>
B. Sbarufatti (PSU), A. Y. Lien (GSFC/UMBC) and S. R. Oates (Uni. of Warwick) report on behalf of the Swift team:

We report on further analysis of BAT trigger 778435/possible GRB 171013A (Sbarufatti et al., GCN Circ. 22007).

The BAT ground analysis using the data set from T-240 to T+962 s shows nothing significant in the mask-weighted light curve. Also, the BAT image significance in 15-350 keV is only 4.3 sigma. 

The XRT ground analysis on 6.5 ks of data observed between T+107 s to T+21.6 ks.  No X-ray source was detected inside the BAT error circle, down to a 3 sigma upper limit of 5.4e-3  cts s^-1, corresponding to a 0.3-10 keV observed flux of 2.7e-13 erg cm^-2 s^-1 (assuming a typical GRB spectrum).

The Swift/UVOT began settled observations of the field of GRB 171013A 110 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

white_FC           110          260          147         >20.2
u_FC               322          572          246         >19.1
white              110          621          167         >20.1
v                  652          672           19         >17.0
b                  577          597           19         >18.0
u                  322          572          246         >19.2
m2                 677          696           19         >17.2
w2                 628          647           19         >17.5

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.97 in the direction of the burst (Schlegel et al. 1998).

We therefore conclude that BAT trigger 778435 was caused by a noise fluctuation.

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