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GCN Circular 37262

Subject
GRB 240821A: Swift/UVOT observations of the XRT sources.
Date
2024-08-24T16:46:55Z (a month ago)
From
Sam Shilling at Lancaster University <shilling.sam@gmail.com>
Via
Web form
S. P. R. Shilling (Lancaster U.) and S. R. Oates (Lancaster U.) reports on behalf 
of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 240821A
91 ks after the Fermi/GBM trigger (Fermi GBM team, GCN Circ. 37219). The
UVOT exposures include the positions of XRT sources 1, 2, 3 and 4
(Bernardini et al., GCN 37249).

We note there is a DSS source (J233641.30-100258.0, Rmag ~20.76) coincident with
the XRT source 2 position, and a USNO-B1.0 source (0798-0668880, Bmag ~18.31)
coincident with the XRT source 4 position. There are no DSS or USNO-B1.0 sources
at the positions of XRT sources 1 and 3.

Preliminary magnitudes, and upper limits, using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the four XRT sources are:

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

XRT source 1    u          91622       93293	    1643        >20.92
XRT source 2	u	   91622       93293        1643	 20.73 +/- 0.30
XRT source 3	u	   91622       93293        1643        >20.92
XRT source 4	u	   91622       93293        1643         18.27 +/- 0.05

XRT source 1    w2         136400      153663       3261        >21.42
XRT source 2    w2         136400      153663       3261	 20.55 +/- 0.18
XRT source 3    w2         136400      153663       3261        >21.43
XRT source 4    w2         136400      153663       3261         20.66 +/- 0.20

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.01 in the direction of the burst
(Schlegel et al. 1998).

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