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GCN Circular 16706

Subject
GRB 140818A: Swift-XRT and UVOT potential afterglow detection
Date
2014-08-18T15:05:41Z (10 years ago)
From
Craig Swenson at PSU/Swift <cswenson@astro.psu.edu>
Swenson, C.A. (PSU) and Sbarufatti, B. (INAF-OAB/PSU) report on behalf
of the Swift team

Swift-XRT has observed the error circle of the MAXI GRB 140818A in a
series of observations tiled on the sky. The total exposure time is 2.3
ks spread over 4 fields; the maximum exposure within the sky observed
was 1.0 ks. The observations started 10.5 ks after the MAXI trigger (Honda
et al., GCN Circ. 16702). Within these data we detect a bright, 
uncatalogued
X-ray source at RA, Dec= 199.56275, 6.92254 which is equivalent to:
   RA (J2000.0)	= 13h 18m 15.06s
   Dec (J2000.0) = +06d 55' 21.1"
with an uncertainty of 2.1 arcsec (radius, 90% confidence).  This is an
enhanced position, determined by using UVOT field stars to determine
the astrometry.The exposure at this location was 746 s.

This previously uncatalogued source is significantly brighter than the
RASS limit, and we thus consider it the likely GRB afterglow.

The results of the automatic processing for this source are available
at http://www.swift.ac.uk/xrt_products/TILED_GRB00029/index_1.php

The Swift/UVOT began settled observations of the field of GRB 140818A
10467 s after the MAXI trigger.
A source consistent with the XRT position is detected in the initial
UVOT exposures.

The preliminary UVOT position is:
     RA  (J2000) =  13:18:15.08 = 199.56285 (deg.)
     Dec (J2000) = +06:55:22.6  =   6.92295 (deg.)
with an estimated uncertainty of 0.50 arc sec. (radius, 90% confidence).

Preliminary detections using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early 
exposures are:

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

u                10467        11094          617         19.21 � 0.12

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.04 in the direction of the burst
(Schlegel et al. 1998).

This circular is an official product of the Swift team.
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