GCN Circular 22752

S. B. Cenko (GSFC), J.D. Gropp (PSU), A. Y. Lien (GSFC/UMBC),
A. Melandri (INAF-OAB), K. L. Page (U Leicester), D. M. Palmer (LANL)
and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 08:14:50 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180602A (trigger=835200).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 142.339, +32.093 which is 
   RA(J2000) = 09h 29m 21s
   Dec(J2000) = +32d 05' 34"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The burst appears to have been in progress
as Swift was slewing to the pre-planned pointing target before the
nominal trigger time.  The BAT light curve showed a single peak
structure with a duration of about 50 seconds, however the start
of the peak may be due to the GRB entering the FOV during the slew. 
The peak count rate was ~1300 counts/sec (15-350 keV), at ~20 sec 
before the trigger. 

The XRT began observing the field at 08:16:55.2 UT, 124.3 seconds after
the BAT trigger. The position determined from promptly downlinked data
differs significantly from the on-board position, suggesting that the
XRT may have centroided on a cosmic ray; the initial XRT position
notice should be treated with caution. Using promptly downlinked data
we find an uncatalogued X-ray source with an enhanced position: RA, Dec
142.3319, 32.0661 which is equivalent to:
   RA(J2000)  = 09h 29m 19.65s
   Dec(J2000) = +32d 03' 58.1"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 99 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.56 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 6.1
(+2.76/-2.42) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 133 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.02. 

Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)