TITLE:   GCN CIRCULAR
NUMBER:  33592
SUBJECT: GRB 230409B: Swift detection of a burst
DATE:    23/04/09 05:21:45 GMT
FROM:    Kim Page at U.of Leicester  <kimlpage1978@gmail.com>

A. P. Beardmore (U Leicester), C. Gronwall (PSU),
A. Y. Lien (U Tampa), K. L. Page (U Leicester), T. Sakamoto (AGU),
M. H. Siegel (PSU) and M. A. Williams (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 04:56:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230409B (trigger=1163401). Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 329.530, +52.854 which is 
   RA(J2000) = 21h 58m 07s
   Dec(J2000) = +52d 51' 15"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The immediately available BAT light curve
starts at ~ a few seconds after the BAT trigger time. The light 
curve shows a small hump from ~50 to 100 s, however, further ground 
data will be required to determine whether the hump is due to 
background fluctuation. 

The XRT began observing the field at 04:58:38.0 UT, 101.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 329.51486, 52.83228
which is equivalent to:
   RA(J2000)  = 21h 58m 03.57s
   Dec(J2000) = +52d 49' 56.2"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 84 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 https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 7.56
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 386 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 none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT error circle. No correction has been made for
the large, but uncertain, extinction expected. 

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). 
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/)