GCN Circular 36162

M. H. Siegel (PSU), S. Dichiara (PSU), J.D. Gropp (PSU),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and
M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 20:24:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240418A (trigger=1222885).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 239.322, +23.607 which is 
   RA(J2000) = 15h 57m 17s
   Dec(J2000) = +23d 36' 26"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of at least 20 sec.  The peak count rate
was ~700 counts/sec (15-350 keV), at ~7 sec after the trigger. 

The XRT began observing the field at 20:26:10.4 UT, 122.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 239.37228, 23.59581 which is equivalent
to:
   RA(J2000)  = 15h 57m 29.35s
   Dec(J2000) = +23d 35' 44.9"
with an uncertainty of 6.5 arcseconds (radius, 90% containment). This
location is 170 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.  We cannot determine whether the source
is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 5.26
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 125 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. 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 0.058. 

Burst Advocate for this burst is M. H. Siegel (siegel AT swift.psu.edu). 
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/)