GCN Circular 15749

W. H. Baumgartner (GSFC/UMBC), V. D'Elia (ASDC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), C. Pagani (U Leicester),
K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of
the Swift Team:

At 08:39:19 UT, the Swift Burst Alert Telescope (BAT) triggered 
(trigger=584155) and located an outburst from a recently discovered 
transient, MAXI J1421-613 (Morooka et al., ATel #5750).  Swift slewed 
immediately to the source.  The BAT on-board calculated location is 
RA, Dec 215.411, -61.608 which is 
   RA(J2000) = 14h 21m 39s
   Dec(J2000) = -61d 36' 29"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 10 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~7 sec after the trigger. 

The XRT began observing the field at 08:40:43.9 UT, 84.2 seconds after
the BAT trigger. Using promptly downlinked data we find an
X-ray source with an enhanced position: RA, Dec 215.4049, -61.6073
which is equivalent to:
   RA(J2000)  = 14h 21m 37.17s
   Dec(J2000) = -61d 36' 26.1"
with an uncertainty of 2.4 arcseconds (radius, 90% containment). This
location is 10 arcseconds from the BAT onboard position, within the BAT
error circle and consistent with the position of the MAXI J1421-613 transient. 
This position may be improved as more data are received;
the latest position is available at http://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 in excess of the Galactic value (1.71 x
10^22 cm^-2, Kalberla et al. 2005), with an excess column of 2
(+2.01/-1.51) x 10^22 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 88 seconds after the BAT trigger. No credible optical counterpart 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 large, but uncertain extinction expected.