TITLE:   GCN CIRCULAR
NUMBER:  33487
SUBJECT: Trigger 1160641: Swift detection of 4U 1630-472 (a.k.a. SWIFT J1633.8-4724A)
DATE:    23/03/20 08:30:09 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien@nasa.gov>

E. Ambrosi (INAF-IASFPA), V. D'Elia (SSDC & INAF-OAR),
A. Y. Lien (U Tampa) and A. Melandri (INAF-OAR) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 07:53:05 UT, the Swift Burst Alert Telescope (BAT) triggered and
located emission from the source 4U 1630-472 (a.k.a. SWIFT J1633.8-4724A)
(trigger=1160641).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 248.508, -47.365 which is 
   RA(J2000) = 16h 34m 02s
   Dec(J2000) = -47d 21' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for long image triggers,
there is no obvious variation in immediately available BAT lightcurve. 

The XRT began observing the field at 08:01:45.3 UT, 520.3 seconds after
the BAT trigger. XRT found a bright, X-ray source located
at RA, Dec 248.5015, -47.3926 which is equivalent to:
   RA(J2000)  = 16h 34m 0.36s
   Dec(J2000) = -47d 23' 33.4"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 100 arcseconds from the BAT onboard position, within the
BAT error circle. No event data are yet available to determine the
column density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 531 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 large, but uncertain, extinction expected. 

BAT transient monitor shows 4U 1630-472 has brighten up significantly 
in the last few weeks: 
https://swift.gsfc.nasa.gov/results/transients/weak/4U1630-472/

The on-going outburst activity was also reported in Bouchet et al. 2023
(ATEL #15924) and Jiang et al. 2022 (ATEL #15575).