GCN Circular 8777

Subject
Swift triggered on the HMXB 1A 1118-61
Date
2009-01-04T04:34:58Z (18 years ago)
From
Vanessa Mangano at INAF-IASFPA <vanessa@ifc.inaf.it>
V. Mangano (INAF-IASFPA), W. H. Baumgartner (GSFC/UMBC),
N. Gehrels (NASA/GSFC), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
S. Immler (CRESST/GSFC/UMD), J. A. Kennea (PSU),
V. La Parola (INAF-IASFPA), W.B Landsman (GSFC),
C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC),
A. M. Parsons (GSFC) and P. Schady (MSSL-UCL) report on behalf of the
Swift Team:

At 03:52:35 UT, the Swift Burst Alert Telescope (BAT) triggered on the 
known X-ray source 1A 1118-61 (trigger=339131).  
Swift slewed immediately to the BAT position. 
The BAT on-board calculated location is 
RA, Dec 170.299, -61.913 which is 
   RA(J2000) = 11h 21m 12s
   Dec(J2000) = -61d 54' 46"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  This is an image-triggered event. 

The XRT began observing the field at 04:04:30.3 UT, 714.7 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
170.2399, -61.9175 which is equivalent to:
   RA(J2000)  = 11h 20m 57.57s
   Dec(J2000) = -61d 55' 02.9"
with an uncertainty of 5.8 arcseconds (radius, 90% containment). This
location is 101 arcseconds from the BAT onboard position, within the
BAT error circle. 
The source position is only 3.7 arcsec far from the known HMXB 1A 1118-61. 

No event data are yet available to determine the column density using
X-ray spectroscopy. 

The initial flux in the 2.5 s image was 8.20e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the U filter  
starting 872 seconds after the BAT trigger.  There is a UVOT source  
inside the XRT error circle at the position coincident with 1A 1118-61,
which is a known optical source.  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. 

Given the positional coincidence with the HMXB 1A 1118-61
we conclude that Swift triggered on this known transient source.