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GCN Circular 36653

Subject
Trigger 1236151: Swift detection of EXO 0748-676
Date
2024-06-10T08:04:47Z (3 days ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email

V. D'Elia (SSDC & INAF-OAR), J. A. Kennea (PSU),
K. L. Page (U Leicester) and T. M. Parsotan (GSFC) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 07:36:36 UT, the Swift Burst Alert Telescope (BAT) triggered and
located EXO 0748-676 (trigger=1236151).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 117.175, -67.804 which is 
   RA(J2000) = 07h 48m 42s
   Dec(J2000) = -67d 48' 13"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 25 sec.  The peak count rate
was ~500 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 07:37:56.0 UT, 79.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright X-ray
source with an enhanced position: RA, Dec 117.13960, -67.75271 which is
equivalent to:
   RA(J2000)  = 07h 48m 33.50s
   Dec(J2000) = -67d 45' 09.7"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 190 arcseconds from the BAT onboard position, outside 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. This position is 2.7 arcseconds from a
known X-ray source: LSXPS J074833.8-674507. This source is in the Swift
XRT LSXPS  catalogue with a mean 0.3-10 keV count-rate of 0.07018 +/-
0.00060 ct/sec; see https://www.swift.ac.uk/LSXPS
/LSXPS%20J074833.8-674507 for details of these previous observations. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.50
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 6.37e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 879 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 expected extinction corresponding to E(B-V) of
0.126. 


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