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GCN Circular 29743

Subject
GRB 210402A: Swift detection of a burst
Date
2021-04-02T22:30:12Z (3 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
K. L. Page (U Leicester), S. D. Barthelmy (GSFC),
F. E. Marshall (NASA/GSFC) and D. M. Palmer (LANL) report on behalf of
the Neil Gehrels Swift Observatory Team:

BAT has triggered on a new GRB 210402A (Trigger #1040123) causing 
Swift to slew to the location and make follow-up observations.  
Due to a ground pass at the time of the GRB, no precise trigger 
time, location, or light curve data is immediately available. 
These details will be given in a later circular. 

We note that Fermi triggered on a GRB at around the same time at a 
consistent location (Fermi Trigger #639093861) confirming its GRB status. 

Using promptly downlinked data we find a bright, uncatalogued X-ray
source located at RA, Dec 198.25866, 44.51291 which is equivalent to:
   RA(J2000)  = 13h 13m 02.08s
   Dec(J2000) = +44d 30' 46.5"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
position may be improved as more data are received; the latest position
is available at https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.51
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.31e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of nominal 150 seconds with the
White  filter. 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. 
No correction has been made for the extinction in the Milky Way. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk). 
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/)
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