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GCN Circular 27701

Subject
GRB 200509A: Swift detection of a burst with a possible optical counterpart
Date
2020-05-09T07:08:31Z (4 years ago)
From
Kim Page at U.of Leicester <klp5@leicester.ac.uk>
E. Troja (NASA/GSFC/UMCP), V. D'Elia (SSDC), J.D. Gropp (PSU),
N. J. Klingler (PSU), K. L. Page (U Leicester), B. Sbarufatti (PSU),
M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 06:47:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 200509A (trigger=970886).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 116.450, -4.641 which is 
   RA(J2000) = 07h 45m 48s
   Dec(J2000) = -04d 38' 28"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 20 sec. There is also possible 
further activity for several hundred seconds  The peak count rate
was ~1400 counts/sec (15-350 keV), at ~4 sec after the trigger. 

The XRT began observing the field at 06:49:16.9 UT, 119.6 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 116.42333, -4.63131 which is equivalent
to:
   RA(J2000)  = 07h 45m 41.60s
   Dec(J2000) = -04d 37' 52.7"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 101 arcseconds from the BAT onboard position, within 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.  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 (1.58 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 5.2
(+4.12/-3.39) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of nominal 150.000 seconds with the White
filter starting 123 seconds after the BAT trigger. There is a candidate
afterglow in the list of sources generated on-board at
 RA(J2000)  =	07:45:41.20 = 116.42166
 DEC(J2000) = -04:37:49.4  =  -4.63040
with a 90%-confidence error radius of about 1.10 arc sec. This position is 6.4
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.15. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.13. The 2.7'x2.7' sub-image does not cover the region of
this potential counterpart. 

Burst Advocate for this burst is E. Troja (eleonora.troja AT nasa.gov). 
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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