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GCN Circular 31972

Subject
GRB 220430A: Swift detection of a burst or Swift J0630.4+0932
Date
2022-04-30T14:22:26Z (2 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
E. Ambrosi (INAF-IASFPA), A. D'Ai (INAF-IASFPA),
V. D'Elia (SSDC & INAF-OAR), C. Gronwall (PSU), J.D. Gropp (PSU),
J. A. Kennea (PSU), A. Y. Lien (U Tampa), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 13:53:15 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 220430A (trigger=1104692).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 97.611, +9.510 which is 
   RA(J2000) = 06h 30m 27s
   Dec(J2000) = +09d 30' 36"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve appears to show a
double-peaked structure with the second peak larger, and a total
duration of about 70 sec. The peak count rate was ~35000 counts/sec
(15-350 keV), at ~38 sec after the trigger. 

However, near the time of the GRB the Sun also produced an X-class 
solar flare which may be the cause of some or all of the apparent
lightcurve activity.  The GRB emission will be independently
determined once the mask-tagged BAT data has been received. 

The XRT began observing the field at 13:54:07.7 UT, 51.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 97.61267,
9.54730 which is equivalent to:
   RA(J2000)  = 06h 30m 27.04s
   Dec(J2000) = +09d 32' 50.3"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 134 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (6.47 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 1.1
(+0.75/-0.59) x 10^22 cm^-2 (90% confidence). 

The initial flux in the 0.1 s image was 4.31e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 60 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. 

Although this looks like a GRB both in terms of its lightcurve and
the presence of strong flux above 100 keV, its location near the
Galactic plane (lon=202.05, lat=-0.26) raises the possibility of it
being a Galactic transient.  In addition, as noted above, the
apparent lightcurve may be due to solar activity. If further 
analysis shows that it is a Galactic transient, we will use the 
name Swift J0630.4+0932. 

Burst Advocate for this burst is E. Ambrosi (elena.ambrosi AT inaf.it). 
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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