GRB 220518A

J.D. Gropp (PSU), K. L. Page (U Leicester),
T. M. Parsotan (GSFC/UMBC/CRESSTII), B. Sbarufatti (PSU) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 06:19:20 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 220518A (trigger= 1107050).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 55.369, -47.571 which is 
   RA(J2000) = +03h 41m 29s
   Dec(J2000) = -47d 34' 15"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  There are currently no BAT light curve data available
at this time. 

The XRT began observing the field at 06:21:27.1 UT, 126.4 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 55.34297, -47.58392
which is equivalent to:
   RA(J2000)  = 03h 41m 22.31s
   Dec(J2000) = -47d 35' 02.1"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 78 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 (9.33 x
10^19 cm^-2, Willingale et al. 2013), with an excess column of 5
(+3.25/-2.72) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 129 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 none of
the XRT error circle. 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 0.008. 

Burst Advocate for this burst is J.D. Gropp (jdg44 AT psu.edu). 
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/)

V. Lipunov, V.Kornilov, E.Gorbovskoy, K.Zhirkov, N.Tyurina, P.Balanutsa, A.Kuznetsov, D. Vlasenko, 
G.Antipov, D.Zimnukhov, V.Senik, E.Minkina, A.Chasovnikov, V.Topolev, D.Kuvshinov,  D.Cheryasov, Ya.Kechin
(Lomonosov Moscow State University, SAI, Physics Department),

R. Podesta, C.Lopez, F. Podesta, C.Francile 
(Observatorio Astronomico Felix Aguilar OAFA),

R. Rebolo, M. Serra 
(The Instituto de Astrofisica de Canarias),

D. Buckley 
(South African Astronomical Observatory),

O.A. Gres, N.M. Budnev
(Irkutsk State University, API),

L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez 
(INAOE, Guillermo Haro Astrophysics Observatory),

A. Tlatov, D. Dormidontov 
(Kislovodsk Solar Station of the Pulkovo Observatory),

A. Gabovich, V.Yurkov 
(Blagoveschensk Educational State University)


MASTER-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 220518A ( J.D. Gropp et al., GCN 32060
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 ) errorbox  6501 sec after notice time and 6534 sec after trigger time at 2022-05-18 08:08:15 UT, with upper limit up to  15.6 mag. The observations began at zenith distance = 87 deg. The sun  altitude  is -40.1 deg. 

The galactic latitude b = -51 deg., longitude l = 256 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=1977169

We obtain a following upper limits.  

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

    6565 |         MASTER-OAFA |   C |    60 | 14.3 |        
    7687 |         MASTER-OAFA |   C |   180 | 15.3 |        
    7947 |         MASTER-OAFA |   C |    60 | 15.6 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 1717 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 220518A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 55.34331, -47.58376 which is equivalent
to:

RA (J2000): 03h 41m 22.39s
Dec (J2000): -47d 35' 01.5"

with an uncertainty of 2.2 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

S. R. Oates (U.Birmingham) and J. D. Gropp (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 220518A
129 s after the BAT trigger (Gropp et al., GCN Circ. 32060
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 ).
A source consistent with the enhanced XRT position
(Beardmore et al. GCN Circ. 32062
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 )
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
   RA  (J2000) =  03:41:22.29 =  55.34289 (deg.)
   Dec (J2000) = -47:35:00.3  = -47.58342 (deg.)
with an estimated uncertainty of 0.55 arc sec. (radius, 90% confidence).

Preliminary detections and 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: 

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag
white (FC)        129          279         147         20.00 +/- 0.25
white             621         1717         334         20.49 +/- 0.23
v                 671         1767         136         >19.0
b                 597         1861         131         >20.1
u (FC)            341          591         246         >19.7
u                 341         1841         324         >19.9
w1               1103         1296          39         >18.4


The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.008 in the direction of the burst
(Schlegel et al. 1998).

J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester), A. Melandri (INAF-OAB), T. Sbarrato (INAF-OAB), P.
D'Avanzo (INAF-OAB) and J.D. Gropp report on behalf of the Swift-XRT
team:

We have analysed 6.5 ks of XRT data for GRB 220518A (Gropp et al. GCN
Circ. 32060
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 ), from 133 s to 53.3 ks after the  BAT trigger. The data
are entirely in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Beardmore et al. (GCN Circ. 32062
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 ).

The late-time light curve (from T0+7.3 ks) can be modelled with a
power-law decay with a decay index of alpha=1.3 (+/-0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.85 (+0.15, -0.14). The
best-fitting absorption column is  2.7 (+3.1, -1.8) x 10^20 cm^-2,
consistent with the Galactic value of 9.3 x 10^19 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.5 x 10^-11 (3.7 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.7 (+3.1, -1.8) x 10^20 cm^-2
Galactic foreground: 9.3 x 10^19 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.85 (+0.15, -0.14)

If the light curve continues to decay with a power-law decay index of
1.3, the count rate at T+24 hours will be 3.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-13 (1.3 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01107050.

This circular is an official product of the Swift-XRT team.

S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC), 

H. A. Krimm (NSF), A. Y. Lien (U Tampa), 

C. B. Markwardt (GSFC), D. M. Palmer (LANL), 

T. Parsotan (GSFC/UMBC), T. Sakamoto (AGU), 

M. Stamatikos (OSU), (i.e. the Swift-BAT team):

 

Using the data set from T-239 to T+963 sec from the recent telemetry downlink,

we report further analysis of BAT GRB 220518A (trigger #1107050)

(Gropp et al., GCN 32060
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 ).  The BAT ground-calculated position is

RA, Dec = 55.340, -47.593 deg which is 

   RA(J2000)  =  03h 41m 21.6s 

   Dec(J2000) = -47d 35' 36.3" 

with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment).

The partial coding was 51%.

 

The light curve has a peak at the trigger time and potential precursor 

emission occuring slightly before the BAT trigger time. 

T90 (15-350 keV) is 12.29 +- 3.07 sec (estimated error including systematics).

The time-averaged spectrum from T-8.36 to T+7.58 sec is best fit by a simple

power-law model.  The power law index of the time-averaged spectrum is

1.71 +- 0.21.  The fluence in the 15-150 keV band is 5.7 +- 0.8 x 10^-07 erg/cm2.

The 1-sec peak photon flux measured from T+0.07 sec in the 15-150 keV band

is 1.8 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90% confidence

level. 

 

The results of the batgrbproduct analysis are available at

http://gcn.gsfc.nasa.gov/notices_s/1107050/BA/