GRB 220427A

A. D'Ai (INAF-IASFPA), E. Ambrosi (INAF-IASFPA),
V. D'Elia (SSDC & INAF-OAR), C. Gronwall (PSU), J.D. Gropp (PSU),
J. A. Kennea (PSU), S. Laha (GSFC/UMBC), D. M. Palmer (LANL) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 21:00:34 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 220427A (trigger=1104343).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 275.869, -56.253 which is 
   RA(J2000) = 18h 23m 29s
   Dec(J2000) = -56d 15' 09"
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 80 sec.  The peak count rate
was ~4000 counts/sec (15-350 keV), at ~10 sec after the trigger. 

The XRT began observing the field at 21:03:33.0 UT, 178.1 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 275.88860, -56.25508 which is
equivalent to:
   RA(J2000)  = 18h 23m 33.26s
   Dec(J2000) = -56d 15' 18.3"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 39 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 (1.02 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 2.9
(+3.18/-2.73) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White 
filter  starting 181 seconds after the BAT trigger. There is a 
candidate afterglow in  the rapidly available 2.7'x2.7' sub-image at  
RA(J2000) = 18:23:33.32 = 275.88882    
DEC(J2000) = -56:15:18.3 = -56.25507    
with a 90%-confidence error radius of about 0.61 arc sec. 
This position is 1.4  arc sec. from the center of the XRT error
circle. The estimated magnitude is  17.23 with a 1-sigma error of
about 0.14. No correction has been made for the  expected extinction
corresponding to E(B-V) of 0.097. 

The original BAT Position Notice found an approximate coincidence
(~10 arcmin) with the position of IGR J18244-5622 (which in turn 
is coincident with the Sey2 galaxy IC 4709 at z=0.017). 
However, the position of the X-ray and Optical counterpart 
excludes this source and galaxy, indicating that this is a new GRB. 

Burst Advocate for this burst is A. D'Ai (antonino.dai 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/)

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

Using 1603 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 220427A, 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 = 275.88811, -56.25541 which is equivalent
to:

RA (J2000): 18h 23m 33.15s
Dec (J2000): -56d 15' 19.5"

with an uncertainty of 2.5 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.

A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), J.P. Osborne
(U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi	(INAF-IASFPA) ,
M. Capalbi (INAF-IASFPA), J. D. Gropp (PSU), J.A. Kennea (PSU), A.
Tohuvavohu (U. Toronto) and A. D'Ai report on behalf of the Swift-XRT
team:

We have analysed 6.1 ks of XRT data for GRB 220427A (D'Ai et al. GCN
Circ. 31960
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 ), from 162 s to 30.4 ks after the  BAT trigger. The data
comprise 37 s in Windowed Timing (WT) mode (the first 10 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Goad et al.
(GCN Circ. 31962
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.05 (+0.05, -0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.01 (+/-0.15). The
best-fitting absorption column is  1.6 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 1.0 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.4 x 10^-11 (4.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.6 (+0.5, -0.4) x 10^21 cm^-2
Galactic foreground: 1.0 x 10^21 cm^-2
Excess significance: 2.1 sigma
Photon index:	     2.01 (+/-0.15)

If the light curve continues to decay with a power-law decay index of
1.05, the count rate at T+24 hours will be 6.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.1 x
10^-13 (2.8 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/01104343.

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

D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long GRB 220427A (Swift detection: D'Ai et al., GCN Circ. 31960
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 )
triggered Konus-Wind (KW) at T0=75645.009 s UT (21:00:45.009).

The burst light curve shows a single pulse,
which starts at ~T0-3 s and has a total duration of ~15 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB220427_T75645/

As observed by Konus-Wind, the burst had
a fluence of (1.0 �� 0.3)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 0.256 s,
of (2.5 �� 0.7)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

A time-averaged spectrum of the burst (measured from T0 to T0+8.448 s)
is well fit in the 20 keV - 15 MeV range
by a power law with exponential cutoff (CPL) model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.04(-0.30,+0.43) and Ep = 503(-204,+757) keV (chi2 = 77/97 dof).
A fit to this spectrum by the Band GRB function is poorly constrained,
it results in the best fit values of alpha~-0.6, Ep~260 keV, and
the high energy photon index beta ~ -2.0 (chi2 = 77/96 dof).

All the quoted errors are estimated at the 90% confidence level.
All the presented results are preliminary.

A. de Ugarte Postigo (Artemis/OCA), H. Fausey (George Washington
University), D. A. Kann (HETH/IAA-CSIC), D. B. Malesani (Radboud
Univ. and DAWN/NBI), P. Jakobsson (U. of Iceland), A. Saccardi
(GEPI, Observatoire de Paris) and B. Schneider (CEA Paris-Saclay),
report on behalf of the Stargate Consortium:

We observed the field of GRB 220427A (D'Ai et al., GCN 31960
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 )
using the ESO VLT UT3 (Melipal) equipped with the X-shooter
spectrograph. In images taken with the acquisition camera 7.9 hr
after the GRB we measure for the optical afterglow the following
AB magnitudes (calibrated against 2 field stars from the SkyMapper
catalogue):

g' = 22.40 +/- 0.09 mag
r' = 22.04 +/- 0.08 mag
z' = 20.81 +/- 0.08 mag

No useful spectroscopy could be secured.

We acknowledge excellent support from the ESO observing staff in
Paranal, in particular Jonathan Smoker and Rob van Holstein.

M. Stamatikos (OSU) S. D. Barthelmy (GSFC),
A. D'Ai (INAF-IASFPA), H. A. Krimm (NSF),
S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa),
C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Parsotan (GSFC/UMBC), T. Sakamoto (AGU)
(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 220427A (trigger #1104343)
(D'Ai et al., GCN Circ. 31960
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 ).  The BAT ground-calculated position is
RA, Dec = 275.922, -56.257 deg which is
   RA(J2000)  =  18h 23m 41.2s
   Dec(J2000) = -56d 15' 23.7"
with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 62%.

The mask-weighted light curve shows a single-peaked structure that
starts at ~T+3 s and peaks at ~T+8 s, and followed by a long tail
emission that ends at ~T+70 s. T90 (15-350 keV) is 57.2 +- 12.2 sec
(estimated error including systematics).

The time-averaged spectrum from T+3.09 to T+68.06 sec is best fit
by a simple power-law model.  The power law index of the time-averaged
spectrum is 1.23 +- 0.13.  The fluence in the 15-150 keV band is
2.1 +- 0.2 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from
T+7.83 sec in the 15-150 keV band is 3.0 +- 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/1104343/BA/

A. Klotz (CNRS-OMP-IRAP), M. Blazek (FZU), N. Christensen (OCA/Artemis), 
D. Datashvili (AbAO), P. A. Duverne (IJCLAB), F. Z. Guo (THU), N. 
Kochiashvili (AbAO), M. Lamoureux (UCLouvain), N. Leroy (IJCLAB), T. 
Sadibekova (AIM/CEA-UPS), P. Thierry (AGORA), A. Simon (Kyiv Univ), L. 
Eymar, S. Antier, M. Boer, A. de Ugarte Postigo (all CNRS-OCA-ARTEMIS), 
D. A. Kann (IAA-CSIC), A. Iskandar (XAO), A. Baransky (Kyiv Univ),
N. Sasaki (NEPA/UEA), W. Corradi (LNA), I. Tosta e Melo (INFN-LNS), J. 
P. Vignes (KNC), V. Godunova (IC ICAMER) report on behalf of GRANDMA 
collaboration:

The GRANDMA telescope network responded promptly to the alert of GRB 
220427A (A. D'Ai et al. GCN 31960
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 , M. R. Goad et al. GCN 31962
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 , A. P. 
Beardmore et al. GCN 31963
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 , M. Stamatikos et al. GCN 31968
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 ).

The first observations with TAROT La Reunion started 2.35 min after the 
Swift/BAT trigger time, and were followed by observations with the Les 
Makes T60. Later observations were obtained by the CDK40 0.4m telescope 
that is part of the Kilonova-Cather citizen science programme. The light 
curve peaked around 4.5 min after the burst and then decayed steadily 
with a decay slope of alpha ~ 1.3, where F ~ t^-alpha.

The following table displays part of our photometry with magnitudes 
given in the AB system and calibrated with respect to field stars from 
the SkyMapper catalogue, using the MUPHOTEN pipeline (Duverne et al. 
2021).


T-T0 (hr)| MJD   | Obser.     |Exposure| Filter| Mag +/- err (AB)
_________________________________________________________________
0.04 |59696.878276| TRE       |     60s| Clear | 15.7 +/- 0.2
1.38 |59696.932836| Makes-T60 | 15x120s| Clear | 20.1 +/- 0.1
1.65 |59696.944294| Makes-T60 | 15x120s| Clear | 20.4 +/- 0.2
11.37|59697.3491  | KNC-CDK400| 10x300s| R     | > 21.0

T-T0 is the mid-time of observations related to the BAT trigger time.

These detections are consistent with the previous report of detections 
by the VLT (A. de Ugarte Postigo et al. GCN 31967
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 ).

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) 
devoted to the observation of transients in the context of 
multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). 
Kilonova-Catcher (KNC) is the citizen science program of GRANDMA 
(http://kilonovacatcher.in2p3.fr/).

R. Strausbaugh (U. of the Virgin Islands), A. Cucchiara (College of Marin) report on behalf of a larger collaboration:

We observed Swift GRB220427A (D'Ai et al. GCN 31960
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 ) with the LCO 1-m Sinistro instrument at the South African Astronomical Observatory site, on April 27, from 22:57 to 23:24 UT (corresponding to 1.95 to 2.40 hours from the GRB trigger time) with the Bessel R and I filters.

We performed a series of 3x300s exposures in I and R bands. We detect an optical counterpart at the location of the Swift optical counterpart that is consistent with other optical detections (Ugarte Postigo, et al., GCN 31967
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 ; Klotz, et al., GCN 31991
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 ). The following magnitudes are calculated using the USNO-B.1 catalog as reference:

R=20.35+/-0.12
I=20.15+/-0.30

These magnitudes are not corrected for galactic extinction.

R.S. is funded by NSF AST grant #1831682

S. de Wet (UCT), D.B. Malesani (Radboud), P.M. Vreeswijk (Radboud)
and P.J. Groot (Radboud/UCT/SAAO) report on behalf of the MeerLICHT
consortium:

Following the detection of GRB 220427A by Swift and its X-ray counterpart
(D'Ai et al., GCN 31960
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 ), the 0.6m MeerLICHT telescope, located at
Sutherland, South Africa obtained a repeating series of 60 s images in the
q,u,q,g,q,r,q,i,q,z bands. Observations began at 2022-04-27, 23:41:48 UT,
2 hours 41 minutes after the Swift detection, and continued for a further
4.5 hours. The median seeing was 2" under clear skies.

No source is detected at the optical transient position (D'Ai et al., GCN
31960
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 )
in any individual 60 s exposure, hence we co-added all the images in each
filter and performed forced photometry at the transient position. The
afterglow
is detected with a SNR of at least 7 in the g,r,i,z and q bands. We
report the following upper limit and AB magnitudes, at 3 hours 50 minutes
post-trigger:

u > 21.83
g = 21.26 +/- 0.11
q = 21.14 +/- 0.09
r = 20.88 +/- 0.12
i = 20.49 +/- 0.13
z = 19.59 +/- 0.15

MeerLICHT is built and run by a consortium consisting of Radboud
University, University of Cape Town, the South African Astronomical
Observatory, the University of Oxford, the University of Manchester
and the University of Amsterdam.