EP250615a, GRB 250615A

S. Dichiara (PSU), J. J. DeLaunay (PSU), R. Gupta (NASA GSFC) and
T. Sakamoto (AGU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 22:25:19.59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250615A (trigger=1324646). Swift slewed immediately to the burst.
The BAT on-board calculated location is RA, Dec 290.344, +4.672 which is 
   RA(J2000) = 19h 21m 23s
   Dec(J2000) = +04d 40’ 21"
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 55 sec.  The peak count rate 
was ~5000 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 22:26:46.4 UT, 86.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 290.38466, 4.68467
which is equivalent to:
   RA(J2000)  = 19h 21m 32.32s
   Dec(J2000) = +04d 41' 04.8"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 152 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 does not constrain the column density. 

The initial flux in the 2.5 s image was 9.89e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
152 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. Results from the 2.7'x2.7' sub-image are
not available at this time. 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 18.0 mag. No correction has been made for the large, but
uncertain, extinction expected.

Burst Advocate for this burst is S. Dichiara (sbd5667 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/)

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using 1.0 ks of promptly downlinked XRT event data for GRB 250615A, we
find an enhanced XRT position of the afterglow: RA, Dec: 290.38555,
4.68527 which is equivalent to:
   RA (J2000)  = 19 21 32.53
   Dec (J2000) = +04 41 07.0
with an uncertainty of 2.0 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/1324646.

Position enhancement is described by Goad et al. (2007, A&A, 476, 1401)
and Evans et al. (2009, MNRAS, 397, 1177).

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

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

Using 1622 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 250615A, 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 = 290.38565, +4.68515 which is equivalent
to:

RA (J2000): 19h 21m 32.56s
Dec (J2000): +04d 41' 06.5"

with an uncertainty of 2.3 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.

G. J. Yang, M. J. Liu (NAO, CAS), Y. Q. Zhao (USTC,PRIC), Y. P. Zhou (NJU), X. L. Chen (YNU), Q. Y. Wu, C. C. Jin (NAO,CAS) on behalf of the Einstein Probe team:

We report on the detection of a fast X-ray transient, designated EP250615a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2025-06-15T22:25:17 (UTC). The position of the source is R.A. = 290.379 deg, DEC = 4.678 deg (J2000) with an uncertainty of 2.181 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for more than 50 seconds. The lightcurve of the transient observed by the WXT exhibits a rapid brightening profile. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 2.2 (+1.5/-1.4) with a column density value of 2.1 (+1.7/-1.6) x 10^22 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 1.0 (+4.3/-0.6) x 10^-8 erg/s/cm^2. The peak flux is around 6.3 x 10^-7 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. The trigger time and position of this X-ray transient are consistent with that of GRB 250615A (GCN 40736
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 , GCN 40737
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 , GCN 40739
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 ).

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

D.N. Burrows (PSU), M.A. Williams (PSU), S. Dichiara (PSU), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester), M.G. Bernardini
(INAF-OAB), E. Ambrosi	(INAF-IASFPA) , M. Capalbi (INAF-OAR) and P.A.
Evans report on behalf of the Swift-XRT team:

We have analysed 6.2 ks of XRT data for GRB 250615A, from 93 s to 40.3
ks after the   trigger. The data comprise 106 s in Windowed Timing (WT)
mode with the remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.62 (+0.07, -0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.62 (+0.27, -0.26). The
best-fitting absorption column is  2.4 (+0.6, -0.5) x 10^22 cm^-2, in
excess of the Galactic value of 5.5 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.0 (+0.4, -0.3) and
a best-fitting absorption column of 2.4 (+0.7, -0.6) x 10^22 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 6.7 x 10^-11 (1.5 x 10^-10) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.4 (+0.7, -0.6) x 10^22 cm^-2
Galactic foreground: 5.5 x 10^21 cm^-2
Excess significance: 5.3 sigma
Photon index:	     2.0 (+0.4, -0.3)

If the light curve continues to decay with a power-law decay index of
1.62, the count rate at T+24 hours will be 2.2 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.5 x
10^-14 (3.3 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01324646.

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

N.P.M.Kuin (UCL/MSSL) and S. Dichiara (PSU) report on behalf of the
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250615A
152 s after the BAT trigger (Dichiara et al., GCN Circ. 40736
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 ).
No optical afterglow consistent with the enhanced XRT position (Goad et
al., GCN Circ. 40739
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 ) is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

u_FC               152          402          246         >19.8
white              432         1722          284         >21.0
v                  483         1772          156         >19.5
b                  408         1697          136         >20.7
u                  152         1672          363         >20.0
w1                 532         1821          136         >19.5
w2                 804         1747           78         >19.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 1.165 in the direction of the burst
(Schlegel et al. 1998).

Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Sahil Atri (U Roma), Camila Angulo Valdez (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM), Margarita Pereyra (UNAM) and Eleonora Troja (U Roma) report:

We observed the field of GRB 250615A detected by Swift (Dichiara et al., GCN Circ. 40736
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 ) with the DDOTI/OAN wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir (http://ddoti.astroscu.unam.mx) on the night of 2025-06-16 UTC. 

DDOTI observed the enhanced Swift-XRT position (Goad et al., GCN Circ. 40739
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 ) from 05:18 UTC to 09:55 UTC (from T+6.9 h to T+ 11.5 h after the trigger), obtaining a total exposure of 1.6 hours, alternating with other scientific programs.

Comparing our observations to Pan-STARRS PS1 DR2 catalogues, we detect no uncatalogued fading sources within the observed field down to a 3-sigma limiting AB magnitude of: 

w >19.3

This upper limit is consistent with the non-detection reported by Swift/UVOT (Kuin et al., GCN Circ. 40747
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 ).

This value is not corrected for Galactic extinction, which is significant due to the burst’s proximity to the Galactic plane.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra of San Pedro Mártir.

Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):


We imaged the field of the Swift GRB 250615A (Dichiara et al., GCN Circ. 40736
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 ) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-06-16 08:40 to 2025-06-16 10:46 UTC (from 10.3 to 12.3 hours after the trigger) and obtained 96 minutes of exposure in the i filter.

The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

In our stacked image, we do not detect any new source at the enhanced Swift-XRT position (Goad et al., GCN Circ. 40739
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 ) down to a 3-sigma limiting AB magnitude of: 

i > 22.8

Our upper limit is consistent with the non-detection reported by Swift/UVOT (Kuin et al., GCN Circ. 40747
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 ) and DDOTI (Becerra et al., GCN Circ. 40748
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 ).

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.

M. Tembhurnikar (IUCAA), G. Waratkar (IITB), Harsha K. H. (IUCAA), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long duration GRB 250615A / EP250615a which was also detected by Swift/BAT (Dichiara et al., GCN Circ. 40736
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 ) and Einstein Probe/WXT (Yang et al., GCN Circ. 40743
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 ). The burst location was at an angle of 84 deg from the pointing axis of AstroSat CZTI.

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-06-15 22:25:17.50 UTC. The measured peak count rate associated with the burst is 99 (+31, -11) counts/s above the background in the combined data of two quadrants (out of four), with a total of 983 (+231, -222) counts. The local mean background count rate was 155 (+1, -2) counts/s. Using cumulative rates, we measure a T90 of 22 (+15, -5) s.

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-06-15 22:25:22.52 UTC. The measured peak count rate associated with the burst is 490 (+70, -50) counts/s above the background in the combined data of all quadrants, with a total of 7176 (+826, -898) counts. The local mean background count rate was 1329 (+7, -8) counts/s. We measure a T90 of 34 (+6, -10) s from the cumulative Veto light curve.

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb

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-duration GRB 250615A (Swift detection: Dichiara et al., GCN 40736
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 ;
AstroSat CZTI detection: Tembhurnikar et al., GCN 40752
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 ),
coincident with the X-ray transient EP250615a (Yang et al., GCN 40743
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 ),
triggered Konus-Wind (KW) at T0=80722.429 s UT (22:25:22.429).

The burst light curve shows a multi-peaked emission pulse
which starts at ~T0-5 s and has a total duration of ~50 s.
The emission is seen up to ~4 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250615_T80722/

As observed by Konus-Wind, the burst had
a fluence of (3.57 ± 0.50)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 5.248 s,
of (5.32 ± 0.53)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+49.408 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -0.96 (-0.09,+0.11),
the high energy photon index beta = -2.41 (-1.15,+0.35),
the peak energy Ep = 908 (-187,+218) keV,
chi2 = 84/97 dof.

The spectrum near the peak count rate(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -0.92 (-0.09,+0.09),
the high energy photon index beta = -2.62 (-7.38,+0.41),
the peak energy Ep = 1290 (-234,+320) keV,
chi2 = 89/93 dof.

All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.

Giovanni Calapai at Calapai Astronomical Observatory, Massa S. Giorgio, (Messina) Italy 
Member of: GRB/UAI Gamma Ray Burst Section of Unione Astrofili Italiani. 

Report:

We imaged the field of GRB 250615A detected by Swift/BAT (Dichiara et al. GCN 40736
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 ), with the 11 inches Schmidt-Cassegrain (Celestron 11) telescope F/D=6,3. 

The observations were started at 2025-06-15 23:21:59 UT (approximately 56 minutes after burst) stacking a set of unfiltered CCD image.

We co-added 180 exposures of 60 sec each.

Start T0+      End T0+     CR lim
0.94 hour     4.33 hour     19.6

We did not found any optical uncatalogued object within the Swift error circle.

Magnitudes were estimated with the PanSTARRS cat. and are not corrected for galactic dust extinction.

Our upper limit is consistent with other observations reported by Kuin et al. (GCN 40747
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 ), Becerra et al. (GCN 40748
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 ), Magnani et al. (GCN 40749
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 ).

The message may be cited.

S. D. Barthelmy (GSFC), S. Dichiara (PSU), R. Gupta (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa),
C. B. Markwardt (GSFC), M. J. Moss (GSFC), D. M. Palmer (LANL),
T. Parsotan (GSFC), D. Sadaula (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 250615A (trigger #1324646)
(Dichiara, et al., GCN Circ. 40736
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 ).  The BAT ground-calculated position is
RA, Dec = 290.384, 4.679 deg which is
   RA(J2000)  =  19h 21m 32.1s
   Dec(J2000) = +04d 40' 44.0"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 56%.

The mask-weighted light curve shows a complex structure that
starts at T-5 sec, peaks at T+5 sec, and ends at T+77 sec.
T90 (15-350 keV) is 52.5 +- 10.3 sec (estimated error including systematics).

The time-averaged spectrum from -5.2 to 77.3 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.22 +- 0.08.  The fluence in the 15-150 keV band is 3.8 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+3.29 sec in the 15-150 keV band
is 4.1 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1324646

Malte Busmann (LMU), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU), and Antonella Palmese (Carnegie Mellon U.) report:

We observed the improved localization (Evans et al., GCN 40737
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 ; Goad et al., GCN 40739
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 ) of GRB 250615A/EP250615a (Dichiara et al., GCN 40736
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 ; Yang et al., GCN 40743
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 ; Burrows et al., GCN 40745
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 ; Tembhurnikar et al., GCN 40752
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 ; Frederiks et al., GCN 40753
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 ; Barthelmy et al. GCN 40765
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 ) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 20 x 180 s starting at 2025-06-17T21:29:37 UT (1.96 days after the trigger). We detect no new source in the 90% Swift/XRT localization (Evans et al., GCN 40737
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 ; Goad et al., GCN 40739
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 ) down to a 3-sigma depth of

r > 23.9 mag
i > 23.3 mag
J > 21.8 mag.

These upper limits are consistent with the previous observations by Kuin et al. (GCN 40747
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 ), Becerra et al. (GCN 40748
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 ),  Magnani et al. (GCN 40749
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 ), and Calapai & Giorgio (GCN 40759
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 ).

The r and i band magnitudes are calibrated against the PS1 catalog and the J band is calibrated with the 2MASS Catalog. All magnitudes are provided in the AB system and are not corrected for the significant Galactic extinction at this location.

We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.

D. Turpin (CEA-Saclay/Irfu), R. Hellot,  M. Freeberg (KNC), C. Andrade(UMN), M. Pillas (ULiege), M. Tanasan (NARIT), S. Antier (OCA/IJCLAB) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 250615A (Dichiara et al., GCN 40736
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 ) detected by Swift/BAT with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the TEC160FL telescope operated by M. Freeberg and a CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot. Our observations started at TGRB+7.6hr. 

In our stacked frames, subtracted from the PanSTARRS DR2 template image, we do not detect any optical counterpart inside the Swift/XRT refined position (Goad et al., GCN 40739
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 ).

We report our follow-up results in the table below:
+---------------+-------------+---------+---------------------+--------------+
| Tmid-TGRB (hr)| Exp (s)     | Filter  | Magnitude           | Instrument   |
+===============+=============+=========+=====================+==============+
| 7.9           | 6 x 300s    | r (AB)  | 20.1 (U.L., 5sigma) | TEC160FL     | 
| 8.1           | 11 x 300s   | g (AB)  | 20.8 (U.L., 5sigma) | CDK17-AITP   | 
| 8.5           | 6 x 300s    | g (AB)  | 20.4 (U.L., 5sigma) | TEC160FL     | 
| 9.0           | 6 x 300s    | i (AB)  | 19.2 (U.L., 5sigma) | TEC160FL     | 
| 9.6           | 11 x 300s   | r (AB)  | 20.2 (U.L., 5sigma) | CDK17-AITP   | 
+---------------+-------------+---------+---------------------+--------------+

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan images were calibrated using the PanSTARRS DR1 catalog.

We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). 

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/).