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GRB 250424A

GCN Circular 40920

Subject
GRB 250424A: VIRT Optical Upper Limit
Date
2025-07-03T15:22:30Z (4 months ago)
From
Priya Gokuldass at ERAU <gokuldap@my.erau.edu>
Via
Web form
K. Smith (UVI), P. Gokuldass (ERAU), N. Orange (OrangeWave Innovative Science, LLC), B. Gendre (UVI), D. Morris (NASA), T. Lombardi (Eckerd College), F. George (ERAU), D. Smith (UVI), C. Watson (UVI) report:

We observed the field of GRB 250424A (Cenko et al., GCN 40224) detected by Swift/BAT with the 0.5m Virgin Island Robotic Telescope (VIRT) at the University of the Virgin Islands' Etelman Observatory on 2025-05-06 starting at 02:22:01.68 UTC (T-mid ~ T0 + 7d 3.45h). We performed a series of exposures in an R filter with a total exposure of 2860s. The weather conditions were partly cloudy during the hours of observation with an average airmass of 1.8. 

We do not detect any source at the enhanced XRT position reported (Evans et al., GCN 40232). This non-detection is consistent with reported detections (Francile et al., GCN 40222; Brivio et al., GCN 40225; de Wet et al., GCN 40229; Becerra et al., GCN 40226; Saccardi et al., GCN 40228; Siegel et al., GCN 40244; Dutton et al., GCN 40241; Schneider et al., GCN 40250; Ducoin et al., GCN 40230; Turpin et al., GCN 40240; Elkabir et al., GCN 40251; Ghosh et al., GCN 40263; de Ugarte Postigo et al., GCN 40461 ) and upper limits  (Lipunov et al., GCN 40223; Siegel et al., GCN 40244). We report the following 3-sigma upper limit:

T_mid                ||Exposure   ||Filter   ||Limit 
T+ 7d 3.45h          || 2860 s    || R       ||> 20.7

The limit is estimated from comparison to nearby USNO B1 stars and is not corrected for Galactic extinction. 

We acknowledge financial support from NASA EPSCoR award 80NNSC22M0063, NSF PAARE award 2319415, and NASA EPSCoR award 80NSSC24M0112. This message can be cited.

GCN Circular 40461

Subject
GRB 250424A: COLIBRÍ detection of the associated supernova
Date
2025-05-15T16:39:06Z (6 months ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
Antonio de Ugarte Postigo (LAM), Benjamin Schneider (LAM), Stéphane Basa (UAR Pytheas), Ny Avo Rakotondrainibe (LAM), Alan M. Watson (UNAM), Kin Ocelotl López (UNAM), William H. Lee (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), Noémie Globus (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), and Margarita Pereyra (UNAM):

We have continued imaging the field of GRB 250424A (Cenko et al., GCN Circ. 40224 <https://gcn.nasa.gov/circulars/40224>; Harsha et al. GCN Circ. 40231; Ridnaia et al. GCN Circ. 40243; McKenna et al. GCN Circ. 40249; Zhang et al. GCN Circ. 40252; Nakahira et al. GCN Circ. 40298) at a redshift of z = 0.310 (Saccardi et al. GCN Circ. 40228) to monitor the evolution of the optical transient (Brivio et al. GCN Circ. 40225; Becerra et al. GCN Circ. 40226; de Wet et al. GCN Circ. 40229; Ducoin et al. GCN Circ. 40230; Turpin et al. GCN Circ. 40240; Dutton et al. GCN Circ. 40241; Siegel et al. GCN Circ. 40244; Hu et al. GCN Circ. 40246; Schneider et al. GCN Circ. 40250; Elkabir et al. GCN Circ. 40251; Ghosh et al. GCN Circ. 40263), in search for the possible SN emission, using the DDRAGO wide-field imager on the COLIBRÍ telescope. 

The field is covered by the Legacy Survey, which reveals an underlying host galaxy with AB magnitudes g=22.60, r=21.98, i=22.05 (Pérez-Fournon et al. GCN Circ. 40227). 

A GRB-SN at that redshift would be expected to reach peak light between the 8th and the 15th of May (Cano et al. 2017). We hereby report two photometric observations of the optical counterpart obtained in the r- and i-bands. The data were reduced and coadded with the COLIBRÍ pipeline and analyzed 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.

An r-band observation consisting of 112x60s exposures was obtained with mean epoch 2025-05-11 06:49:32 UTC (16.998 d after the burst, 12.976 d in the rest frame). We measure r = 21.54 +/- 0.21 mag.

An i-band observation consisting of 111x60s exposures was obtained with mean epoch 2025-05-15 06:42:39 UTC (20.993 d after the burst, 16.025 in the rest frame). We measure i = 21.46 +/- 0.12 mag.

Both values are clearly in excess of the underlying host galaxy, and indeed an excess emission is obtained when performing image subtraction with respect to the archival Legacy Survey image. 

Subtracting the host contribution we obtain values of r ~ 22.7 mag and i ~ 22.4 mag which are in agreement to the expectations of a GRB-SN at this redshift with a certain amount of extinction. We consequently suggest that COLIBRÍ is currently detecting the supernova associated with GRB 250424A.

Further observations to confirm and monitor the evolution of this GRB-SN are encouraged.

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.

GCN Circular 40298

Subject
GRB 250424A: CALET Gamma-Ray Burst Monitor detection
Date
2025-05-01T03:19:12Z (6 months ago)
From
Yuta Kawakubo at Aoyama Gakuin University <kawakubo@phys.aoyama.ac.jp>
Via
Web form
GRB 250424A: CALET Gamma-Ray Burst Monitor detection

S. Nakahira (JAXA), A. Yoshida, T. Sakamoto, S. Sugita, Y. Kawakubo (AGU),
K. Yamaoka (Nagoya U), Y. Asaoka (ICRR), S. Torii, Y. Akaike, 
K. Kobayashi (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U),
N. Cannady (GSFC/UMBC), M. L. Cherry (LSU), S. Ricciarini (U of Florence),
P. S. Marrocchesi (U of Siena),
and the CALET collaboration:

The long GRB 250424A (Swift detection: Cenko et al., GCN Circ. 40224;
Swift-BAT refined analysis: Markwardt et al., GCN Circ. 40255; AstroSat
CZTI detection: Harsha et al., GCN Circ. 40231; Konus-Wind detection:
Ridnaia et al., GCN Circ. 40243; EIRSAT-1 GMOD Detection: McKenna et
al., GCN Circ. 40249; SVOM/GRM observation: Jin-Peng et al.,  GCN Circ.
40252) triggered the CALET Gamma-ray Burst Monitor (CGBM) at 
06:52:04.323 UTC on 24 April 2025
(https://cgbm.calet.jp/cgbm_trigger/flight/1429512582/index.html).
The burst signal was seen by all CGBM detectors. 

The burst light curve shows a single pulse that starts
at T+2.5 sec, peaks at T+8.4 sec, and ends at T+27.5 sec.
The T90 and T50 durations measured by the SGM data are 13.5 +/- 0.8 sec
and 4.6 +/- 0.2 sec (40-1000 keV), respectively.

The ground-processed light curve is available at
https://cgbm.calet.jp/cgbm_trigger/ground/1429512582/

The CALET data used in this analysis are provided by
the Waseda CALET Operation Center located at Waseda University.

GCN Circular 40263

Subject
GRB 250424A: LCO optical observation
Date
2025-04-27T13:05:34Z (6 months ago)
From
ankur ghosh at CAPP, University of Johannesburg <ghosh.ankur1994@gmail.com>
Via
Web form
Ankur Ghosh, Soebur Razzaque (CAPP, University of Johannesburg), Alexander Moskvitin, Yulia Sotnikova (SAO RAS), Naveen Dukiya (ARIES), Rahul Gupta (NASA GSFC) on behalf of a larger collaboration.
 
We observed the field of the GRB 250424A triggered by Swift (Cenko et al., GCN 40224), AstroSat CZTI (Harsha et al., GCN 40231), Konus-Wind (Ridnaia et al., GCN 40243) and EIRSAT-1 GMOD (McKenna et al., GCN 40249)in B, V, r  filters of  the 1-meter Sinistro at the Las Cumbres Observatory Global Telescope (LCOGT) node located at  Cerro Tololo Interamerican Observatory, Chille. The 1-m Sinistro telescope is equipped with a 4K x 4K CCD (FOV: 26 x 26 arcmin, scale: 0.39 arcsec/pixel).

Observations began on , starting from 2025-04-24, 17.8 hours after the GRB trigger. Observation for later epochs are still going on.

We clearly detect the optical transient (OT) reported by GCNs (Francile et al., GCN 40222; Cenko et al., GCN 40224; Brivio et al., GCN 40225; Becerra et al., GCN 40226; Saccardi et al., GCN 40228; de Wet et al., GCN 40229; Ducoin et al., GCN 40230; and D. Turpin et al., GCN 40240, Dutton et al., GCN Circ. 40241, Siegel et al., GCN 40244)  in our B, V, r band images. 

------------------------------------------------------------------------------------------------------
|Date|		Start JD	|t-T0 (hours)|	|Exp (sec)|	|Filter|	|Magnitude|  
------------------------------------------------------------------------------------------------------     
2025-04-25	2460790.52730	17.78		1 x 900 	V		V = 21.31+/- 0.08
2025-04-25	2460790.78105	23.87		1 x 900 	B		B = 22.07+/- 0.08
2025-04-25	2460790.79155	24.12		1 x 900		r		r = 21.04 +/- 0.05
------------------------------------------------------------------------------------------------------

The field was calibrated against nearby APASS stars, with magnitudes converted using Lupton (2005) equations, and has not been corrected for Galactic extinction.

GCN Circular 40255

Subject
GRB 250424A: Swift-BAT refined analysis
Date
2025-04-26T22:48:38Z (6 months ago)
From
Rahul Gupta at NASA GSFC <rahul.gupta@nasa.gov>
Via
Web form
C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC), 
S. B. Cenko (GSFC), R. Gupta (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
A. Y. Lien (U Tampa), 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 250424A (trigger #1306404)
(Cenko, et al., GCN Circ. 40224).  The BAT ground-calculated position is
RA, Dec = 217.528, -35.025 deg which is 
   RA(J2000)  =  14h 30m 06.7s 
   Dec(J2000) = -35d 01' 30.6" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 39%.
 
The mask-weighted BAT light curve of the burst (began during a slew) shows a faint precursor emission followed by a bright main pulse. 
T90 (15-350 keV) is 19.03 +- 1.06 sec (estimated error including systematics), with the T90 starting at T0-20.76 sec due to the slew-delayed T0.

 
The time-averaged spectrum from T-38.75 to T+262.67 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.54 +- 0.13, 
and Epeak of 106.5 +- 33.7 keV (chi squared 47.05 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 3.2 +- 0.1 x 10^-05 erg/cm2
and the 1-sec peak flux measured from T-15.38 sec in the 15-150 keV band is
47.0 +- 1.3 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.79 +- 0.03 (chi squared 58.54 for 57 d.o.f.).  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/1306404

GCN Circular 40252

Subject
GRB 250424A: SVOM/GRM observation of a long burst
Date
2025-04-26T13:30:39Z (6 months ago)
From
zhangjinpeng@ihep.ac.cn
Via
Web form
SVOM/GRM team: Jin-Peng Zhang, Chen-Wei Wang, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Jean-Luc Atteia (IRAP), Olivier Godet (IRAP), Stéphane Schanne (CEA), Frédéric Daigne (IAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 250424A (SVOM trigger reference: sb25042401) at 2025-04-24T06:52:08.000 (T0). This burst was also detected by Swift (Cenko et al., GCN 40224), AstroSat CZTI (Harsha et al., GCN 40231), Konus-Wind (Ridnaia et al., GCN 40243) and EIRSAT-1 GMOD (McKenna et al., GCN 40249).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multi-pulses with a T90 of 18.8 +1.0/-1.0 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250424A.png

In addition, the position of this burst, as determined by Swift (Cenko et al., GCN 40224, RA: 217.50010 deg, DEC: -35.02493 deg, Error: 1.9 arcseconds), is located at about 63.7 degrees from the SVOM optical axis, and outside the ECLAIRs field of view.

The time-averaged spectrum from T0-5 to T0+18 s is best fitted by Band function. The alpha is -1.14 +/- 0.06, the beta is -2.44 +/- 0.08, and the cutoff energy, parameterized as Epeak, is 89 +/- 5 keV. The event fluence (10-1000 keV) in this time interval is (6.63 +/- 0.12)E-05 erg/cm^2. With a redshift of 0.310 (Saccardi et al., GCN 40228), GRB 250424A is consistent with Type II GRBs in the 'Amati' relation diagram, as shown at:
https://www.bursthub.cn//admin/static/grb250424A_amati.png

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.

The SVOM/GRM point of contact for this burst is: Jin-Peng Zhang (IHEP) (zhangjinpeng@ihep.ac.cn)

GCN Circular 40251

Subject
GRB 250424A: PRIME near-infrared detection
Date
2025-04-26T00:30:37Z (6 months ago)
Edited On
2025-04-28T14:09:12Z (6 months ago)
From
O. Guiffreda at UMD <oriogui@umd.edu>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of O. Guiffreda at UMD <oriogui@umd.edu>
Via
Web form

M. Elkabir (U Rome), O. Guiffreda (UMD), J. Durbak (UMD), N. Passaleva (U Rome), E. Troja (U Rome), A. S. Kutyrev (NASA/GSFC), S. B. Cenko (NASA/GSFC)

Following the Swift BAT detection (GCN 40224

Loading...
 
 
), we observed the transient field in J and H filters with PRIME ~12 hours after the initial Swift detection.

At the position of the optical counterpart reported by Swift UVOT (GCN 40244

Loading...
 
 
), we detect an uncatalogued source in both J and H band. Using nearby VISTA Hemispherical Survey (VHS) for preliminary calibration we derive the following magnitudes and limits, not corrected for Galactic extinction:

FilterMag(AB)
J19.2 +/- 0.06
H18.8 +/- 0.05

PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024).

We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations.

GCN Circular 40250

Subject
GRB 250424A: WINTER J-band detection
Date
2025-04-25T20:51:47Z (6 months ago)
From
Benjamin Schneider at MIT <bschn@mit.edu>
Via
Web form
Benjamin Schneider (LAM), Geoffrey Mo (MIT), Tomas Ahumada (Caltech), Viraj Karambelkar (Caltech), Danielle Frostig (CfA), Robert Stein (UMD), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:

We observed the GRB 250424A (Cenko et al., GCN 40224; Harsha et al., GCN 40231; Ridnaia et al., GCN 40243) in the near-infrared with the Palomar 1-m telescope, equipped with the 1-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). Observations started on 2025-04-24 at 07:33:24 UT (40.92 min after the Swift trigger) and consisted of 15 exposures of 120 s in the J-band.

In the stacked image, we detect the optical counterpart reported by Cenko et al., GCN 40224; Brivio et al., GCN 40225; Becerra et al., GCN 40226; Saccardi et al., GCN 40228; de Wet et al., GCN 40229; Ducoin et al., GCN 40230; Turpin et al., GCN 40240, Dutton et al., GCN 40241; Siegel et al., GCN 40244. The preliminary AB magnitude derived for that source is:

J = 18.1 +/- 0.2

The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565). The photometric calibration was performed using nearby stars from the 2MASS catalog and the magnitude is not corrected for Galactic extinction.

WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.

GCN Circular 40249

Subject
GRB 250424A: EIRSAT-1 GMOD Detection
Date
2025-04-25T20:36:05Z (6 months ago)
From
Caimin McKenna at University College Dublin <caimin.mckenna@ucdconnect.ie>
Via
Web form

C. McKenna, P. McDermott, D. Murphy, C. de Barra, A. Ulyanov, G. Finneran, G. Corcoran, L. Cotter, A. Empey, J. Fisher, F. Gibson Kiely, J. Thompson, D. McKeown, A. Martin-Carrillo, L. Hanlon, S. McBreen, on behalf of the EIRSAT-1 team:

EIRSAT-1 reports the detection of the long gamma-ray burst GRB 250424A by the Gamma-ray Module (GMOD) instrument, which was also detected by Swift-BAT (GCN 40224), Calet/CGBM (Trigger No. 1429512582), AstroSat CZTI (GCN 40231), and Konus-Wind (GCN 40243). The GMOD detection was made starting at 2025-04-24 06:52:12.6 UTC.

The GMOD light curve for GRB 250424A, with 1.2s binning, shows a long, smooth, single pulse, consistent with other observations.

The spacecraft location at time of detection was 23.407 S, 123.088 W and an altitude of 402.9 km.

The light curve for this event as measured by GMOD can be found here: https://grb.eirsat1.ie/250424A/250424A_LC_onboard_preliminary.png

EIRSAT-1 is Ireland’s first satellite (Doyle et al. Proceedings of the 4th SSEA, 2022). It is a 2U CubeSat and carries onboard a number of experiments including the Gamma-Ray Module (GMOD), a novel, compact, gamma-ray detector (Murphy et al, Experimental Astronomy, 53, 961–990, 2022). GMOD consists of a 25 mm × 25 mm × 40 mm Cerium Bromide scintillator coupled to SiPMs and is designed to detect gamma-ray bursts in the ~ 60 keV - 1.5 MeV range. EIRSAT-1 was developed in University College Dublin with support from ESA’s Fly Your Satellite! programme and was launched on 1st December 2023.

GCN Circular 40246

Subject
GRB 250424A: SVOM/VT optical observation
Date
2025-04-25T11:38:33Z (6 months ago)
Edited On
2025-04-25T13:32:26Z (6 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y. D. Hu(GXU), L. Zhang(IHEP), X. L. Chen(YNU),  L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, Z. H. Yao, X. H. Han,Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC) report on behalf of the SVOM/VT team.

SVOM/VT conducted ToO follow-up observations of the GRB 250424A(Francile et al., GCN 40222). The observation started on 2025 Apr 24 09:26:29 UT in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously.

The candidate (Francile et al., GCN 40222; Cenko et al., GCN 40224; Brivio et al., GCN 40225; Becerra et al., GCN 40226; Saccardi et al., GCN 40228; de Wet et al., GCN 40229; Ducoin et al., GCN 40230; D. Turpin et al., GCN 40240; Dutton et al., GCN 40241; and Siegel et al., GCN 40244) was clearly detected in stacked images of both channels.

The brightness in AB magnitude was estimated to be:

Mid time (hour)   | Band | Exposure Time (second) | Magnitude | Magnitude error
2.98          | VT_B | 50x60  | 19.89 | 0.03
2.98          | VT_R | 50x60  | 18.96 | 0.02

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC),CAS.

GCN Circular 40244

Subject
GRB 250424A: Swift/UVOT Detection
Date
2025-04-24T23:09:21Z (6 months ago)
From
Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>
Via
Web form
M.H. Siegel (PSU) and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250424A 254 s after the BAT trigger (Cenko et al., GCN Circ. 40224). A source consistent with the XRT position (Evans et al., GCN Circ. 40232) and the optical transient (Francile et al., GCN Circ. 40222, Brivio et al., GCN Circ. 40225; Becerra et al., GCN Circ. 40226; de Wet et al., GCN Circ. 40229; Ducoin et al., GCN Circ. 40230; Turpin et al., GCN Circ. 40240; Dutton et al., GCN Circ. 40241) is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  14:29:60.00 = 217.49998 (deg.)
    Dec (J2000) = -35:01:30.6  = -35.02518 (deg.)
with an estimated uncertainty of 0.47 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              254          404          147         18.96 +/- 0.07
v                  411         1918          175        >18.7
b                  509         1844          136         19.97 +/- 0.26
u                  485         1647          136         19.15 +/- 0.20
w1                 460         1795          156         19.44 +/- 0.31
m2                 436         1598           97        >18.4
w2                 559         1894          156        >19.9

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.064 in the direction of the burst
(Schlegel et al. 1998).

GCN Circular 40243

Subject
Konus-Wind detection of GRB 250424A
Date
2025-04-24T22:12:37Z (6 months ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB GRB 250424A
(Swift-BAT detection: Cenko et al., GCN 40224;
AstroSat CZTI detection: Harsha et al., GCN 40231)
triggered Konus-Wind at T0=24733.431 s UT (06:52:13.431).

The burst light curve shows a single pulse
which starts at ~T0-17.9 s and has a total duration of ~40.5 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/GRB250424_T24733/

As observed by Konus-Wind, the burst
had a fluence of 5.83(-0.16,+0.16)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+4.192 s,
of 8.88(-0.94,+0.95)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+21.760 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.30(-0.06,+0.06),
the high energy photon index beta = -2.87(-0.15,+0.11),
the peak energy Ep = 104(-3,+3) keV
(chi2 = 94/82 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+5.376 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.09(-0.06,+0.07),
the high energy photon index beta = -2.91(-0.15,+0.12),
the peak energy Ep = 129(-4,+5) keV
(chi2 = 72/66 dof).

Assuming the redshift z=0.310 (Saccardi et al., GCN 40228)
and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315,
and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is 1.49(-0.04,+0.04)x10^52 erg,
the peak luminosity L_iso is 2.97(-0.31,+0.32)x10^51 erg/s,
the rest-frame peak energy of the time-averaged spectrum
Ep,i,z is 137(-4,+4) keV and the spectrum near the maximum count rate
Ep,p,z is 169(-5,+7) keV.

With the obtained estimates, GRB 250424A is inside 68% prediction bands for
both 'Amati' and 'Yonetoku' relations derived for the sample of >300 long
KW GRBs with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021),
see http://www.ioffe.ru/LEA/GRBs/GRB250424_T24733/GRB250424A_rest_frame.pdf

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.

GCN Circular 40241

Subject
GRB 250424A: Skynet Optical Observations
Date
2025-04-24T20:17:01Z (6 months ago)
From
Dylan Dutton at UNC Chapel Hill <ddutton59@gmail.com>
Via
Web form
Dylan Dutton, Daniel Reichart, Joshua Haislip, Vladimir Kouprianov, and Donovan Schlekat report on behalf of the Skynet Robotic Telescope Network at the University of North Carolina at Chapel Hill.

We observed the field of GRB 250424A detected by Swift (Cenko, GCN 40224) with one of Skynet's PROMPT telescopes located at the Cerro Tololo Inter-American Observatory.

We detect the optical afterglow (Francile et al., GCN 40222; Cenko et al., GCN 40224; Brivio et al., GCN 40225; Becerra et al., GCN 40226; Saccardi et al., GCN 40228; de Wet et al., GCN 40229; Ducoin et al., GCN 40230; and D. Turpin et al., GCN 40240) in the B, V, and R bands and report the initial photometry below. Exposure lengths were calculated using our automated exposure length scaling model.

Tmid - T0 (s)| Telescope | Filter | Exposure (s) | Mag	   | Mag Error
------------------------------------------------------------------
1224         | PROMPT-5  | B      | 264          | 19.610  | 0.054
1467	     | PROMPT-5  | V      | 216          | 19.253  | 0.065
1649	     | PROMPT-5  | R      | 144          | 18.500  | 0.051

Our images have been calibrated using stars from the APASS catalog. Magnitudes were not corrected for dust extinction.

GCN Circular 40240

Subject
GRB 250424A: Kilonova-Catcher optical afterglow detection
Date
2025-04-24T18:25:31Z (6 months ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
D. Turpin (CEA-Saclay/Irfu), R. Hellot, P. Jaquiery, M. Freeberg (KNC), C. Andrade(UMN), S. Antier (OCA), M. Coughlin (UMN),S. Karpov (FZU), I. Tosta e Melo (UniCT-DFA), P. Hello (IJCLAB), P-A Duverne (APC), T. Pradier (Unistra/IPHC), N. Guessoum (AUS), M. Pillas (ULiege) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 250424A (Cenko et al., GCN 40224) detected by Swift/BAT with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the iT30 telescope at the Siding Spring Observatory, a Celestron C14 telescope at Beverly Begg Observatory (New Zealand) and a CDK17 telescope located at AITP San Pedro Chile Observatory starting from TGRB+1.3hr. 

In our stacked frames, subtracted from the Legacy Survey DR10 template image, we detect the optical afterglow at the position reported by Swift/UVOT (Cenko et al., GCN 40224), REM (Brivio et al., GCN 40225), DDOTI (Becerra et al., GCN 40226), VLT/X-Shooter (Saccardi et al., GCN 40228), BlackGEM (de Wet et al., GCN 40229)and COLIBRI (Ducoin et al., GCN 40230).

We report some of our detections in the table below:
+---------------+-----------+-----------+----------------+------------+
| Tmid-TGRB (hr)| Exp (s)   | Filter    | Magnitude      | Instrument |
+===============+===========+===========+================+============+
| 1.3           | 1 x 300s  | r (AB)    | 19.34 +/- 0.09 | CDK17-AITP | 
| 4.0           | 20 x 120s | r (AB)    | 19.74 +/- 0.07 | C14-BBO    | 
| 6.8           | 17 x 180s | Rc (Vega) | 19.95 +/- 0.09 | iT30       |
+---------------+-----------+-----------+----------------+------------+

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot catalog while the sloan images were calibrated using the SkyMapper DR4 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/).

GCN Circular 40233

Subject
GRB 250424A: Swift-XRT refined Analysis
Date
2025-04-24T13:59:17Z (6 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB),
J.A. Kennea (PSU), D.N. Burrows (PSU), M. A. Williams (PSU), A.P.
Beardmore (U. Leicester), J.P. Osborne (U. Leicester) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 6.9 ks of XRT data for GRB 250424A, from 259 s to 18.9
ks after the  BAT trigger. The data are entirely in Photon Counting
(PC) mode. 

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=0.20 (+0.12, -0.13). At T+2382 s  the
decay steepens to an alpha of 0.53 (+0.16, -0.11) before breaking again
at T+17.4 ks to a final decay with index alpha=4.5 (+3.5, -2.8).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.05 (+/-0.11). The
best-fitting absorption column is  1.04 (+0.11, -0.10) x 10^22 cm^-2,
in excess of the Galactic value of 6.4 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.9 x 10^-11 (9.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.04 (+0.11, -0.10) x 10^22 cm^-2
Galactic foreground: 6.4 x 10^20 cm^-2
Excess significance: 15.7 sigma
Photon index:	     2.05 (+/-0.11)

If the light curve continues to decay with a power-law decay index of
4.5, the count rate at T+24 hours will be 5.3 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.6 x
10^-14 (5.1 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/01306404.

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

GCN Circular 40232

Subject
GRB 250424A: Enhanced Swift-XRT position
Date
2025-04-24T12:21:10Z (6 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
P.A. Evans, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 3429 s of XRT Photon Counting mode data and 5 UVOT
images for GRB 250424A, 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 = 217.49920, -35.02504 which is equivalent
to:

RA (J2000): 14h 29m 59.81s
Dec (J2000): -35d 01' 30.2"

with an uncertainty of 2.1 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.

GCN Circular 40231

Subject
GRB 250424A: AstroSat CZTI detection of a bright long burst
Date
2025-04-24T11:37:06Z (6 months ago)
From
Gaurav Waratkar at IIT Bombay <gauravwaratkar@iitb.ac.in>
Via
Web form
Harsha K. H. (IUCAA), M. Tembhurnikar (IUCAA), G. Waratkar (IITB), 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 ML pipeline (Abraham et al., 2021, MNRAS, 504, 3084) and the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a bright long-duration GRB 250424A which was also detected by Swift/BAT (Cenko et. al., GCN Circ. 40224), and Calet/CGBM (Trigger No. 1429512582).

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-04-24 06:52:13.50 UTC. The measured peak count rate associated with the burst is 332.3 (+35.7 -27.5) counts/s above the background in the combined data of two quadrants (out of four), with a total of 2298 (+184 -190) counts. The local mean background count rate was 157.4 (+2.7 -3.0) counts/s. Using cumulative rates, we measure a T90 of 9.2 (+1.5 -1.0) s. In the preliminary analysis, we find 665 Compton events associated with this event.

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-04-24 06:52:12.48 UTC. The measured peak count rate associated with the burst is 1364 (+88, -94) counts/s above the background in the combined data of all quadrants, with a total of 8511 (+491, -536) counts. The local mean background count rate was 1507 (+7, -8) counts/s. We measure a T90 of 14 (+1, -3) 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

GCN Circular 40230

Subject
GRB 250424A: COLIBRÍ Detection of a Bright Optical Counterpart
Date
2025-04-24T10:48:17Z (6 months ago)
From
J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr>
Via
Web form
Jean-Grégoire Ducoin (CPPM), Benjamin Schneider (LAM), Ramandeep Gill (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA/IJCLAB), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM) and Antonio de Ugarte Postigo (LAM) report:

We imaged the field of Swift/BAT GRB 250424A (Cenko et al., GCN Circ. 40224) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico.

We started observing at 2025-04-24T08:39:07 UTC (1.78h after the trigger). The data were coadded with the COLIBRÍ pipeline and analyzed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against SkyMapper DR4. Our photometry is in the AB system and is not corrected for Galactic extinction.

In our first 780 seconds of exposure, At the position of the optical counterpart (Francile et al. GCN Circ. 40222; Cenko et al. GCN Circ. 40224; Brivio et al. GCN Circ. 40225; Becerra et al. GCN Circ. 40226; Saccardi et al. GCN Circ. 40228; de Wet et al. GCN Circ. 40229), we detect a source with 

	i = 18.93 +/- 0.05
	
Further observations and analysis are ongoing.

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

GCN Circular 40229

Subject
GRB 250424A: BlackGEM optical afterglow detection
Date
2025-04-24T09:43:29Z (6 months ago)
From
Simon de Wet at University of Cape Town <simdewet@gmail.com>
Via
Web form
S. de Wet (DTU), P.J. Groot (Radboud/UCT/SAAO) and P.M. Vreeswijk (Radboud) report on behalf of the BlackGEM consortium:

The BlackGEM Unit Telescope 4 (BG4) located at ESO La Silla, Chile, responded automatically to the Swift trigger on GRB 250424A (Cenko et al., GCN 40224) and obtained a repeating series of 60 s exposures in the q,u,g,r,i,z bands. The first exposure started 264 s after the trigger time at 06:56:53 UT on 2025 April 24. A total of 26 exposures were obtained. 

We detect the optical afterglow with the following AB magnitudes:

q = 18.30 +/- 0.03 at 06:57:24 UT
u = 19.33 +/- 0.18 at 06:58:47 UT
g = 18.98 +/- 0.05 at 07:01:32 UT
r = 18.58 +/- 0.04 at 07:04:17 UT
i = 18.26 +/- 0.04 at 07:07:02 UT
z = 18.22 +/- 0.10 at 07:09:46 UT

BlackGEM is an array of wide-field telescopes designed, built and operated by a consortium consisting of Radboud University, the Netherlands Research School for Astronomy NOVA, KU Leuven, the University of Manchester, Tel Aviv University, the Weizmann Institute, the Hebrew University of Jerusalem, the University of Potsdam, Texas Tech University, the University of California at Davis, the Danish Technical University and the Armagh Observatory and Planetarium.

GCN Circular 40228

Subject
GRB 250424A: VLT/X-shooter spectroscopic redshift z = 0.310
Date
2025-04-24T09:42:20Z (6 months ago)
From
Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Via
Web form
A. Saccardi (CEA/Irfu), D. B. Malesani (DAWN/NBI and Radboud Univ.), G. Corcoran (UCD), S. Covino (INAF/OAB), N. Habeeb (Leicester), L. Izzo (INAF/OACN and DARK/NBI), A. J. Levan (Radboud and Warwick), A. Martin-Carrillo (UCD), J. T. Palmerio (CEA/Irfu), G. Pugliese (API), B. Schneider (LAM), N. R. Tanvir (Leicester), S. D. Vergani (LUX-Paris Obs.), K. Wiersema (Hertfordshire), report on behalf of the Stargate collaboration:

We observed the optical and NIR afterglow of GRB 250424A (Francile et al., GCN 40222; Cenko et al., GCN 40224; Brivio et al., GCN 40225; Becerra et al., GCN 40226) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Observations started on 2025 Apr 24.322 UT (0.87 hr after the Swift/BAT trigger) and consisted of a sequence of increasing exposure times (175, 300, 600, 1200, 1920 s).

In the acquisition image (taken at a mid time of 0.851 hr after the Swift/BAT trigger), the counterpart is well detected with a magnitude r = 19.09 +- 0.02, calibrated against nearby stars from the SkyMapper catalog (Wolf et al. 2018, doi:10.4225/41/593620ad5b574).

We detect a bright continuum over the wavelength range 3100 to 24,500 AA. Many absorption features are visible which we interpret as Mn II, Fe II, Mg II, Mg I, Ti II, Ca II, Na I, including a number of fine-structure transitions from Fe II*, all at a common redshift z = 0.310. At a consistent redshift, we also detect several emission lines, due to [O II], [O III], [S II], [N II], and the Balmer lines from the GRB host galaxy (Perez-Fournon et al., GCN 40227). We note that our redshift is consistent with the photometric determination z = 0.33 +/- 0.12 of the host redshift from the Legacy Survey (Zhou et al. 2021, doi:10.1093/mnras/staa3764).

We acknowledge excellent support from the ESO observing staff in Paranal.

GCN Circular 40227

Subject
GRB 250424A: likely host galaxy
Date
2025-04-24T08:54:58Z (6 months ago)
From
Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>
Via
Web form

I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), and A. López-Oramas (IAC and ULL)

We report on the likely host galaxy of the Swift/BAT GRB 250424A (Cenko et al., GCN circ. 40224), with Swift/XRT and Swift/UVOT detections (Cenko et al., GCN circ. 40224) and detections in the optical by Global MASTER-net (Francile et al. GCN circ. 40222) and DDOTI (Becerra et al., GCN circ. 40226) and in the optical and near-IR by REM (Brivio et al., GCN circ. 40225). A catalogued Legacy Surveys DR10 (LS DR10) galaxy (RA, Dec = 217.4999, -35.0252) is visible at the position of the optical and near-IR counterpart of GRB 250424A, with magnitudes in the LS DR10 catalog of g=22.60, r=21.98, i=22.05, and z=21.70. This galaxy is located at about 0.3" from the Swift/UVOT position and 0.2" from the Global MASTER-net position. We propose that this galaxy is the host of GRB 250424A.

GCN Circular 40226

Subject
GRB 250424A: DDOTI Afterglow Detection
Date
2025-04-24T08:30:40Z (6 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Roma), Sahil Atri (U Roma), Alan M. Watson (UNAM), Eleonora Troja (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) and Margarita Pereyra (UNAM) report:

We observed the field of the GRB 250424A detected by Swift/BAT (Cenko
 et al., GCN Circ. 40136) 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 2025-04-24 UTC from 07:23 UTC to 7:45 UTC (from T+30.6 h to T+53.5 min after the trigger) and obtained a total exposure of 20 minutes. 

Comparing our observations to the USNO-B1 and Pan-STARRS PS1 DR2 catalogues, we
detect a source consistent with the UVOT position (Cenko et al., GCN  Circ. 40136) and the previous optical detections (Francile et al., GCN Circ. 40222, Brivio et al., GCN Circ. 40225), with an AB magnitude of: 

w = 19.4 +/- 0.1

Further observations and analysis are ongoing.

This value is not corrected for the Galactic extinction.

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

GCN Circular 40225

Subject
GRB 250424A: REM optical/NIR afterglow detection
Date
2025-04-24T08:19:08Z (6 months ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio (INAF-OAB), A. Melandri (INAF-OAR), M. Ferro, P. D'Avanzo, S. Covino, G. Tagliaferri, D. Fugazza (INAF-OAB) report on behalf of the REM team:

We observed the field of GRB 250424A detected by Swift/BAT (Cenko et al., GCN 40224) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, J, H, and K bands, started on 2025 April 24 at 06:54:31 UT (i.e. 123 s after the burst).

From preliminary photometry, we detect the optical/NIR counterpart (Francile et al., 40222) at the Swift/UVOT position (Cenko et al., GCN 40224) with the following magnitudes:

r = 17.7 +/- 0.1 (AB; calibrated against the SkyMappercatalogue),
at a mid-time of 205 s after the trigger,

H = 14.5 +/- 0.2 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 164 s after the trigger.

GCN Circular 40224

Subject
GRB 250424A: Swift detection of a burst with an optical counterpart
Date
2025-04-24T07:12:30Z (6 months ago)
Edited On
2025-07-12T17:23:53Z (4 months ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
S. B. Cenko (GSFC), V. D'Elia (SSDC & INAF-OAR), J. J. DeLaunay (PSU),
R. Gupta (NASA GSFC), A. Melandri (INAF-OAR), K. L. Page (U Leicester)
and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 06:52:28.51 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250424A (trigger=1306404). Swift slewed immediately to the burst.
The BAT on-board calculated location is RA, Dec 217.513, -35.022 which is 
   RA(J2000) = 14h 30m 03s
   Dec(J2000) = -35d 01' 19"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
structure with a duration of about 30 sec.  The peak count rate
was ~25000 counts/sec (15-350 keV), at ~15 sec before the trigger.

The XRT began observing the field at 06:56:39.3 UT, 250.9 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 217.50010, -35.02493
which is equivalent to:
   RA(J2000)  = 14h 30m 00.03s
   Dec(J2000) = -35d 01' 29.7"
with an uncertainty of 1.9 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 does not constrain the column density.

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 254 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	14:30:00.00 = 217.49998
  DEC(J2000) = -35:01:30.6  = -35.02518
with a 90%-confidence error radius of about 0.63 arc sec. This position is 1
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.80 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.064.

Burst Advocate for this burst is S. B. Cenko (brad.cenko 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/)

GCN Circular 40223

Subject
Swift GRB 250424A: Global MASTER-Net observations report
Date
2025-04-24T07:08:43Z (6 months ago)
Edited On
2025-05-16T14:27:21Z (6 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, 
G.Antipov,  A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

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 GRB250424.29 (trigger No 1306404,14h 30m 03.12s , -35d 01m 19.2s, R=0.05) errorbox  94 sec after notice time and 178 sec after trigger time at 2025-04-24 06:55:26 UT, with upper limit up to  19.4 mag. The observations began at zenith distance = 25 deg. The sun  altitude  is -52.5 deg. 

The galactic latitude b = 23 deg., longitude l = 325 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2852168

We obtain a following upper limits.  

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

     198 |         MASTER-OAFA |   C |    40 | 19.4 |        
     244 |         MASTER-OAFA |   C |    40 | 19.4 |        
Filter C is a clear (unfiltred) band. 


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

GCN Circular 40222

Subject
Swift GRB 250424A: Global MASTER-Net OT detection
Date
2025-04-24T07:06:22Z (6 months ago)
Edited On
2025-05-16T14:26:39Z (6 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>
Via
email
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),
V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, D.Vlasenko, I.Panchenko, A.Kuznetsov, G.Antipov, 
A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina (Lomonosov MSU),
O.Gress, N.Budnev(ISU),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)

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  GRB250424.29 99 sec after notice time and 178 sec after trigger time at 2025-04-24 06:55:26 UT. On our first (40s exposure)  set we  found 1 optical transient within Swift error-box (ra=217.512 dec=-35.0214 r=0.05) brighter than  18.9.


 T-Tmid      Date      Time       Expt.        Ra                Dec           Mag
---------|---------------------|-------|-----------------|-----------------|-------
     198   2025-04-24 06:55:26      40   (14h 29m 59.96s , -35d 01m 30.7s)     18.0

The 5-sigma upper limit has been about  18.9mag
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