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GRB 260509A

GCN Circular 44598

Subject
GRB 260509A: SVOM/VT optical observations
Date
2026-05-13T15:17:36Z (2 days ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, Y. L. Qiu, C. Wu,  L. P. Xin, Y. N. Ma, Z. H. Yao, J. R. Xu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio, D. Götz and D. Adrien (CEA/Irfu) report on behalf of the SVOM/VT team.

SVOM/VT performed ToO observations of GRB260509A triggered by SVOM/ECLAIRs (sb26050905, Götz et al., GCN 44504). The observation started at 2026-05-09T22:12:49 UTC, about 21.66 minutes post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. 

The afterglow (O’Neill et al., GCN 44505; Saccardi et al., GCN 44506; Wu et al., GCN 44507; Alvarez et al., 44509; Zheng et al., GCN 44510; Lipunov et al., GCN 44512; Belkin et al., GCN 44513; Corcoran et al., GCN 44514; Bochenek et al., GCN 44516; Moskvitin et al., GCN 44563; Mo et al., GCN 44590) was detected clearly in both channels. The following measurements are in the AB magnitude and are not corrected for Galactic extinction:

Mid time | Band | Exposure Time | Brightness
 22.08 m   VT_B        50 s      20.05 +/- 0.07 mag
 22.08 m   VT_R        50 s      19.47 +/- 0.06 mag    
  8.17 h   VT_B    16*100 s      21.76 +/- 0.09 mag
  8.24 h   VT_R    11*100 s      21.48 +/- 0.09 mag
  
The light curves shows fading and brightening at early epochs, followed by a temporal decay with a slope of −1 in the late phase during our observations.

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 44590

Subject
GRB 260509A: Infrared J and Hs observations with MDM/MIRAGE
Date
2026-05-12T20:45:37Z (3 days ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
Via
Web form

Geoffrey Mo (Carnegie/Caltech), K. De, V. Karambelkar, S. Ibrahim, D. Schiminovich (Columbia University) report:

We observed the field of GRB 260509A (Gotz et al., GCN 44504

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) in the near-infrared J and Hs bands with the MDM InfraRed Astronomy inGaas Explorer (MIRAGE) instrument on the MDM 1.3 m telescope.

Observations began at 2026-05-10T04:00:51 (+6.2 hours after the GRB trigger) in the J band and 2026-05-10T03:27:31 (+5.6 hours) in the Hs band, lasting 1560 s in J and 1260 s in Hs.

The optical counterpart (O’Neill et al., GCN 44505

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; Saccardi et al., GCN 44506; Wu et al., GCN 44507; Alvarez et al., 44509; Zheng et al., GCN 44510; Lipunov et al., GCN 44512; Belkin et al., GCN 44513; Corcoran et al., GCN 44514; Bochenek et al., GCN 44516; Moskvitin et al., GCN 44563) is detected in the J filter at low significance and undetected in H band, with the following AB magnitudes: J = 20.91 ± 0.40, Hs > 19.6.

MIRAGE is a new YJHs-band near-infrared imager for the MDM 1.3m telescope. We thank the MDM Observatory staff for supporting the observations.

GCN Circular 44563

Subject
GRB 260509A: SAO RAS optical observations
Date
2026-05-11T18:46:55Z (4 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin and O. Spiridonova (SAO RAS),
report on behalf of the GRB follow-up team.

We observed the field of the GRB 260509A (Götz et al., GCN 44504)
with the Zeiss-1000, 1m telescope of the SAO RAS equipped
with the CCD photometer. We obtained 12 * 300 sec. images in Rc
band on May 10, 18:18:31--19:49:52 UT (21.22 hr after the trigger).

The OT (O'Neill et al., GCN 44505; Saccardi et al., GCN 44506;
Wu et al., GCN 44507; Sánchez Álvarez et al., GCN 44509;
Zheng et al., GCN 44510; Kechin et al., GCN 44512;
Belkin, GCN 44513; Corcoran et al., GCN 44514;
Bochenek and Perley, GCN 44516) is marginally detected with the
brightness of R = 24.0 +/- 0.3.

This preliminary photometry is based on nearby stars from the USNO-B1
catalogue (R2 magnitudes) and is not corrected for the Galactic
extinction.

GCN Circular 44516

Subject
GRB 260509A: Liverpool Telescope optical observations
Date
2026-05-10T13:20:53Z (5 days ago)
Edited On
2026-05-11T00:01:08Z (5 days ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
Web form
A. Bochenek, D. A. Perley (LJMU), report:

We observed the field of GRB 260509A (Götz et al., GCN 44504) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 4x100s exposures in SDSS griz filters, starting at 2026-05-09 23:08:01 UT, approximately 1.28 hours after trigger.

We detect a source in all filters at the position reported by O'Neill et al. (GCN 44505), observed also by Saccardi et al. (GCN 44506), Wu et al. (GCN 44507), Sánchez Álvarez et al. (GCN 44509), Zheng et al. (GCN 44510), Lipunov et al. (GCN 44512), Belkin (GCN 44513), and Corcoran et al. (GCN 44514):

MJD (mid)          T_mid-T_0       Filter       Mag. (AB)
61169.96654        1.34 h           g         20.00 ± 0.05
61169.97218        1.48 h           r         19.83 ± 0.06
61169.97779        1.61 h           i         19.81 ± 0.09
61169.98272        1.73 h           z         19.61 ± 0.09

The photometry was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.

GCN Circular 44514

Subject
GRB 260509A: NOT spectroscopic redshift upper limit z < 1.95
Date
2026-05-10T11:07:26Z (5 days ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
Web form
G. Corcoran (UCD), A. Saccardi (CEA/Irfu), B. Schneider (LAM), D. B. Malesani (DAWN/NBI and Radboud), N. R. Tanvir (Univ. Leicester), Kostas Valeckas (NOT and NBI), V. Vuolteenaho (NOT and Oulu Univ.), report on behalf of a larger collaboration:

We observed the optical counterpart (O'Neill et al., GCN 44505; Saccardi et al. GCN 44506; Wu et al. GCN 44507; Sánchez Álvarez et al. GCN 44509; Zheng et al. GCN 44510; Lipunov et al. GCN 44512; Belkin GCN 44513) of the SVOM/ECLAIRs GRB 260509A (Götz et al., GCN 44504) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC spectrograph.

A spectrum consisting of 4x1200 s exposures was secured starting on 2026 May 10 at 00:22:16 UT (2.52 hr after the SVOM trigger), using grism #4 with the WG345 356_LP order blocker which covers the wavelength range 3400-9600 AA.

Based on a preliminary reduction of the spectra using PyLongslit (Valeckas et al. 2025 10.21105/joss.09264), a continuum is detected down to ~3600 AA, setting an upper limit to the redshift z < 1.95 based on the lack of HI absorption. We do not identify clear metal absorption features despite good S/N.

Further analysis is ongoing.

GCN Circular 44513

Subject
GRB 260509A: LCO optical observations
Date
2026-05-10T09:43:56Z (5 days ago)
From
Sergey Belkin at Monash University <sergey.belkin@monash.edu>
Via
Web form
S. Belkin (Monash U) reports:

We observed the optical counterpart candidate GOTO26emi of GRB 260509A, detected by SVOM/ECLAIRs (Götz et al., GCN 44504), with the Las Cumbres Observatory 1m telescope at Cerro Tololo Observatory equipped with the Sinistro instrument.

In the stacked images, we clearly detect the reported optical counterpart candidate GOTO26emi, first identified by GOTO (O'Neill et al., GCN 44505) and subsequently reported by Saccardi et al. (GCN 44506), Wu et al. (GCN 44507), Sánchez Álvarez et al. (GCN 44509), Zheng et al. (GCN 44510), and Lipunov et al. (GCN 44512).

The preliminary photometry from the stacked images is:

Date (UT)                T-T0 (days)   Filter   Exp. time (s)   Magnitude
============================================================================
2026-05-10 01:53:25       0.16825       r        60 x 5          20.67 +/- 0.13
2026-05-10 02:00:37       0.17324       i        60 x 5          20.51 +/- 0.17
2026-05-10 02:07:48       0.17823       g        60 x 5          20.88 +/- 0.12

The photometry was calibrated against Pan-STARRS field stars. The quoted magnitudes are in the AB system and are not corrected for Galactic extinction.

Our r-band measurement is consistent with the fading behaviour reported by Saccardi et al. (GCN 44506) and Sánchez Álvarez et al. (GCN 44509). Further follow-up is encouraged.

GCN Circular 44512

Subject
SVOM GRB 260509A: MASTER-OAFA optical observation
Date
2026-05-10T08:23:16Z (5 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
Ya.Kechin (Lomonosov MSU),
F.Podesta, M.J. Segura, C.Francile, R.Podesta, E. Gonzalez (OAFA, SJNU),
A.Kuznetsov, V.Lipunov, I.Panchenko, P.Balanutsa, G.Antipov, N.Tiurina,K.Zhirkov, E.Gorbovskoy, V.Senik, A.Chasovnikov, V.Topolev, D.Vlasenko,A.Sankovich, I.Gorbunov(Lomonosov MSU),
N.Budnev, O.Gress (ISU),
D.Buckley (SAAO),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich  (Blagoveschensk Educational StateUniversity)
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez,J.Martinez,A.R.Corella,
J.Tanori, L. Villalobos, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)

MASTER-OAFA robotic telescope of MASTER Global Robotic Net [1-4]
was pointed to the SVOM GRB260509A (trigger No 1778363469,11h 47m 32.88s , +00d 12m 34.2s, R=0.0786667 Gotz et al. GCN 44504) errorbox  8054 sec after notice time and 10202 sec after trigger time at 2026-05-10 00:41:11 UT,
with upper limit up to  20.5 mag. The observations began at zenith distance = 31 deg. The sun  altitude  is -36.5 deg.

MASTER-OAGH robotic telescope  located in Mexico (OAGH National Institute for Astrophysics, Optics and Electronics)
was pointed to the SVOM GRB260509.91 errorbox  24424 sec after notice time (26572 sec after trigger time at 2026-05-10 05:14:01 UT),
 with upper limit up to  19.3 mag. The observations began at zenith distance = 36 deg. The sun  altitude  is -33.4 deg.

The galactic latitude b = 59 deg., longitude l = 272 deg.

There is MASTER OT J114735.9135.91+001717.7 optical counterpart at first images with m_OT~20.9 at 2026-05-10 00:41:11UT
discovered by GOTO (O'Neill et al. GCN 44505)
and observed by LCO (Saccardi et al. GCN 44506), SVOM|VT (Wu et al. GCN 44507), KAIT (Zheng et al. GCN 44510)

Real time updated cover map and OT are available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3303991

MASTER upper limits:

Tmid-T0  |      Date Time      |          Site       |             Coord J2000)          |Filt.| Expt. | Limit| Comment
   10233 | 2026-05-10 00:41:11 |         MASTER-OAFA | (11h 44m 48.88s , -00d 51m 54.0s) |   C |    60 | 20.2 |
   10293 | 2026-05-10 00:41:11 |         MASTER-OAFA | (11h 44m 48.87s , -00d 51m 54.0s) |   C |   180 | 20.5 |  Coadd
   10311 | 2026-05-10 00:42:29 |         MASTER-OAFA | (11h 44m 48.21s , -00d 50m 56.7s) |   C |    60 | 20.2 |
   10389 | 2026-05-10 00:43:47 |         MASTER-OAFA | (11h 44m 52.24s , -00d 51m 59.4s) |   C |    60 | 20.2 |
   10465 | 2026-05-10 00:45:03 |         MASTER-OAFA | (11h 44m 46.21s , -00d 53m 02.8s) |   C |    60 | 20.2 |
   10525 | 2026-05-10 00:45:03 |         MASTER-OAFA | (11h 44m 46.20s , -00d 53m 02.8s) |   C |   180 | 20.5 |  Coadd
   10540 | 2026-05-10 00:46:19 |         MASTER-OAFA | (11h 44m 52.06s , -00d 52m 51.7s) |   C |    60 | 20.2 |
   10618 | 2026-05-10 00:47:36 |         MASTER-OAFA | (11h 44m 49.58s , -00d 51m 21.4s) |   C |    60 | 20.2 |
   10697 | 2026-05-10 00:48:55 |         MASTER-OAFA | (11h 44m 49.29s , -00d 53m 05.6s) |   C |    60 | 20.2 |
   10757 | 2026-05-10 00:48:55 |         MASTER-OAFA | (11h 44m 49.28s , -00d 53m 05.6s) |   C |   180 | 20.4 |  Coadd
   10774 | 2026-05-10 00:50:12 |         MASTER-OAFA | (11h 44m 52.95s , -00d 51m 27.6s) |   C |    60 | 20.2 |
   10852 | 2026-05-10 00:51:31 |         MASTER-OAFA | (11h 44m 45.40s , -00d 52m 20.8s) |   C |    60 | 20.2 |
   10930 | 2026-05-10 00:52:48 |         MASTER-OAFA | (11h 44m 45.82s , -00d 51m 24.8s) |   C |    60 | 20.2 |
   10990 | 2026-05-10 00:52:48 |         MASTER-OAFA | (11h 44m 45.81s , -00d 51m 24.8s) |   C |   180 | 20.4 |  Coadd
   11007 | 2026-05-10 00:54:06 |         MASTER-OAFA | (11h 44m 49.85s , -00d 52m 28.2s) |   C |    60 | 20.2 |
   11086 | 2026-05-10 00:55:24 |         MASTER-OAFA | (11h 44m 45.55s , -00d 53m 32.1s) |   C |    60 | 20.2 |
   11160 | 2026-05-10 00:56:39 |         MASTER-OAFA | (11h 44m 49.59s , -00d 53m 31.6s) |   C |    60 | 20.2 |
   11220 | 2026-05-10 00:56:39 |         MASTER-OAFA | (11h 44m 49.59s , -00d 53m 31.6s) |   C |   180 | 20.4 |  Coadd

[1] Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L
[2] Lipunov et al. 2022, Universe, Vol. 8(5), id.271
[3] Lipunov et a. 2019, ARep, vol.63, 293
[4] Lipunov et al. 2023, Astronomical Robotic Networks and Operative Multichanel
Astrophysics,Lomonosov MSU PRESS, 591pp.
 http://www.pereplet.ru/lipunov/625.html#625

GCN Circular 44510

Subject
GRB 260509A: KAIT optical observations
Date
2026-05-10T06:13:38Z (5 days ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and

Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team:


The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at

Lick Observatory, observed the field of GRB 260509A (Götz et al.,

GCN 44504 starting at 04:56 UT, 7.08 hours after the burst. A set

of clear (roughly R) filter images were obtained. We detected the

afterglow (O'Neill et al., GCN 44505; Saccardi et al., GCN 44506;

Wu et al., GCN 44507; Álvarez et al., GCN 44509) in our coadd image.

We measure its brightness to be 21.1 +/- 0.2 mag (Vega) at mid time

of 7.37 hours after the burst.

GCN Circular 44509

Subject
GRB 260509A: SVOM/COLIBRÍ (FM-GFT) observations
Date
2026-05-10T04:54:27Z (5 days ago)
From
freddalvarez@astro.unam.mx
Via
Web form

Fredd Sánchez Álvarez (UNAM), Jean-Grégoire Ducoin (CPPM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM),  Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM),Nikos Mandarakas (LAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and D. Adrien (CEA/Irfu) report:

We imaged the field of the SVOM/ECLAIRs GRB 260509A (Götz et al. GCN 44504) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-05-10 03:31 to 03:59 UTC and obtained 8 minutes of exposure in g, r, and i, and simultaneously 24 minutes of exposure in z.

The data were reduced, coadded, calibrated, and analyzed with the COLIBRÍ ASU pipeline.
The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

We detect the optical counterpart reported by O’Neill et al.(GCN 44505), Saccardi et al. (GCN 44506), and Wu et al. (GCN 44507) with preliminary magnitudes of: 


g = 21.25 +/- 0.05

r = 21.31 +/- 0.08

i = 20.97 +/- 0.08

z = 20.81 +/- 0.09

Our strong detection of the source in g suggests that the redshift is less than 3.0.

We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.

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 44507

Subject
GRB 260509A: SVOM/VT optical candidate with VHF data
Date
2026-05-10T01:47:02Z (6 days ago)
Edited On
2026-05-11T13:39:51Z (4 days 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

C. Wu, H. L. Li, Y. L. Qiu, L. P. Xin (NAOC), J. T. Palmerio (CEA/Irfu), D. Götz (CEA/Irfu), A. Saccardi (CEA/Irfu), L. Zhang (IHEP), Y. H. Cheng (YNU) on behalf of the SVOM/VT team.

After the trigger by SVOM/ECLAIRs (SVOM burst-id sb26050905) at 2026-05-09T21:51:09 UTC (T0), SVOM/VT began observing the field after the automatic slew on the burst (Götz et al., GCN 44504

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) in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.

From a preliminary analysis of the 1-bit subimage and source list generated by on-board data processing and downloaded via VHF network, one credible candidate is identified, the details of which are presented below.

VT_ID 722: The position of this candidate is R.A., Dec. 176.89974, 0.28822 degrees, corresponding to:
R.A. (J2000) = 11:47:35.94
Dec. (J2000) = 00:17:17.6
with an uncertainty of 0.50 arcsec.

This candidate was detected in both VT_R and VT_B, and was flagged as an uncatalogued source whose brightness faded during the VT observations. The candidate's magnitudes are:

date-obs (UTC)mid-timeexposureVT_B mag(AB)VT_R mag(AB)
2026-05-09T23:18:3189.87 min6x50 sec20.38 ± 0.0319.80 ± 0.04
2026-05-09T23:58:59129.08 min3x50 sec20.67 ± 0.0520.18 ± 0.06

Magnitudes were not corrected for Galactic extinction.

The mean color of VT_B - VT_R = 0.54 is in line with the expectations of an event at redshift z < 4.

The position of this candidate is consisent with the reports (O'Neill et al., GCN 44505

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; Saccardi et al., GCN 44506) .

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

GCN Circular 44506

Subject
GRB 260509A: LCO optical counterpart detection
Date
2026-05-10T01:00:45Z (6 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
A. Saccardi (CEA/Irfu), C. Wu (NAOC), D. Adrien (CEA/Irfu), L. Zhang (IHEP), Y. H. Cheng (YNU), report on behalf of the SVOM mission team:

We observed the field of GRB 260509A detected by SVOM (Götz et al., GCN 44504) with the LCO 1m telescope at Cerro Tololo Observatory equipped with the Sinistro instrument.

Our observation started on 2026-05-09 at 23:41:26 UT (about 1.84 hr after the trigger) and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters.

The optical candidate reported by O'Neill et al. (GCN 44505) is clearly detected in our images. We measure the following magnitudes calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:

r = 19.79 +/- 0.05 AB (mid-time 1.92 hr after the trigger);
z = 19.76 +/- 0.15 AB (mid-time 2.65 hr after the trigger).

This project is funded by the SVOM collaboration.

GCN Circular 44505

Subject
GRB 260509A: GOTO optical counterpart candidate
Date
2026-05-09T23:23:47Z (6 days ago)
From
Sergey Belkin at Monash University <sergey.belkin@monash.edu>
Via
Web form
D. O'Neill, S. Belkin, K. Ulaczyk, A. Kumar, K. Ackley, M. Dyer, J. Lyman, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, R. Starling, B. Gompertz, B. Godson, T. Killestein, M. Pursiainen, on behalf of GOTO collaboration.
 
We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the  SVOM/ECLAIRS  alert GRB 260509A (sb26050905, Götz et al. GCN 44504). Observations covering the localisation area were taken at 2026-05-09 21:56:19 UT, (t0 +0.09h) with an average 5-sigma depth of 20.8 mag.

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations (Lyman et al. 2026). Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks.

A new optical source GOTO26emi is identified near the edge of the ECLAIRS  localisation region, at an angular separation of 4.79 arcmin from the reported GRB position, marginally outside the quoted 4.72 arcmin error radius. The source coordinates:

    RA,DEC (J2000) = 176.899834, 0.288276,
                    11:47:35.96, +00:17:17.80

The rapidly rising source was detected with average L = 20.13 ± 0.16 AB mag (+0.09h). We find no evidence of the source prior to the GRB trigger time in previous GOTO observations taken at 2026-05-09 10:38:10 (-11.21h) down to a depth of >20.40 AB mag, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021).

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).

GCN Circular 44504

Subject
GRB 260509A: SVOM detection of a burst
Date
2026-05-09T22:26:24Z (6 days ago)
From
Diego Gotz at CEA <diego.gotz@cea.fr>
Via
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GRB 260509A: SVOM detection of a burst

D. Götz (CEA/Irfu), A. Saccardi (CEA/Irfu), H. Goto (RIKEN), F. Lacreu (IAP), D. Adrien (CEA/Irfu), report on behalf of the SVOM mission team:

At 2026-05-09T21:51:09 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260509A (SVOM burst-id sb26050905).

The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.

The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 18 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 18.00 in the [5-20] keV energy band over a time window of 20.40 seconds starting at 2026-05-09T21:51:03.

The localization of the best alert is R.A., Dec. 176.8870, 0.2095 degrees:
R.A. (J2000) = 11h47m32.9s
Dec. (J2000) = 0d12m34.2s
 with a 90% confidence level (C.L.) radius of 4.72 arcmin (including systematic error of 2 arcmin added in quadrature).

We note the presence of a high proper motion star G 10-49 at a distance of 4.48 arcmin from the ECLAIRs position.

The SVOM/ECLAIRs light curve showed a single peak structure with a T90 duration of about 14.7 +/-0.6 s.

This burst was also detected by SVOM/GRM with a significance of 6.40.
The SVOM/GRM light curve showed a double peak structure with a T90 duration of about 18 (-6/+10) s.

SVOM slewed to the burst.

MXT and VT began observing the field after the slew. The analysis of the data will be published in  future circulars.

The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.

The Burst Advocate (BA) on shift for this alert is Dylan Adrien: dylan.adrien@cea.fr.
Please contact the BA by email if you require additional information.
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