GRB 240911A

SVOM/ECLAIRs commissioning team: Stéphane Schanne, Hervé Le Provost, Nicolas Dagoneau, Frédéric Chateau (CEA), Olivier Godet, Laurent Bouchet, Sebastien Guillot, Juliette Alaux (IRAP), Frédéric Piron (LUPM), Floriane Cangemi (APC), Karine Mercier, Marie-Claire Charmeau, Stefano Crepaldi (CNES)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Jean-Luc Atteia (IRAP), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Bing Zhang (UNLV)

report on behalf of the SVOM team:

During the commissioning phase, the SVOM/ECLAIRs telescope triggered and located a weak GRB at 2024-09-11T20:38:48 (TtimeTb), called GRB 240911A. The burst was detected by the Count-Rate Trigger (CRT) onboard ECLAIRs. The signal to noise ratio in the reconstructed image was 6.8 in the 20-120 keV energy band during a single time window of 10.24 s starting at TimeTb. The image transmitted in real-time over the SVOM VHF network is clean and shows a point-like source.

The GRB location is RA, Dec = 351.30, 21.33 (J2000). This is about 41 degrees from the ECLAIRs optical axis. The statistical uncertainty on this position is 13 arcminutes, to which we recommend to add 10 arcminutes of systematic uncertainty.

SVOM did not slew to the burst since automated slewing is not yet enabled.

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), French Space Agency (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. ECLAIRs was developed jointly by APC, CEA, CNES and IRAP.

The SVOM point of contact for this burst is: s.schanne AT cea.fr

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

Swift has initiated a series of observations, tiled on the sky, of the
SVOM/ECLAIRs GRB 240911A. Automated analysis of the XRT data will
be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00128

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. The probability of finding
serendipitous sources, unrelated to the SVOM/ECLAIRs event is high: any X-ray source
considered to be a probable afterglow candidate will be reported via a GCN Circular
after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

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

M. A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T. Sbarrato
(INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), J.A.
Kennea (PSU) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of the
SVOM/ECLAIRs-detected burst GRB 240911A in a series of observations
tiled on the sky. The total exposure time is 2.9 ks, distributed over 3
tiles; the maximum exposure at a single sky location was 1.6 ks. The
data were collected between T0+73.0 ks and T0+79.5 ks, and are entirely
in Photon Counting (PC) mode. 

Two uncatalogued X-ray sources have been detected, however none of them
is above the RASS limit or shows definitive signs of fading. Therefore,
at the present time we cannot identify which, if any, is the afterglow.
Details of these sources are given below:

Source 1:
  RA (J2000.0):  351.4579  =  23:25:49.91
  Dec (J2000.0): +21.3760  =  +21:22:33.6
  Error: 13.5 arcsec (radius, 90% conf.)
  Count-rate: (3.7 [+2.3, -1.6])e-3 ct s^-1   
  Distance: 554 arcsec from SVOM/ECLAIRs position.

Source 2:
  RA (J2000.0):  351.5268  =  23:26:6.43
  Dec (J2000.0): +21.2944  =  +21:17:39.8
  Error: 8.2 arcsec (radius, 90% conf.)
  Count-rate: (5.5 [+3.6, -2.6])e-3 ct s^-1   
  Distance: 771 arcsec from SVOM/ECLAIRs position.

The results of the XRT-team automatic analysis of the tiled XRT
observations, including a position-specific upper limit calculator, are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00128.

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

A. Kumar, Y. Julakanti, B. P. Gompertz, J. Lyman, R. Starling, K. Ackley, M. J. Dyer, K. Ulaczyk, F. Jimenez-Ibarra, D. O'Neill, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Palle and D. Pollacco report on behalf of the GOTO collaboration:

The Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022) serendipitously covered part of the localisation region of the SVOM/ECLAIRs-detected and Swift/XRT followed-up GRB 240911A (Schanne et al. GCN 37462
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 ; Williams et al. GCN 37483
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 ) in survey mode at 2024-09-12 04:40:43 UT (8.03 hours after trigger). Each observation consisted of 4x45s exposures in the GOTO L-band (400-700 nm).

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations of the same paintings.

No candidate optical counterpart is detected at the positions of the 2 uncatalogued Swift/XRT detected sources (Williams et al. GCN 37483
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 ) to a 5-sigma limiting magnitude of L > 19.3.

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and were not corrected for Galactic extinction.

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 and the Instituto de Astrofisica de Canarias (IAC).

S.Y. Fu, S.Q. Jiang, J. An, Z.P. Zhu, X. Liu, D. Xu (NAOC), J.Z. Liu (XAO) report on behalf of a large collaboration:

We observed the field of GRB 240911A detected by SVOM/ECLAIRs (Schanne et al., GCN 37462
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 ), using the 50cm-A/B telescopes (50A, 50B) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 16:27:18 UT on 2024-09-12, i.e., 19.8 hr after the trigger, and a series of frames were obtained in the Sloan g-, r-, i- and z-bands.

No new optical source is detected within the error circles of two uncatalogued Swift/XRT candidates (Williams et al., GCN 37483
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 ), down to the following 5-sigma upper limits:

T_mid (UT) | T_mid (hr) | filter | 5-sigma U.L. (AB) 
----------------------------------------------------
2024-09-12T17:18:03.5 | 20.65 | g | 21.1
2024-09-12T16:42:30.0 | 20.06 | r | 21.1
2024-09-12T16:49:22.0 | 20.18 | i | 20.1
2024-09-12T17:24:54.0 | 20.77 | z | 18.8

calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction. 

We acknowledge the excellent support from X. Yao, S.W. Luo, and Z.K. Feng for enabling these observations.

L. Izzo (INAF-OACn and DARK/NBI) and D. B. Malesani (DAWN/NBI and Radboud Univ.) report:

We observed the field of GRB 240911A (Schanne et al., GCN #37462
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 ) with the Sinistro instrument mounted on the 1-m telescope of the LCO network, located in Sutherland, South Africa. Observations started on 2024 September 11 at 22:30 UT (1.87 hr after the GRB trigger). We obtained a series of 3x180 s images in the SDSS-r filter, and of 5x120 s in the PS1-z filter.

We did not detect any source consistent with the position of either of the two X-ray sources reported by Swift-XRT (Williams et al., GCN #37483
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 ) up to a limit of r = 21.0 mag and z = 19.5 mag. 

This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 101004719.