SVOM/ECLAIRs likely detection of a burst from SGR Swift J1818.0-1607

ATel #16837; Miguel Llamas Lanza, Jean-Luc Atteia, Sebastien Guillot, Laurent Bouchet (IRAP, Toulouse, France), Floriane Cangemi, Alexis Coleiro (APC, Paris, F), Stéphane Schanne (CEA, F), Yong-Wei Dong, Jiang-Tao Liu, Lu Li, Shi-Jie Zheng, Yue Huang, Hao-Li Shi, Ping Wang, Bo-Bing Wu (IHEP, CN) and the SVOM JSWG: Jian-Yan Wei (NAOC, China), Bertrand Cordier (CEA, F), Shuang-Nan Zhang (IHEP, CN), Stéphane Basa (LAM, F), Olivier Godet (IRAP, F), Arnaud Claret (CEA, F), Zi-Gao Dai (USTC, CH), Frédéric Daigne (IAP, F), Jin-Song Deng (NAOC, CN), Andrea Goldwurm (APC, F), Diego Götz (CEA, F), Xu-Hui Han (NAOC, CN), Cyril Lachaud (APC, F), En-Wei Liang (GXU, CN), Yu-Lei Qiu (NAOC, CN), Susanna Vergani (Obs. Paris, F), Jing Wang (NAOC, CN), Chao Wu (NAOC, CN), Li-Ping Xin (NAOC, CN), Bing Zhang (UNLV, CN)
on 30 Sep 2024; 14:22 UT
Credential Certification: Miguel Llamas Lanza (Miguel.Llamas.Lanza@gmail.com)

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We report on the detection of a burst likely from Swift J1818.0-1607 by the ECLAIRs coded-mask instrument onboard the SVOM mission. This source was detected with the offline trigger (ground analysis) at 2024-08-17T21:34:06.36UTC (T0), and is coincident in time with an in-flight SVOM/GRM trigger (SVOM trigger reference: sb24081709). We report here the analysis of ECLAIRs data. The signal-to-noise ratio in the reconstructed image is 37.6 in the 4 -120 keV during a time window of 0.32s from T0. The source is located at RA=274.530 +/- 0.048°, Dec = -16.052 +/- 0.043° (J2000), corresponding to l = 14.8985° ; b = -0.1250° in Galactic coordinates. The center of ECLAIRs position lies 5.05’ away from the previously reported location of this source, well within the statistical position uncertainties. The lightcurve has a double-peak structure lasting about 0.3 s followed by a tail of about 0.5 s. Its spectrum is soft with most detected counts below 50 keV. We fitted the spectrum by a two-black-body model with kT1 = 2.10 +/- 0.15 keV and kT2 = 4.77 +/- 0.64 keV (Chi2/dof = 12.19/11). The uncertainties are quoted at the 1 sigma level. From the black-body fit, we found an average flux of 1.65 e-7 erg/s/cm^2 within the 4-50 keV energy range in a two second window, resulting in a fluence of 3.3 e-7 erg/cm^2. We advise caution regarding these spectral parameters as we are still checking the instrumental spectral performances, and the presence of several other bright sources in the FoV makes the spectrum extraction process challenging. The softness, duration and position lead us to suggest the association of this transient with the SGR Swift J1818.0-1607. 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. ECLAIRs was developed jointly by APC, CEA, CNES and IRAP.