TITLE: GCN CIRCULAR NUMBER: 42985 SUBJECT: GRB 251129A/EP251129a : SVOM/ECLAIRs refined analysis DATE: 25/12/04 09:35:09 GMT FROM: SVOM_group <svomgroup@bao.ac.cn> H. Yang, M. Brunet, O. Godet, J.-L. Atteia (IRAP), W. J. Xie (NAOC), W. J. Tan (IHEP), D. F. Kong (GXU) Using the event-by-event data downloaded through the X-band ground stations, we report further analysis of ECLAIRs observations of GRB 251129A/EP251129a (SVOM burst-id sb25112901, GCN 42877) detected at T0 = 2025-11-29T02:32:05 UT, which was also detected by EP/WXT (GCN 42884). The burst that triggered ECLAIRs shows a featureless light curve, and is detected from T0-41 s to T0+3200 s in the 4-120 keV energy band, indicating that it may be an ultra-long GRB or a very bright afterglow or a combination of both. The peculiar multi-wavelength temporal behavior of this event makes it a burst of great interest. ECLAIRs observations of GRB 251129A/EP251129a were interrupted by a slew and by a pass through the SAA, which divides the data into three time intervals for the time-averaged spectra analysis. The time-averaged spectrum from T0-41 s to T0+77 s (before the slew) in the 4-120 keV is well fitted by a power-law model with a photon index of 1.7+/-0.1 and a corresponding 4-120 keV flux of (1.5+/-0.2)e-8 erg/cm^2/s. The spectrum from T0+190 s to T0+585 s (after the slew and before the SAA transit) in the 4-120 keV is well fitted by a power-law model with a photon index of 2.1+/-0.1, with a 4-120 keV flux of (3.9+0.5-0.4)e-9 erg/cm^2/s. The spectrum from T0+1600 s to T0+3200 s (after the SAA transit) in the 4-120 keV is well fitted by a power-law model with a photon index of 2.0+/-0.1, with a 4-120 keV flux of (1.5+0.1-0.3)e-9 erg/cm^2/s. Overall, the ECLAIRs spectral analysis shows that GRB 251129A/EP251129a is decreasing in fluxes, and shows no significant spectral evolution from T0+190 s to T0+3200 s. The photon index measured from T0+190 s to T0+3200 s is consistent with that measured from EP/FXT at T0+2643 s (GCN 42902). The long-lasting emission seen by ECLAIRs matches the X-ray emission of the uncatalogued source seen by SVOM/MXT (GCN 42877). All the quoted errors are at the 68% confidence level. 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. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. The SVOM/ECLAIRs point of contact for this burst is: Hui Yang (IRAP) (hui.yang@irap.omp.eu) View this GCN Circular online at https://gcn.nasa.gov/circulars/42985. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...