GCN 44753: GRB 260518B: SVOM/ECLAIRs detection of a soft burst possibly associated with EP 260518a and AT 2026ndp

M. Brunet, O. Godet (IRAP) report on behalf of the SVOM/ECLAIRs team

The SVOM/ECLAIRs telescope detected an X-ray source starting at 2026-05-18T07:53:22 UTC (trigger time T0), labelled GRB 260518B, through an offline targeted search with the event-by-event data downloaded through the X-band ground station. 

The source localization is RA, Dec = 299.003 , -48.620 degrees:

RA (J2000) =19h56m00.7s    
Dec (J2000) = -48d37m12.0s 

with a 90% C.L. radius of 11.1 arcmin (including a systematic error of 6 arcmin added in quadrature). This position is consistent with those of EP 260518a (Liang et al., GCN 44690) and of the optical transient AT 2026ndp  with an angular separation of 1.1 and 1.0 arcmin, respectively.

This event was detected within several energy ranges and timescales. The best detection is obtained by the on-ground Image Trigger with a signal-to-noise ratio of 8.2 within 4-20 keV over a time window of 40.96 seconds starting at T0-7.68 s.

The burst that triggered ECLAIRs on-ground shows a multiple peak lightcurve. The burst duration is estimated to be around 52 s in the 4-20 keV energy band through imaging. The source being detected when the source was emerging from behind the Earth, ECLAIRs may have missed part of the event emission. 

The ECLAIRs light curve can be found here: https://docs.google.com/document/d/1DvqHDMZI5uQY-K7kINk7iSFot71MQIoEbDZZ7YvvAEc/edit?tab=t.0

The time-averaged spectrum from T0-11 s to T0+41 s in the 5-120 keV energy range is best fitted by a broken powerlaw model with alpha = -1.14 +0.28/-0.26, beta = -2.98 +0.72/-1.12 and a break energy of 21.0 +3.5/-6.7 keV. With this model, the 4-120 keV fluence is (7.8 +1.6/-2.1)e-7 erg/cm^2 and the 4-120 keV photon flux is 0.63 +0.05/-0.17 ph/cm^2/s. 

This event is very soft and if it is a GRB, then it should be probably classified as an X-Ray Flash. 

Assuming a redshift of z=2.28 (Corcoran et al., GCN 44695) and a standard cosmology with H_0 = 67.3 km/s/Mpc and Omega_M = 0.315 (Planck Collaboration, 2014), we estimate the following rest-frame parameters: the isotropic energy release E_iso is (1.25 +0.27/-0.34)e+52 erg, and the rest-frame peak energy of the time-averaged spectrum Ep,z is 68.9 +11.5/-22.0 keV. With these estimates, GRB 260518B is consistent with the distribution of Type II (collapsar-like) GRBs on the Amati relation reported by Willingale et al. 2017 (figure available with the light curve). This further strengthens the nature of the event to be a Gamma-ray burst. 

If GRB 260518B is related to EP 260518a and the optical transient AT 2026ndp, ECLAIRs may have detected the prompt phase 5.8 h prior to the EP detection while  EP and ATLAS may have observed the afterglow phase. 

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: Marius Brunet (IRAP) (marius.brunet at utoulouse.fr)