SVOM detection of a thermonuclear Type I bursts from EXO 0748-676

ATel #17015; A. Coleiro (APC, France), D. Götz (CEA, F), M. Brunet (IRAP, F), J.-L. Atteia (IRAP, F), F. Cangemi (APC, F), P. Maggi (ObAS, F), S. Guillot (IRAP, F), Z. Li (XTU, China), L. Tao (IHEP, C), L. Zhang (IHEP, C)
on 5 Feb 2025; 16:42 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)

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On Saturday, February 1st 2025, at 12:23:59 UTC, the SVOM/ECLAIRs (4-150 keV) coded-mask telescope detected a burst from EXO 0748-676 via its onboard trigger. The transmitted light-curve showed a single peak with a rise time of approximately 5 s followed by a decay lasting approximately 15 s in the 5-20 keV band.

A preliminary spectral analysis with SVOM/ECLAIRs indicates that the 4-50 keV spectrum is well fitted (chi2 = 15.8 / 14 dof) with an unabsorbed blackbody (bbodyrad) model with a temperature of kT = 2.3 +0.3/-0.2 keV (90% confidence level) and a radius of 4 +3/-1 km, assuming a distance of 7.1 kpc. The integrated 4-20 keV flux is approximately 7.4 (+0.6/-0.5)e-09 erg/s/cm^2 corresponding to a luminosity of about 4.5e37 erg/s.

The SVOM/ECLAIRs lightcurve and spectrum are consistent with a thermonuclear Type I X-ray burst. This observation hence suggests the detection of a new Type I burst from this source, following several similar Type 1 bursts reported by different facilities (ATels #16790, #16703, #16654, #16678, #16753) after the source re-entered an active state (see GCN #36653, ATels #16646, #16648, #16649, #16651) in June 2024.

SVOM slewed automatically to the position of EXO 0748-676, and MXT observed the field starting at 12:27:56 UTC. At 12:41:05 UTC a second, fainter, type I X-ray burst, lasting about 40 s has been detected. The integrated flux in the 0.5-10 keV energy range is 6.0(+/-0.1)e-9 erg/s/cm^2, corresponding to 4.5e-09 erg/s/cm^2 in the 4-20 keV energy range. The spectrum of the burst is compatible with a blackbody of temperature 1.6+/-0.3 (90% c.l.) keV, and a radius of 6.7+/-2.2 (90% c.l.) km.

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. MXT has been developed jointly by CNES, CEA, IJCLab, University of Leicester and MPE.