SVOM/ECLAIRs detection of an X-ray burst from a new source SVOM J17308-1702 or possibly from GX 9+9

ATel #17270; Sebastien Guillot (IRAP, France), Marius Brunet (IRAP, France), Floriane Cangemi (APC, France), Alexis Coleiro (APC, France), Sebastien Le Stum (APC, France), Jerome Rodriguez (CEA, France), Liang Zhang (IHEP, China)
on 5 Jul 2025; 07:54 UT
Credential Certification: Alexis Coleiro (alexis.coleiro@u-paris.fr)

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We report on the detection of an X-ray burst at 2025-06-28T11:11:26.6 (UTC) by the ECLAIRs telescope onboard the Space-based multi-band astronomical Variable Objects Monitor (SVOM). The burst was first seen by visual inspection of the lightcurve and then through an offline search with the event-by-event data downloaded via the X-band ground station. The most significant trigger is in the 4-20 keV band with a signal-to-noise ratio SNR=15.4.

The position of this trigger with the best SNR is RA=262.744 degrees and Dec.=-17.038 degrees, with an R90 positional uncertainties of 7.80 arcmin, which includes 6 arcminutes of systematic uncertainties (a conservative value). The average position of all offline-trigger detections weighted by SNR is RA = 262.7075 degrees and Dec=-17.0465 degrees (R90=9.15 arcminutes), or RA = 17h30.8m and Dec = - 17d02m.

The average position is 14 arcminutes from the known neutron star low-mass X-ray binary GX 9+9 (also 4U 1728-16). Therefore, it is likely that this X-ray burst does not originate from GX 9+9, but an origin from GX 9+9 is not fully excluded.

A preliminary analysis covering the entire 10-second burst duration, shows that the 4-20 keV ECLAIRs spectrum is well fitted (chi2/ddof = 0.6) by a blackbody model with kT = 2.4 +/- 0.4 keV (90% confidence) and norm = 0.17 +/- 0.03 (90% confidence).

Follow-up X-ray observations are planned to confirm the detection of this source, to improve its localization, and determine whether it originates from a new X-ray burster (SVOM J17308-1702) or from GX 9+9.

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.