SVOM/ECLAIRs Detection of a possible photospheric radius expansion Type I X-ray burst from 4U 1850-086 in the globular cluster NGC 6712
ATel #17233; F. Cangemi (APC, France), O. Godet (IRAP, F), S. Guillot (IRAP, F), J.-L. Atteia (IRAP, F), L. Zhang (IHEP, China)
on 18 Jun 2025; 07:52 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)
Subjects: X-ray, Binary, Neutron Star
On June 16th 2025 at 11:37:51.8 UTC (Tb), the onboard trigger of SVOM/ECLAIRs detected a Type I thermonuclear X-ray burst at a position coincident with the globular cluster NGC 6712 in which lies the known X-ray burster 4U 1850-086. The trigger localisation at RA=283.241 deg. and Dec=-8.727 deg. is 2.2 arcminutes from 4U 1850-086, well within the 3.85 arcminutes uncertainty on the position (90% confidence).
In the past, this source has exhibited a few X-ray bursts, mostly of several minutes durations or longer (with RXTE, Galloway et al. 2020; with Swift-BAT, ATel #5972, ATel #5978; with MAXI, ATel #7500, and with MAXI+NICER, ATel #16600), and three bursts with order 20-sec durations (with Integral JEM-X, Galloway et al. 2020).
A preliminary time-resolved spectral analysis shows a decrease in the blackbody temperature from kT = 2.7 +/- 0.2 keV (90% confidence) to kT = 1.5 +/-0.5 keV over the ~10 seconds of the burst duration. A drop in temperature is observed, decreasing from 2.8 +/- 0.2 keV to 2.0 +/- 0.2 keV keV between Tb+8.02 seconds and Tb+10.02 seconds, while at the same time, the photospheric radius increases from 0.80 +/- 0.07 km to 1.29 +/- 0.12 km, assuming a distance of 7.4 +/- 0.6 kpc (Baumgardt & Vasiliev 2021). The increase in blackbody normalization coincident with the drop in the temperature suggests that the event could be a photospheric radius expansion (PRE) burst.
The integrated 4-30 keV flux is dropping from (7.8 +/- 0.5)e-08 erg/s/cm^2 (68% confidence) to (8.6 +/- 1.2)e-09 erg/s/cm^2 between Tb+4.02 and Tb+12.02 s. These fluxes correspond to a luminosity of 5.1e38 erg/s and 5.6e37 erg/s assuming a distance of 7.4 kpc.
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.
