SVOM/ECLAIRs detection of a photospheric radius expansion burst from 4U 1702-429
ATel #17079; F. Cangemi (APC, France), A. Coleiro (APC, F), M. Brunet (IRAP, F), S. Guillot (IRAP, F), Z. Li (XTU, China), L. Tao (IHEP, C), L. Zhang (IHEP, C)
on 14 Mar 2025; 13:22 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)
Subjects: X-ray, Binary, Neutron Star
On Monday March 10th at 13:23:19.0 UTC (Tb), the SVOM/ECLAIRs (4-150 keV) coded-mask telescope detected a Type I thermonuclear burst from 4U 1702-429 via an offline search with the event-by-event data downloaded through the X-band ground station.
The lightcurve showed two peaks, the first one lasting about 2 seconds followed by an exponential decay-like shape peak with a duration of about 20 seconds in the 4-20 keV energy range. This suggests that ECLAIRs detected a photospheric radius expansion (PRE) burst.
A preliminary time-resolved spectral analysis shows a decrease in the blackbody temperature from kT = 2.4 +/- 0.2 keV to kT = 0.8 (-0.2 / +0.3) keV during the first 4 seconds of the burst. Then, the temperature increases and reaches a "plateau" value of approximately 2.5 keV. This is typical of a PRE burst in which the photosphere cools at maximum expansion and heats up again as it recedes. Due to poor statistics, we are unable to constrain the size of the emission region. However, if the temperature is fixed to the value obtained with the previous best fit, we derive a radius for a spherical emission region of 64 +/- 36 km, assuming a distance of 4.19 kpc (Galloway et al. 2008), and 128 +/- 17 km, assuming a distance of 8.43 kpc (Varun et al. 2024). After the first 4 seconds, the radius decreases and reaches a mean value of about 7 km and 15 km, respectively, for the two distances.
The integrated 4-30 keV flux is (6.7 +/- 0.6)e-08 erg/s/cm^2 during the expansion phase, whereas it reaches (9.4 +/- 0.7)e-08 erg/s/cm^2 at about Tb + 20 s. These fluxes correspond to a luminosity of 1.4e38 erg/s and 2.0e38 erg/s assuming a distance of 4.19 kpc ; and 5.7e38 erg/s and 8.0e38 erg/s assuming a distance of 8.43 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.
