SVOM/ECLAIRs detection of a thermonuclear burst in Terzan 1

ATel #17201; S. Le Stum (APC, France), M. Brunet (IRAP, F), S. Guillot (IRAP, F), A. Coleiro (APC, F), F. Cangemi (APC, F), L. Tao (IHEP, China), L. Zhang (IHEP, C)
on 23 May 2025; 15:47 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)

Tweet

We report on the detection of a type I thermonuclear burst located in the globular cluster Terzan 1 by the ECLAIRs coded-mask instrument (4-150 keV) on board the SVOM mission. The source was detected by the offline trigger at 2025-05-20T15:24:36.2 on a timescale of 20.48 seconds.

The source has been located at R.A. = 263.953 deg, Dec = -30.537 deg (J2000) with a 90% statistical uncertainty of 6.6' in radius. We tentatively associate the ECLAIRs trigger source with XB 1732-30 (1RXS J173546.9-302859), whose coordinates are located 3.3' away from the trigger location.

The 5-20 keV time-integrated spectrum can be well fitted (chi2/dof = 5.0/12) using a bbodyrad model, with a temperature of kT = 2.3 +/- 0.1 keV and a radius of 6.6 +/- 0.6 km, assuming a distance of 6.7 kpc (Baumgardt et al, MNRAS 482-4, 2019). The derived unabsorbed flux in the 5-20 keV band is 1.6 +/- 0.2 photons/s/cm^2, or (2.2 +/- 0.2)e-8 erg/s/cm^2.

The source XB 1732-30 is a known burster that has been in quiescence for over 20 years, this new burst could indicate that the source is exhibiting renewed activity. Moreover, MAXI has detected an apparent increase in the X-ray flux from Terzan 1, similar to what was observed during a previous period of activity reported in ATel #16124 (http://maxi.riken.jp/star_data/J1734-304/J1734-304.html). This increase might be associated with XB 1732-30, as also suggested by MAXI in their previous ATel.

Further multi-wavelength observations are encouraged to confirm the source identification.

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.