SVOM/ECLAIRs detection of a thermonuclear burst in Terzan 1

ATel #17385; M. Brunet (IRAP, France), F. Cangemi (APC, F), S. Guillot (IRAP, F), A. Coleiro (APC, F), L. Zhang (IHEP, China)
on 11 Sep 2025; 08:47 UT
Credential Certification: Sebastien Guillot (Sebastien.guillot@gmail.com)

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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 2025-09-05T10:20:02.2 on a timescale of 10.24 seconds.

The source has been located at R.A. = 263.973 deg, Dec = -30.462 deg (J2000) with a 90% statistical uncertainty of 6.7' in radius. We associate the ECLAIRs trigger source with XB 1732-30 (1RXS J173546.9-302859), whose coordinates are located 1.9' away from the trigger location.

The 4-20 keV time-integrated spectrum can be well fitted (chi2/dof = 12.67/9) using a bbodyrad model, with a temperature of kT = 2.44 +/- 0.07 keV and a radius of 7.66 +0.50/-0.44 km, assuming a distance of 6.7 kpc (Baumgardt et al, MNRAS 482-4, 2019). The derived unabsorbed flux in the 4-20 keV band is 3.21 +0.15/-0.09 photons/s/cm^2, or (4.26 +0.16/-0.13)e-8 erg/s/cm^2.

The lightcurve shows a double peak structure in the 8-12 keV energy band, not visible in the 4-8 keV energy band. This could indicate that ECLAIRs detected a photospheric radius expansion (PRE) burst. However, a preliminary time-resolved spectral analysis does not show the characteristic signatures of a PRE burst, namely a significant drop in blackbody temperature (kT) together with a strong increase in the emission radius (blackbody normalisation) during the expansion phase, followed by a contraction back to the neutron star radius.

The source XB 1732-30 is a known burster that was in quiescence for over 20 years until the 20th May 2025 when ECLAIRs detected a type I burst from this sources (Atel #17201). A decline in the source's activity had since been observed by NICER (ATel #17215). 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).

Further multi-wavelength observations are encouraged.

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.