SVOM/ECLAIRs detection of the symbiotic low-mass X-ray binary 4U 1700+24

ATel #17997; H. Yang, (IRAP, France), F. Cangemi (APC, F), S. Guillot (IRAP, F), A. Coleiro (APC, F), C. Plasse (HKU, China), L. Tao, (IHEP, C), L. Zhang (IHEP, C)
on 21 Aug 2026; 13:01 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)

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On Tuesday, August 18, 2026, at 14:41:30.099 UT, the onboard trigger software of the SVOM/ECLAIRs telescope (4-150 keV) detected and localized a source at R.A. = 256.787 deg and Dec. = 23.897 deg (J2000), with an uncertainty radius of 6.95 arcmin (90% confidence level) in the 5-20 keV energy band (SNR = 11.56).

Following this detection, SVOM autonomously slewed to the trigger direction, and observations were performed with MXT (0.2-10 keV) starting at 14:54:46 UT, for a total exposure of 6.7 ks. The MXT position R.A. = 256.649 deg, Dec. = 23.976 deg (J2000) with an uncertainty radius of 44.4 arcsec (90% confidence level) is consistent with the symbiotic low-mass X-ray binary 4U 1700+24.

A preliminary joint spectral analysis of the simultaneous ECLAIRs (exposure of 4621 s) and MXT (exposure of 6701 s) data shows that the 0.5-50 keV spectrum is well fitted (fit statistic = 1121.12/984 dof) by a black body plus Comptonization model, using bbody+compST in XSPEC, with a black body temperature of kT_0 = 1.5 +/- 0.2 keV, a Comptonization temperature of kT_e = 5.6 (+2.3 / -1.5) keV and optical depth tau = 17 (+6 / -4). The 0.5-50 keV observed flux is (7.7 +/- 0.3) e-10 erg/cm2/s.

In addition, 4U 1700+24 was in the field-of-view of ECLAIRs on August 19, 2026, and the source was detected by the ECLAIRs Quick Look Analysis (Goldwurm et al. 2026). A preliminary spectral analysis of the 2026 August 19 spectrum (exposure of 6970 s) is well fitted (chi2/dof = 5.35/10) with a powerlaw model, yielding a photon index of 1.9 +0.2 / -0.1 and a normalisation of norm = 0.23 (+0.10 / -0.07) photons/keV/cm2/s. The observed 4-50 keV flux is (1.2 +/- 0.1) e-9 erg/cm2/s. All errors are at the 90% confidence level.

This is the first detection of 4U 1700+24 with ECLAIRs. The preliminary analysis presented above indicates an increase in the source flux between August 18 and August 19 observations, suggesting a potential increase in activity from this symbiotic X-ray binary. The last reported outbursts from this source were on 2014 (Atels #6482, #6850, GCNs 17173, 16829, and 17280).

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.