SVOM follow-up of the X-ray transient EP260531a

ATel #17842; F. Cangemi (APC, France), L. Tao (IHEP, China), A. Coleiro (APC, F.), L. Zhang (IHEP, C)
on 13 Jun 2026; 08:37 UT
Credential Certification: Floriane Cangemi (cangemi@apc.in2p3.fr)

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We report on SVOM ToO follow-up observations of EP260531a, a new X-ray transient discovered by the Einstein Probe mission on 2026 May 31 (ATel #17828). SVOM performed observations at the position RA = 265.71186, Dec = -31.200138 between 2026 June 7 at 23:49:29 UT and 2026 June 8 at 04:40:42 UT.

The SVOM/MXT spectrum (5747 s of exposure) in the 0.3-10 keV energy range can be well described (chi2/dof = 1073.74/1000) by an absorbed diskbb model with a temperature of kT = 0.47 +/- 0.04 keV and a column density of nH = 2.90 (+0.28 / -0.25) 10^22 cm^-2. The 0.5-10 keV flux is 1.73 (+0.06 / -0.59) e-10 ergs/cm2/s. The addition of a power-law component does not significantly improve the fit. All uncertainties are quoted at the 90% confidence level.

The temperature and the flux derived from the MXT spectrum are consistent with that reported from Einstein Probe/FXT observations (ATel #17833) on 2026 June 4 suggesting that the source may have reached a plateau. Such a temperature is compatible with a black hole X-ray binary observed in the low-hard state.

No significant emission was detected with SVOM/ECLAIRs. Assuming a power-law spectrum with a photon index of Gamma = 2, we derived a 3-sigma upper limit on the source flux of 2.21 e-10 ergs/cm2/s in the 4-150 keV energy range.

Optical observations performed with SVOM/VT at the position of the X-ray source do not show any significant variability in the VT_R band of the possible optical counterpart reported in ATel #17840. We note, however, that several optical sources are present within the error circle, preventing an unambiguous identification of the counterpart based on the current VT observations alone.

Additional SVOM observations are scheduled for 2026 June 15.

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.