SVOM/VT discovery of an optical counterpart to the new NS-LXMB EP J171159.4-333253

ATel #17272; Sebastien Guillot (IRAP), Liping XIN (NAOC, CAS), Alessio Marino, Francesco Coti Zelati, Nanda Rea (ICE-CSIC), on behalf of the SVOM and EP collaborations
on 5 Jul 2025; 09:38 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Nanda Rea (rea@ice.csic.es)

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Following the discovery of EP J171159.4-333253 (Ni et al., 2025, Atel #17247) as a neutron star low-mass X-ray binary showing eclipses and clocked X-ray bursts (Wu et al., 2025, Atel #17255; Marino et al. 2025, Atel #17271), as well as with the identification of a radio counterpart with MeerKAT (Cowie et al., 2025, Atel #17258), we report here on ToO observations with the SVOM Visible Telescope (VT).

Analyses of the observations taken between 2025-06-28T07:58:05 UT and 2025-06-28T11:59:43 UT (for a total of 102 x 60 seconds of exposure) found a source at the position RA=17h11m59.35, DEC=-33d32m51.29s (+/- 0.5 arcsec). This position is compatible, within the uncertainties, with the position of the MeerKAT radio counterpart, and distinct from the position of Gaia DR3 5979701138818118016, which was initially within the Swift XRT error box of EP J171159.4-333253 (Wu et al., 2025, Atel #17255).

The measured magnitudes are around 18.5 in the VT-Red band, and around 20.0 in the VT-Blue band; although there are some uncertainties due to the high stellar density. The expected extinction in the direction of EP J171159.4-333253 is around Av = 2.1 +/- 0.6 magnitude, based on the hydrogen column density N_H of (0.44+/0.05)e22 cm-2 deduced from the X-ray spectral analysis (Marino et al. 2025, Atel #17271).

We encourage further monitoring of this source in all wavelengths.

We kindly thank the SVOM ToO Scientist, Cyrill Lachaud, for scheduling these observations.

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. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC),CAS.