2026A&A...705A.135S

Context. Very faint X-ray transients (VFXTs) are a class of X-ray binary systems that exhibit occasional outbursts with peak X-ray luminosities (L_X ≲ 10³⁶ erg s^–1) much lower than those of typical X-ray transients. On 22 February 2024, during its daily Galactic center monitoring, Swift-XRT detected a VFXT 7 arcmin from Sgr A*, dubbing it Swift J174610-290018.
Aims. We aim to characterize the outburst that occurred in 2024 and a second, distinct outburst in 2025 to understand the nature and accretion flow properties of this new VFXT.
Methods. Swift-XRT light curves were used to constrain the duration of the two events. We carried out X-ray spectral analysis, exploiting XMM-Newton and NuSTAR data. We used Chandra and XMM-Newton observations from the last 25 years to constrain the quiescent luminosity of the source and to compare the two most recent outbursts with previous detections of the source.
Results. During the 2024 outburst, which lasted about 50 days, the source reached a luminosity in the 2-10 keV band (L_2–10) of ≃1.2 × 10³⁵ erg s^–1 (assuming it is located at the Galactic center, i.e., at a distance of 8.2 kpc). The 2025 outburst is shorter (about 5 days) and reached L_2–10 ≃ 9 × 10³⁴ erg s^–1. The spectral features of the source include an excess at 6.5-7 keV, which can be associated either with a single reflection line or with the ionized Fe XXV and XXVI lines. The same source was identified in both the XMM-Newton and Chandra catalogs of point sources (known as 4XMM J174610.7-290020 and 2CXO J174610.7-290019). During previous detections the source displayed luminosity levels ranging from L_2–10 ≃ 2 × 10³² to L_2–10 ≃ 3 × 10³³ erg s^–1 between 2000 and 2010. Moreover, it exhibited a potential type I X-ray burst in 2004.
Conclusions. The analysis of the outbursts and the potential type I burst strongly suggests that the VFXT Swift J174610-290018 is a neutron star low-mass X-ray binary. The source can be described as being an accretion disk corona source (as has been recently proposed by an XRISM/Xtend analysis). This scenario explains the overall low luminosity of this transient and the peculiar iron lines in the spectrum.