The very faint X-ray transient Swift J174610–290018 at the Galactic center

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
² Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁴ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1098 XH Amsterdam, The Netherlands
⁵ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Via A. Corti 12, 20133 Milano, Italy
⁶ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
⁷ Department of Astronomy, University of Illinois, 1002 W. Green St., Urbana, IL 61801, USA
⁸ Department of Astronomy, Ohio State University, 140 West 18th Ave., Columbus, OH 43210, USA
⁹ Department of Astronomy, The University of Michigan, 1085 South Univerity Avenue, Ann Arbor, MI, 48103, USA
¹⁰ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
¹¹ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China
¹² Institute for Frontier in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China
¹³ Department of Physics, Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
¹⁴ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
¹⁵ Department of Physics and Astronomy, Michigan State University, East Lansing, 48824, USA
¹⁶ Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
¹⁷ Department of Physics and Astronomy, University of California, Los Angeles, CA, 90095-1547, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 September 2025
Accepted: 10 November 2025

Abstract

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.