Observations inspect the nature of a newly discovered very faint X-ray transient
Tomasz Nowakowski
astronomy writer
Sadie Harley
scientific editor
Robert Egan
associate editor
Using various space telescopes, an international team of astronomers have observed a newly detected very faint X-ray transient designated 4XMM J174610.7–290020. Results of the observational campaign, October 2 on the arXiv pre-print server, yield new insights into the nature of this transient.
Very faint X-ray transients (VFXTs) are X-ray binary systems experiencing occasional outbursts with peak X-ray luminosities lower than an undecillion erg/s, therefore fainter than typical X-ray binaries. To date, only a few tens of VFXTs have been detected in the Milky Way, and about a dozen in the center of our galaxy.
Due to their typical low fluxes, the number of VFXTs that have been investigated in detail is still very small. Therefore, finding new VFXTs and studying them is essential to get a comprehensive view of the population of these transients.
4XMM J174610.7–290020, also known as Swift J174610–290018 is a VFXT in the galactic center discovered with NASA's Swift spacecraft in February 2024. The transient was identified approximately 6.7 arcminutes east of Sagittarius A*, which corresponds to 52.16 light years, at a distance of 26,700 light years away from Earth.
Since its discovery, 4XMM J174610.7–290020 exhibited two significant outbursts and a group of astronomers led by Giovanni Stel of the Brera Astronomical Observatory in Italy, decided to take a closer look at these events, hoping to shed more light on the nature of this VFXT.
"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," the scientists wrote in the paper.
The observations found that the 2024 and 2025 outbursts of 4XMM J174610.7–290020, lasted 50 and five days, while their luminosities (in the 2–10 keV band) were measured to be about 10–120 and 60–90 decillion erg/s, respectively. These two outbursts were the brightest recorded for this source, which confirms its VFXT classification.
According to the study, the spectrum of 4XMM J174610.7–290020 is characterized by a spectral slope of about 2–3, which is common for VFXT that are neutron star low-mass X-ray binaries (NS-LMXBs). Moreover, during the two outbursts, the transient exhibited spectra characterized by a strong iron complex.
By analyzing the archival data of 4XMM J174610.7–290020, the astronomers found evidence of a potential type I X-ray burst in 2004. It turned out that during the 2004 type I burst, immediately preceding the flare, the source was nearly two orders of magnitude fainter than during the 2024 and 2025 outbursts.
The researchers concluded that the results strongly suggest that 4XMM J174610.7–290020 is an NS-LMXB, which is seen almost edge-on, and the radiation from the source is blocked by the thick accretion disk.
"This scenario explains the overall low luminosity of this transient and the peculiar iron lines in the spectrum," the authors of the paper explained.
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More information: Giovanni Stel et al, The Very Faint X-ray Transient 4XMM J174610.7-290020 at the Galactic center, arXiv (2025).
Journal information: arXiv
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