A team of international astronomers has conducted significant observations of a newly discovered very faint X-ray transient (VFXT) located in the center of the Milky Way galaxy. The object, designated 4XMM J174610.7–290020, also known as Swift J174610–290018, has provided new insights into the nature of these dim and elusive celestial bodies. The findings, which were made public on the arXiv pre-print server, detail the transient’s intermittent outbursts and suggest it is a neutron star low-mass X-ray binary. The study of VFXTs is crucial for understanding the full range of behaviors of X-ray binaries and the accretion processes that power them.
VFXTs are a class of X-ray binaries that are characterized by their low peak X-ray luminosity, which is less than 10^36 erg/s, making them significantly fainter than more typical X-ray binaries. Only a few dozen VFXTs have been documented in the Milky Way, with just a handful found in the galaxy’s central region. Their low flux levels make them difficult to detect and study in detail, which is why the discovery and analysis of new VFXTs like 4XMM J174610.7–290020 are so important to the astronomical community. This particular VFXT was first detected in February 2024 by NASA’s Swift spacecraft and is located about 26,700 light-years from Earth, near the supermassive black hole Sagittarius A*.
Observational History and Outbursts
Since its discovery, 4XMM J174610.7–290020 has been the subject of intense scrutiny by a team of astronomers led by Giovanni Stel of the Brera Astronomical Observatory in Italy. The researchers have focused on characterizing the transient’s outbursts to better understand its physical nature and the processes that drive its variability. The team’s observations have revealed two significant outbursts in 2024 and 2025. The first of these events lasted for approximately 50 days, while the second was much shorter, lasting only five days. During these outbursts, the luminosity of the transient in the 2–10 keV energy band reached between 10–120 and 60–90 decillion erg/s, respectively. These luminosity measurements are the highest ever recorded for this source and firmly establish its classification as a VFXT.
Archival Data Analysis
In addition to the recent observations, the research team also delved into archival data to search for past activity from 4XMM J174610.7–290020. This investigation revealed a potential type I X-ray burst from the source in 2004. A type I X-ray burst is a thermonuclear explosion that occurs on the surface of a neutron star as it accretes matter from a companion star. This finding is a key piece of evidence that suggests the transient is a neutron star low-mass X-ray binary. The analysis of the 2004 event also showed that the source’s luminosity before the flare was nearly two orders of magnitude lower than during the more recent outbursts, highlighting the extreme variability of this object.
The Nature of 4XMM J174610.7–290020
Based on the comprehensive analysis of both new and archival data, the astronomers have concluded that 4XMM J174610.7–290020 is likely a neutron star low-mass X-ray binary. In this type of system, a neutron star, which is the incredibly dense remnant of a massive star that has exploded as a supernova, is in a close orbit with a low-mass companion star. The intense gravity of the neutron star pulls material from the companion star into an accretion disk around itself. This material then spirals inward and eventually falls onto the neutron star’s surface, releasing vast amounts of energy in the form of X-rays. The intermittent outbursts observed from 4XMM J174610.7–290020 are thought to be caused by instabilities in the accretion disk, which lead to periods of increased mass transfer onto the neutron star.
An Obscured View
The researchers have also proposed a model for the system’s geometry that explains its faintness and other peculiar features. They suggest that the binary system is viewed nearly edge-on from Earth, meaning that our line of sight passes through the accretion disk. This orientation would cause the disk to partially obscure the X-ray emission from the neutron star, leading to the observed low luminosity. This model also accounts for the presence of strong iron lines in the transient’s X-ray spectrum, which are thought to be produced when X-rays from the neutron star are reprocessed in the surrounding accretion disk. The edge-on viewing angle would enhance the visibility of these iron lines, providing further support for this interpretation.
The Importance of Studying VFXTs
The study of very faint X-ray transients like 4XMM J174610.7–290020 is essential for a complete understanding of the X-ray binary population. These faint systems may represent a significant fraction of the total number of X-ray binaries in the galaxy, but their low luminosities make them difficult to detect. By discovering and characterizing more of these objects, astronomers can gain insights into the full range of accretion processes that can occur in binary systems. This includes studying how accretion proceeds at very low rates, which is not well understood from observations of brighter X-ray binaries alone. Furthermore, VFXTs provide a valuable laboratory for studying the physics of neutron stars and the behavior of matter in extreme gravitational and magnetic fields.
Future Research Directions
The recent findings on 4XMM J174610.7–290020 have opened up new avenues for research. Continued monitoring of this source will be crucial for detecting future outbursts and studying their properties in greater detail. Multi-wavelength observations, from radio to gamma-rays, will also be important for building a more complete picture of the system. For example, optical and infrared observations could help to identify the companion star and determine its properties, which would provide important constraints on the binary system’s parameters. Further theoretical modeling will also be needed to better understand the physical processes that are responsible for the observed behavior of this enigmatic transient source. The study of 4XMM J174610.7–290020 and other VFXTs will undoubtedly continue to be a fruitful area of research in the years to come.
