M10-VLA1
M10-VLA1 is a variable low-mass X-ray binary in the globular cluster Messier 10 that is also a radio source, situated in the equatorial constellation of Ophiuchus about distant. Discovered spectroscopically in 2018 as part of the MAVERIC survey, the system was found to contain an unusual red straggler star orbiting an invisible companion of an uncertain nature, possibly a stellar black hole.
Discovery
M10-VLA1 was detected using the Karl G. Jansky Very Large Array in deep radio continuum imaging at 7.4 GHz, revealing a flux density of 27 ± 4 μJy and a flat to inverted radio spectrum indicative of compact emission from accretion processes. Chandra X-ray Observatory observations identified an X-ray counterpart with a luminosity of ~1031 erg/s, consistent with the radio-X-ray correlation for quiescent black holes. Hubble Space Telescope imaging revealed ultraviolet and optical variability, while spectroscopy from the SOAR Telescope showed double-peaked Hα emission lines, suggesting an accretion disk around the companion. The optical spectrum of the visible star resembles a G-type star.
Charecterstics
The system has an orbital period of 3.339 days, determined through spectroscopic radial velocity measurements. The visible component is a red straggler, a star brighter and redder than typical for its position in M10’s color-magnitude diagram, likely formed through dynamical interactions or a merger in the dense cluster environment. The companion’s low radial velocity semi-amplitude and the system’s properties suggest a massive companion, most likely a black hole with a mass comparable to or greater than similar systems. The binary’s face-on orientation explains its observed characteristics if it is a black hole X-ray binary.
Messier 10 is a relatively loose globular cluster with a metallicity of ≈ -1.5 and an age of ~12–13 billion years, containing hundreds of thousands of stars. Globular clusters like M10 are known for hosting exotic objects such as X-ray binaries due to frequent stellar encounters in their dense cores. M10-VLA1 is a significant example of such systems, potentially representing one of the few confirmed black hole binaries in a Galactic globular cluster.
Further Research
Further observations, including deeper spectroscopy or monitoring for flares or eclipses, are needed to definitively confirm the nature of the compact companion. If confirmed as a black hole, M10-VLA1 would contribute to understanding the formation and evolution of black hole binaries in dense stellar environments.