The Maveric Survey: a Red Straggler Binary with an Invisible Companion in the Galactic Globular Cluster M10

TL;DR

This paper reports the discovery of a binary system in globular cluster M10 with properties suggesting it may host a quiescent black hole or be an active binary, based on multi-wavelength observations and analysis.

Contribution

It provides the first detailed multi-wavelength characterization of M10-VLA1, a candidate black hole binary with unique optical and radio features in a globular cluster.

Findings

M10-VLA1 is a radio-bright binary with a flat to inverted spectrum.

The system's X-ray luminosity aligns with quiescent black hole binaries.

Optical data show an evolved red straggler star with signs of accretion activity.

Abstract

We present the discovery and characterization of a radio-bright binary in the Galactic globular cluster M10. First identified in deep radio continuum data from the Karl G. Jansky Very Large Array, M10-VLA1 has a flux density of $27\pm4$ $\mu$Jy at 7.4 GHz and a flat to inverted radio spectrum. Chandra imaging shows an X-ray source with $L_X \approx 10^{31}$ erg s$^{-1}$ matching the location of the radio source. This places M10-VLA1 within the scatter of the radio--X-ray luminosity correlation for quiescent stellar-mass black holes, and a black hole X-ray binary is a viable explanation for this system. The radio and X-ray properties of the source disfavor, though do not rule out, identification as an accreting neutron star or white dwarf system. Optical imaging from the Hubble Space Telescope and spectroscopy from the SOAR telescope show the system has an orbital period of 3.339 d and an unusual "red straggler" component: an evolved star found redward of M10's red giant branch. These data also show UV/optical variability and double-peaked H$\alpha$ emission characteristic of an accretion disk. However, SOAR spectroscopic monitoring reveals that the velocity semi-amplitude of the red straggler is low. We conclude that M10-VLA1 is most likely either a quiescent black hole X-ray binary with a rather face-on ($ i < $ 4$^{\circ}$) orientation or an unusual flaring RS CVn-type active binary, and discuss future observations that could distinguish between these possibilities.