An infrared FWHM-K2 correlation to uncover highly reddened quiescent black holes

Vol. 679
7. Stellar structure and evolution

An infrared FWHM-K2 correlation to uncover highly reddened quiescent black holes

by V.A. Cúneo, J. Casares, M. Armas Padilla, ey al. 2023, A&A, 679, L11

Black holes and neutron stars can be weighted when in a binary system thanks to Newton's third law. This technique led to the first confirmation of the existence of black holes, and it is the main avenue to assess for the presence of black holes in binary systems. However, this technique requires spectroscopic monitoring of the binary system over the entire orbital period: from the Doppler shift of the lines with respect to the rest value, one can evaluate the radial velocity and in turn the mass of a compact object. In reality, one does not directly measure the mass of a compact object but rather the so-called mass function, which is its minimum mass. If this mass is above ~3 solar masses, we have a firm black hole candidate. This technique is powerful but time-consuming. One has to follow the binary system for an entire orbital period with spectroscopic observations. In this Letter, Cúneo and collaborators extend a relation put forward by J. Casares (in the optical), which links the radial velocity to the width of the emission lines, to near-infrared wavelengths. This relation is much easier to use (albeit somewhat less precise) because it needs only one spectrum to work. This extension to the near-infrared might not at first seem of great consequence. However, the majority of the X-ray binaries in our Galaxy lie in heavily obscured regions, where absorption significantly hampers the possibility of taking optical spectra and, in some cases, even detecting the counterpart. Extending this relation to the near-infrared will allow us to access a much larger number of systems and as such probe the nature of compact objects.