Radial Velocity Orbital Solutions for Candidate Black Hole and Neutron Star Binary Systems in the Gaia Data Release 3 Catalog

Abstract

We present spectroscopic followup observations of binary systems from the Gaia Data Release 3 (DR3) binary catalog that were selected to have large enough mass functions for their companions to be black holes or neutron stars. The selection includes 20 stars that are astrometric and/or spectroscopic binaries, as well as 11 stars with large accelerations both in the plane of the sky and along the line of sight but no DR3 orbital solution. We provide classifications for this entire sample, including radial velocity orbital solutions for 11 binaries. Apart from the previously published binaries Gaia BH1, Gaia BH2, and Gaia NS1, we show that the Gaia orbits are incorrect for all of the stars with candidate dark companions above 2 Msun. We suggest more conservative cuts on the significance and goodness of fit parameters that may be useful for identifying reliable orbital solutions in the tail of the binary star distribution. Although we find no new confirmed black hole or neutron star companions, one accelerating system has a minimum companion mass of 1.16 +/- 0.01 Msun that is likely to be a neutron star or an ultramassive white dwarf. The acceleration catalogs may therefore provide a largely unexplored source of additional wide binaries containing compact objects.

Figures (2)

• Figure 1: (upper left) Secondary mass as a function of primary mass for Gaia DR3 binaries. The confirmed black hole and neutron star binaries Gaia BH1, Gaia BH2, and Gaia NS1 are displayed as yellow circles. The candidates selected from the DR3 orbital catalog are plotted as red circles, those from the AstroSpectroSB1 catalog are plotted as magenta circles, and the spectroscopic binary candidates are plotted as dark blue circles. The stars in the DR3 catalog of binary masses are shown as small gray dots for comparison. (upper right) Orbital solution significance as a function of goodness of fit ($F_{2}$). The symbols are the same as in the upper left panel. Many, although not all, of the candidates investigated in this paper have lower significance orbits and/or worse goodness of fit values than the previously confirmed compact object binaries. (lower left) Velocity semi-amplitudes as a function of binary period. The colored symbols are the same as in the upper left panel, with stars in the DR3 catalog of binary masses shown as small gray dots for comparison. Note that to compute the semi-amplitudes, we used the m2_lower field in the catalog rather than m2 so that each star would have a value and so that spectroscopic and astrometric binaries can be compared on an equal footing. (lower right) Gaia color-magnitude diagram of the candidate sample. The colored symbols are the same as in the upper left panel. A randomly selected 20% of the stars within 150 pc of the sun are shown as small gray dots for comparison. For all stars, we dereddened the photometry using the 3D dust map from wang25. The tendency for the SB1 binaries, and to a lesser extent the accelerating systems, to have hotter and more massive primaries is visually apparent. Almost all of the sample is located close to the main sequence, such that massive secondaries should be easily detectable if they are luminous.
• Figure 2: Properties of the acceleration sample. (left) RV amplitude as a function of the first derivative of the RV. The full sample of stars for which both astrometric and radial velocity accelerations are detected is shown in gray. The 11 stars we selected for spectroscopic followup are plotted as blue circles. (right) RV amplitude against goodness of fit. The symbols are the same as in the left panel. There are a handful of additional stars with RV amplitudes above 50 km s$^{-1}$, but they have worse goodness of fit values.