Weighing the Darkness. II. Astrometric Measurement of Partial Orbits with Gaia

Abstract

Over the course of several years, stars trace helical trajectories as they traverse across the sky due to the combined effects of proper motion and parallax. It is well known that the gravitational pull of an unseen companion can cause deviations to these tracks. Several studies have pointed out that the astrometric mission Gaia will be able to identify a slew of new exoplanets, stellar binaries, and compact object companions with orbital periods as short as tens of days to as long as Gaia's lifetime. Here, we use mock astrometric observations to demonstrate that Gaia can identify and characterize black hole companions to luminous stars with orbital periods longer than Gaia's lifetime. Such astrometric binaries have orbital periods too long to exhibit complete orbits, and instead are identified through curvature in their characteristic helical paths. By simultaneously measuring the radius of this curvature and the orbital velocity, constraints can be placed on the underlying orbit. We quantify the precision with which Gaia can measure orbital accelerations and apply that to model predictions for the population of black holes orbiting stars in the stellar neighborhood. Although orbital degeneracies imply that many of the accelerations induced by hidden black holes could also be explained by faint low-mass stars, we discuss how the nature of certain putative black hole companions can be confirmed with high confidence using Gaia data alone.

Detecting dark compact objects in Gaia DR4: A data analysis pipeline for transient astrometric lensing searches

“…Gaia does not flag sources in binary orbits with a dark (or faint) companion, such as a neutron star, a brown dwarf, an exoplanet, or an astrophysical BH. It is known that the binary orbits of these sources induce a measurable correction to the free trajectory of the source [19,20,[114][115][116]. In particular, it is estimated that Gaia is capable of observing about 75 sources with BH companions [20].…”

“…It is known that the binary orbits of these sources induce a measurable correction to the free trajectory of the source [19,20,[114][115][116]. In particular, it is estimated that Gaia is capable of observing about 75 sources with BH companions [20]. Gaia's binary flagging system is also conservative since flawed binary flagging can hurt Gaia's science output -it is projected that some fully luminous binaries will bypass Gaia's flagging procedure.…”

“…This dark world may also be populated by small DM structures, such as (ultra-compact) minihalos [1][2][3][4][5] or more exotic objects such as primordial black holes (PBHs) [6], boson stars [7][8][9][10][11], and other composite DM objects [10,[12][13][14][15]. These clumps and structures may be invisible to us, but their presence can occasionally be revealed indirectly through gravitational waves [16,17], direct gravitational effects on visible stars [18][19][20][21], pulsar timing arrays [22][23][24][25][26][27][28][29][30], and gravitational lensing of light [31][32][33][34][35] and of gravitational waves [36][37][38][39].…”