130 research outputs found
Constraints on the cosmological coupling of black holes from Gaia
Recent work has suggested that black holes (BHs) could be cosmologically
coupled to the accelerated expansion of the universe, potentially becoming a
candidate for dark energy. This would imply BH mass growth following the
cosmological expansion, with the masses of individual BHs growing as . In this letter, we discuss the binary systems Gaia BH1 and
Gaia BH2, which contain BHs orbited by
stars in widely-separated orbits. The ages of both systems can be constrained
by the properties of the luminous stars. If BH masses are indeed growing as
, the masses of both BHs at formation would have been significantly
smaller than today. We find a 77% probability that the mass of the BH in Gaia
BH2 would have been below at formation. This is below the
classical Tolman-Oppenheimer-Volkov limit, though it is not yet clear if BHs
subject to cosmological coupling should obey this limit. For Gaia BH1, the same
probability is 70%. This analysis is consistent with results from two BHs in
the globular cluster NGC3201, but unlike the NGC3201 BHs, the Gaia BHs have
well-constrained inclinations and thus firm upper mass limits. The discovery of
more BHs in binary systems with Gaia astrometry in the coming years will allow
us to test the cosmological coupling hypothesis decisively.Comment: submitted 8th March 2023; accepted 2nd May 2023; 4 pages, 3 figure
Imprints of white dwarf recoil in the separation distribution of Gaia wide binaries
We construct from Gaia DR2 an extensive and very pure (
contamination) catalog of wide binaries containing main-sequence (MS) and white
dwarf (WD) components within 200 pc of the Sun. The public catalog contains,
after removal of clusters and resolved higher-order multiples, 50,000 MS/MS,
3,000 WD/MS, and nearly 400 WD/WD binaries with projected separations of . Accounting for incompleteness and selection
effects, we model the separation distribution of each class of binaries as a
broken power-law, revealing marked differences between the three populations.
The separation distribution of MS/MS systems is nearly consistent with a single
power-law of slope over at least , with
marginal steepening at AU. In contrast, the separation
distributions of WD/MS and WD/WD binaries show distinct breaks at 3,000
AU and 1,500 AU, respectively: they are flatter than the MS/MS
distribution at small separations and steeper at large separations. Using
binary population synthesis models, we show that these breaks are unlikely to
be caused by external factors but can be explained if the WDs incur a kick of
0.75 km s during their formation, presumably due to asymmetric
mass loss. The data rule out typical kick velocities above 2km s. Our
results imply that most wide binaries with separations exceeding a few thousand
AU become unbound during post-MS evolution.Comment: Accepted to MNRAS. 15 pages, 11 figures, plus appendices. Catalog
available at https://sites.google.com/site/dr2binaries200pc/dat
No X-Rays or Radio from the Nearest Black Holes and Implications for Future Searches
Astrometry from the Gaia mission was recently used to discover the two
nearest known stellar-mass black holes (BHs), Gaia BH1 and Gaia BH2. Both
systems contain stars in wide orbits (1.4 AU,
4.96 AU) around BHs. These objects are among the first
stellar-mass BHs not discovered via X-rays or gravitational waves. The
companion stars -- a solar-type main sequence star in Gaia BH1 and a
low-luminosity red giant in Gaia BH2 -- are well within their Roche lobes.
However, the BHs are still expected to accrete stellar winds, leading to
potentially detectable X-ray or radio emission. Here, we report observations of
both systems with the Chandra X-ray Observatory and radio observations with the
Very Large Array (for Gaia BH1) and MeerKAT (for Gaia BH2). We did not detect
either system, leading to X-ray upper limits of and and radio upper limits of and
. For Gaia BH2, the non-detection implies
that the the accretion rate near the horizon is much lower than the Bondi rate,
consistent with recent models for hot accretion flows. We discuss implications
of these non-detections for broader BH searches, concluding that it is unlikely
that isolated BHs will be detected via ISM accretion in the near future. We
also calculate evolutionary models for the binaries' future evolution using
Modules for Experiments in Stellar Astrophysics (MESA). We find that Gaia BH1
will be X-ray bright for 5--50 Myr when the star is a red giant, including 5
Myr of stable Roche lobe overflow. Since no symbiotic BH X-ray binaries are
known, this implies either that fewer than Gaia BH1-like binaries
exist in the Milky Way, or that they are common but have evaded detection,
perhaps due to very long outburst recurrence timescales.Comment: Submitted to PAS
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