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Register a new userWolf-Rayet stars, black holes and the first detected gravitational wave source
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The recently discovered burst of gravitational waves GW150914 provides a good new chance to verify the current view on the evolution of close binary stars. Modern population synthesis codes help to study this evolution from two main sequence stars up to the formation of two final remnant degenerate dwarfs, neutron stars or black holes [Massevich 1988]. To study the evolution of the GW150914 predecessor we use the Scenario Machine code presented by [Lipunov 1996]. The scenario modelling conducted in this study allowed to describe the evolution of systems for which the final stage is a massive BH+BH merger. We find that the initial mass of the primary component can be $100div 140 M_{odot}$ and the initial separation of the components can be $50div 350 R_{odot}$. Our calculations show the plausibility of modern evolutionary scenarios for binary stars and the population synthesis modelling based on it.
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Population synthesis studies of binary black-hole mergers often lack robust black-hole spin estimates as they cannot accurately follow tidal spin-up during the late black-hole-Wolf-Rayet evolutionary phase. We provide an analytical approximation of the dimensionless second-born black-hole spin given the binary orbital period and Wolf-Rayet stellar mass at helium depletion or carbon depletion. These approximations are obtained from fitting a sample of around $10^5$ detailed MESA simulations that follow the evolution and spin up of close black-hole--Wolf-Rayet systems with metallicities in the range $[10^{-4},1.5Z_odot]$. Following the potential spin up of the Wolf-Rayet progenitor, the second-born black-hole spin is calculated using up-to-date core collapse prescriptions that account for any potential disk formation in the collapsing Wolf-Rayet star. The fits for second-born black hole spin provided in this work can be readily applied to any astrophysical modeling that relies on rapid population synthesis, and will be useful for the interpretation of gravitational-wave sources using such models.
Black holes are unique among astrophysical sources: they are the simplest macroscopic objects in the Universe, and they are extraordinary in terms of their ability to convert energy into electromagnetic and gravitational radiation. Our capacity to probe their nature is limited by the sensitivity of our detectors. The LIGO/Virgo interferometers are the gravitational-wave equivalent of Galileos telescope. The first few detections represent the beginning of a long journey of exploration. At the current pace of technological progress, it is reasonable to expect that the gravitational-wave detectors available in the 2035-2050s will be formidable tools to explore these fascinating objects in the cosmos, and space-based detectors with peak sensitivities in the mHz band represent one class of such tools. These detectors have a staggering discovery potential, and they will address fundamental open questions in physics and astronomy. Are astrophysical black holes adequately described by general relativity? Do we have empirical evidence for event horizons? Can black holes provide a glimpse into quantum gravity, or reveal a classical breakdown of Einsteins gravity? How and when did black holes form, and how do they grow? Are there new long-range interactions or fields in our universe, potentially related to dark matter and dark energy or a more fundamental description of gravitation? Precision tests of black hole spacetimes with mHz-band gravitational-wave detectors will probe general relativity and fundamental physics in previously inaccessible regimes, and allow us to address some of these fundamental issues in our current understanding of nature.
An observable stochastic background of gravitational waves is generated whenever primordial black holes are created in the early universe thanks to a small-scale enhancement of the curvature perturbation. We calculate the anisotropies and non-Gaussianity of such stochastic gravitational waves background which receive two contributions, the first at formation time and the second due to propagation effects. The former contribution can be generated if the distribution of the curvature perturbation is characterized by a local and scale-invariant shape of non-Gaussianity. Under such an assumption, we conclude that a sizeable magnitude of anisotropy and non-Gaussianity in the gravitational waves would suggest that primordial black holes may not comply the totality of the dark matter.
Gravitational microlensing is a powerful tool to search for a population of invisible black holes (BHs) in the Milky Way (MW), including isolated BHs and binary BHs at wide orbits that are complementary to gravitational wave observations. By monitoring highly populated regions of source stars like the MW bulge region, one can pursue microlensing events due to these BHs. We find that if BHs have a Salpeter-like mass function extended beyond $30M_odot$ and a similar velocity and spatial structure to stars in the Galactic bulge and disk regions, the BH population is a dominant source of the microlensing events at long timescales of the microlensing light curve $gtrsim 100~$days. This is due to a boosted sensitivity of the microlensing event rate to lens mass, given as $M^2$, for such long-timescale events. A monitoring observation of $2 times 10^{10}$ stars in the bulge region over 10 years with the Rubin Observatory Legacy Survey of Space and Time (LSST) would enable one to find about $6times 10^5$ BH microlensing events. We evaluate the efficiency of potential LSST cadences for characterizing the light curves of BH microlensing and find that nearly all events of long timescales can be detected.
The LIGO-Virgo gravitational-wave (GW) observation unveiled the new population of black holes (BHs) that appears to have an extended mass spectrum up to around $70M_odot$, much heavier than the previously-believed mass range ($sim 8M_odot$). In this paper, we study the capability of a microlensing observation of stars in the Milky Way (MW) bulge region to identify BHs of GW mass scales, taking into account the microlensing parallax characterized by the parameter $pi_{rm E}propto M^{-1/2}$ ($M$ is the mass of a lens), which is a dimension-less quantity defined by the ratio of the astronomical unit to the projected Einstein radius. First, assuming that BHs follow the same spatial and velocity distributions of stars as predicted by the standard MW model, we show that microlensing events with long light curve timescales, $t_{rm E}gtrsim 100~{rm days}$, and small parallax effects, $pi_{rm E}sim 10^{-2}$, are dominated by BH lenses compared to stellar-mass lenses. Second, using a Markov chain Monte Carlo analysis of the simulated light curve, we show that BH lens candidates are securely identified on individual basis, if the parallax effect is detected or well constrained to the precision of a percent level in $pi_{rm E}$. We also discuss that a microlensing event of an intermediate-mass BH of $sim 1000M_odot$, if it occurs, can be identified in a distinguishable way from stellar-mass BHs.
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