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Register a new userAn alternative interpretation of GW190412 as a binary black hole merger with a rapidly spinning secondary
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The LIGO-Virgo collaboration recently reported the properties of GW190412, a binary black hole merger with unequal component masses (mass ratio $0.25^{+0.06}_{-0.04}$ when using the EOBNR PHM approximant) and a non-vanishing effective spin aligned with the orbital angular momentum. They used uninformative priors to infer that the more massive black hole had a dimensionless spin magnitude between 0.17 and 0.59 at 90% confidence. We argue that, within the context of isolated binary evolution, it is more natural to assume a priori that the first-born, more massive black hole has a negligible spin, while the spin of the less massive black hole is preferentially aligned with the orbital angular momentum if it is spun up by tides. Under this astrophysically motivated prior, we conclude that the lower mass black hole had a dimensionless spin component between 0.64 and 0.99 along the orbital angular momentum.
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We explore the possibility that GW190412, a binary black hole merger with a non-equal-mass ratio and significantly spinning primary, was formed through repeated black hole mergers in a dense super star cluster. Using a combination of semi-analytic prescriptions for the remnant spin and recoil kick of black hole mergers, we show that the mass ratio and spin of GW190412 are consistent with a binary black hole whose primary component has undergone two successive mergers from a population of $sim 10M_{odot}$ black holes in a high-metallicity environment. We then explore the production of GW190412-like analogs in the CMC Cluster Catalog, a grid of 148 $N$-body star cluster models, as well as a new model, behemoth, with nearly $10^7$ particles and initial conditions taken from a cosmological MHD simulation of galaxy formation. We show that the production of binaries with GW190412-like masses and spins is dominated by massive super star clusters with high metallicities and large central escape speeds. While many are observed in the local universe, our results suggest that a careful treatment of these massive clusters, many of which may have been disrupted before the present day, is necessary to characterize the production of unique gravitational-wave events produced through dynamics.
We report the observation of gravitational waves from a binary-black-hole coalescence during the first two weeks of LIGOs and Virgos third observing run. The signal was recorded on April 12, 2019 at 05:30:44 UTC with a network signal-to-noise ratio of 19. The binary is different from observations during the first two observing runs most notably due to its asymmetric masses: a ~30 solar mass black hole merged with a ~8 solar mass black hole companion. The more massive black hole rotated with a dimensionless spin magnitude between 0.22 and 0.60 (90% probability). Asymmetric systems are predicted to emit gravitational waves with stronger contributions from higher multipoles, and indeed we find strong evidence for gravitational radiation beyond the leading quadrupolar order in the observed signal. A suite of tests performed on GW190412 indicates consistency with Einsteins general theory of relativity. While the mass ratio of this system differs from all previous detections, we show that it is consistent with the population model of stellar binary black holes inferred from the first two observing runs.
Long gamma-ray bursts are associated with the core-collapse of massive, rapidly spinning stars. However, the believed efficient angular momentum transport in stellar interiors leads to predominantly slowly-spinning stellar cores. Here, we report on binary stellar evolution and population synthesis calculations, showing that tidal interactions in close binaries not only can explain the observed sub-population of spinning, merging binary black holes, but also lead to long gamma-ray bursts at the time of black-hole formation, with rates matching the empirical ones. We find that $approx$10% of the GWTC-2 reported binary black holes had a long gamma-ray burst associated with their formation, with GW190517 and GW190719 having a probability of $approx$85% and $approx$60%, respectively, being among them.
In this paper we continue the first ever study of magnetized mini-disks coupled to circumbinary accretion in a supermassive binary black hole (SMBBH) approaching merger reported in Bowen et al. 2018. We extend this simulation from 3 to 12 binary orbital periods. We find that relativistic SMBBH accretion acts as a resonant cavity, where quasi-periodic oscillations tied to the the frequency at which the black holes orbital phase matches a non-linear $m=1$ density feature, or ``lump, in the circumbinary accretion disk permeate the system. The rate of mass accretion onto each of the mini-disks around the black holes is modulated at the beat frequency between the binary frequency and the lumps mean orbital frequency, i.e., $Omega_{rm beat} = Omega_{rm bin} - bar{Omega}_{rm lump}$, while the total mass accretion rate of this equal-mass binary is modulated at two different frequencies, $gtrsim bar{Omega}_{rm lump}$ and $approx 2 Omega_{rm beat}$. The instantaneous rotation rate of the lump itself is also modulated at two frequencies close to the modulation frequencies of the total accretion rate, $bar{Omega}_{rm lump}$ and $2 Omega_{rm beat}$. Because of the compact nature of the mini-disks in SMBBHs approaching merger, the inflow times within the mini-disks are comparable to the period on which their mass-supply varies, so that their masses---and the accretion rates they supply to their black holes---are strongly modulated at the same frequency. In essence, the azimuthal symmetry of the circumbinary disk is broken by the dynamics of orbits near a binary, and this $m=1$ asymmetry then drives quasi-periodic variation throughout the system, including both accretion and disk-feeding. In SMBBHs approaching merger, such time variability could introduce distinctive, increasingly rapid, fluctuations in their electromagnetic emission.
The Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) has detected direct signals of gravitational waves (GWs) from GW150914. The event was a merger of binary black holes whose masses are $36^{+5}_{-4}M_{odot}$ and $29^{+4}_{-4}M_{odot}$. Such binary systems are expected to be directly evolved from stellar binary systems or formed by dynamical interactions of black holes in dense stellar environments. Here we derive the binary black hole merger rate based on the nearby ultra-luminous X-ray source (ULX) luminosity function (LF) under the assumption that binary black holes evolve through X-ray emitting phases. We obtain the binary black hole merger rate as $5.8 ({t}_{rm ULX}/{0.1 rm Myr})^{-1} lambda^{-0.6} exp{(-0.30lambda)} {rm Gpc^{-3} yr^{-1}}$, where $t_{rm ULX}$ is the typical duration of the ULX phase and $lambda$ is the Eddington ratio in luminosity. This is coincident with the event rate inferred from the detection of GW150914 as well as the predictions based on binary population synthesis models. Although we are currently unable to constrain the Eddington ratio of ULXs in luminosity due to the uncertainties of our models and measured binary black hole merger event rates, further X-ray and GW data will allow us to narrow down the range of the Eddington ratios of ULXs. We also find the cumulative merger rate for the mass range of $5M_odotle M_{rm BH}le100M_odot$ inferred from the ULX LF is consistent with that estimated by the aLIGO collaboration considering various astrophysical conditions such as the mass function of black holes.
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