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Register a new userDetection of LIGO-Virgo binary black holes in the pair-instability mass gap
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By probing the population of binary black hole (BBH) mergers detected by LIGO-Virgo, we can infer properties about the underlying black hole formation channels. A mechanism known as pair-instability (PI) supernova is expected to prevent the formation of black holes from stellar collapse with mass greater than $sim 40-65,M_odot$ and less than $sim 120,M_odot$. Any BBH merger detected by LIGO-Virgo with a component black hole in this gap, known as the PI mass gap, likely originated from an alternative formation channel. Here, we firmly establish GW190521 as an outlier to the stellar-mass BBH population if the PI mass gap begins at or below $65, M_{odot}$. In addition, for a PI lower boundary of $40-50, M_{odot}$, we find it unlikely that the remaining distribution of detected BBH events, excluding GW190521, is consistent with the stellar-mass population.
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We investigate the potential of ground-based gravitational-wave detectors to probe the mass function of intermediate-mass black holes (IMBHs) wherein we also include BHs in the upper mass gap $sim 60-130~M_odot$. Using the noise spectral density of the upcoming LIGO and Virgo fourth observing (O4) run, we perform Bayesian analysis on quasi-circular non-precessing, spinning IMBH binaries (IMBHBs) with total masses $50mbox{--} 500 M_odot$, mass ratios 1.25, 4, and 10, and (dimensionless) spins up to 0.95, and estimate the precision with which the source-frame parameters can be measured. We find that, at $2sigma$, the source-frame mass of the heavier component of the IMBHBs can be constrained with an uncertainty of $sim 10-40%$ at a signal to noise ratio of $20$. Focusing on the stellar-mass gap, we first evolve stars with massive helium cores using the open-source MESA software instrument to establish the upper and lower edges of the mass gap. We determine that the lower edge of the mass gap is $simeq$ 59$^{+34}_{-13}$ $M_{odot}$, while the upper edge is $simeq$ 139$^{+30}_{-14}$ $M_{odot}$, where the error bars indicate the mass range that follows from the $pm 3sigma$ uncertainty in the ${}^{12}text{C}(alpha, gamma) {}^{16} text{O}$ nuclear rate. We then study IMBHBs with components lying in the mass gap and show that the O4 run will be able to robustly identify most such systems. In this context, we also re-analyze the GW190521 event and show that the 90$%$ confidence interval of the primary-mass measurement lies inside the mass gap. Finally, we show that the precision achieved with the O4 run (and future O5 run) could be crucial for understanding the mass function, the formation mechanism, and evolution history of IMBHs.
Stellar evolution theory predicts a gap in the black hole birth function caused by the pair instability. Presupernova stars that have a core mass below some limiting value, Mlo, after all pulsational activity is finished, collapse to black holes, whereas more massive ones, up to some limiting value, Mhi, explode, promptly and completely, as pair-instability supernovae. Previous work has suggested Mlo is approximately 50 solar masses and Mhi is approximately 130 solar masses. These calculations have been challenged by recent LIGO observations that show many black holes merging with individual masses, Mlo is least some 65 solar masses. Here we explore four factors affecting the theoretical estimates for the boundaries of this mass gap: nuclear reaction rates, evolution in detached binaries, rotation, and hyper-Eddington accretion after black hole birth. Current uncertainties in reaction rates by themselves allow Mlo to rise to 64 solar masses and Mhi as large as 161 solar masses. Rapid rotation could further increase Mlo to about 70 solar masses, depending on the treatment of magnetic torques. Evolution in detached binaries and super-Eddington accretion can, with great uncertainty, increase Mlo still further. Dimensionless Kerr parameters close to unity are allowed for the more massive black holes produced in close binaries, though they are generally smaller.
The primary and secondary masses of the binary black holes (BBH) reported by LIGO/Virgo are correlated with a narrow dispersion that appears to increase in proportion to mass. The mean binary mass ratio $1.45pm0.07$ we show is consistent with pairs drawn randomly from the mass distribution of black holes in our Galaxy. However, BBH masses are concentrated around $simeq 30M_odot$, whereas black holes in our Galaxy peak at $simeq 10M_odot$. This mass difference can be reconciled by gravitational lensing magnification which allows distant events to be detected with typically $zsimeq 2$, so the waveform is reduced in frequency by $1+z$, and hence the measured chirp masses appear 3 times larger than their intrinsic values. This redshift enhancement also accounts for the dispersion of primary and secondary masses, both of which should increase as $1+z$, thereby appearing to scale with mass, in agreement with the data. Thus the BBH component masses provide independent support for lensing, implying most high chirp mass events have intrinsic masses like the stellar mass black holes in our Galaxy, coalescing at $z>1$, with only two low mass BBH detections, of $simeq 10M_odot$ as expected for unlensed events in the local Universe, $zsimeq 0.1$. This lensing solution requires a rapidly declining BBH event rate below $z<1$, which together with the observed absence of BBH spin suggests most events originate within young globular clusters at $z>1$, via efficient binary capture of stellar mass black holes with randomly oriented spins.
Gravitational waves enable tests of general relativity in the highly dynamical and strong-field regime. Using events detected by LIGO-Virgo up to 1 October 2019, we evaluate the consistency of the data with predictions from the theory. We first establish that residuals from the best-fit waveform are consistent with detector noise, and that the low- and high-frequency parts of the signals are in agreement. We then consider parametrized modifications to the waveform by varying post-Newtonian and phenomenological coefficients, improving past constraints by factors of ${sim}2$; we also find consistency with Kerr black holes when we specifically target signatures of the spin-induced quadrupole moment. Looking for gravitational-wave dispersion, we tighten constraints on Lorentz-violating coefficients by a factor of ${sim}2.6$ and bound the mass of the graviton to $m_g leq 1.76 times 10^{-23} mathrm{eV}/c^2$ with 90% credibility. We also analyze the properties of the merger remnants by measuring ringdown frequencies and damping times, constraining fractional deviations away from the Kerr frequency to $delta hat{f}_{220} = 0.03^{+0.38}_{-0.35}$ for the fundamental quadrupolar mode, and $delta hat{f}_{221} = 0.04^{+0.27}_{-0.32}$ for the first overtone; additionally, we find no evidence for postmerger echoes. Finally, we determine that our data are consistent with tensorial polarizations through a template-independent method. When possible, we assess the validity of general relativity based on collections of events analyzed jointly. We find no evidence for new physics beyond general relativity, for black hole mimickers, or for any unaccounted systematics.
Binary black holes on quasicircular orbits with spins aligned with their orbital angular momentum have been testbeds for analytic and numerical relativity for decades, not least because symmetry ensures that such configurations are equilibrium solutions to the spin-precession equations. In this work, we show that these solutions can be unstable when the spin of the higher-mass black hole is aligned with the orbital angular momentum and the spin of the lower-mass black hole is anti-aligned. Spins in these configurations are unstable to precession to large misalignment when the binary separation $r$ is between the values $r_{rm udpm}= (sqrt{chi_1} pm sqrt{q chi_2})^4 (1-q)^{-2} M$, where $M$ is the total mass, $q equiv m_2/m_1$ is the mass ratio, and $chi_1$ ($chi_2$) is the dimensionless spin of the more (less) massive black hole. This instability exists for a wide range of spin magnitudes and mass ratios and can occur in the strong-field regime near merger. We describe the origin and nature of the instability using recently developed analytical techniques to characterize fully generic spin precession. This instability provides a channel to circumvent astrophysical spin alignment at large binary separations, allowing significant spin precession prior to merger affecting both gravitational-wave and electromagnetic signatures of stellar-mass and supermassive binary black holes.
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