--- title: 'GW190517: Rapid Spin BBH Merger' url: https://www.emergentmind.com/topics/gw190517 type: topic --- # GW190517: Rapid Spin BBH Merger GW190517, also designated GW190517\_055101, is a binary-black-hole merger observed on 2019-05-17 05:51:01 UTC and reported as a high-confidence astrophysical event in the LIGO-Virgo catalogs. In early catalog treatments it appeared as a stellar-mass binary black hole with moderate mass ratio and small positive effective inspiral spin, but later spin-focused reanalyses elevated it to a distinctive status: within the GWTC-3 sample, it is the only event whose individual-component spins are both confidently large, and it therefore anchors inferences of a distinct rapidly spinning subpopulation of merging binary black holes [2010.14527] [2108.01045] [2411.02252]. ## 1. Detection, catalog placement, and data analysis GW190517 was detected by multiple search pipelines with network signal-to-noise ratios of order \(10\)–\(11\). In GWTC-2, GstLAL reported a network matched-filter signal-to-noise ratio of \(10.2\) with false-alarm rate \(0.5\,{\rm yr}^{-1}\), PyCBC recovered the event with network SNR \(\approx 9.8\) and FAR \(\approx 0.8\,{\rm yr}^{-1}\), and cWB found a coincident excess with network SNR \(\approx 11.0\) and FAR \(\approx 0.3\,{\rm yr}^{-1}\). In GWTC-2.1, the event retained high significance, with GPS time \(1241135698.0\) s \((\pm 0.004\,{\rm s})\), network SNR \(11.3\) in MBTA, \(10.8\) in GstLAL, and \(10.4\) in PyCBC, and astrophysical probability \(\gtrsim 0.99\) in all pipelines [2010.14527] [2108.01045]. The event was classified as a binary-black-hole merger. GWTC-2.1 states that binary-black-hole candidates are those for which both component masses exceed \(3\,M_\odot\), and GW190517 satisfies this criterion with negligible probability of belonging to neutron-star-containing classes [2108.01045]. The catalog analyses also document the evolution of the data-treatment pipeline. GWTC-2 used aligned-spin and precessing waveform families, including higher-mode models such as SEOBNRv4PHM and NRSur7dq4, and adopted higher-mode runs as fiducial because the Jensen-Shannon divergence between the \(\ell=2\) and higher-mode results exceeded \(0.007\) bit for GW190517’s mass ratio and \(\chi_{\rm eff}\) [2010.14527]. GWTC-2.1 reprocessed the strain data offline with best-available calibration models, subtracted known instrumental lines, applied the same data-quality flags as GWTC-2, and performed Bayesian parameter estimation with waveform models including spin precession and higher harmonics in the LALInference/LALSuite framework [2108.01045]. ## 2. Source parameters and the transition from a catalog event to a spin outlier Published parameter estimates for GW190517 differ across catalog releases and reanalyses. This is explicit in the literature, where different waveform families, priors, and data products are used. The principal published values are summarized below. | Analysis | Quoted parameters | Distinctive point | | --- | --- | --- | | GWTC-2 fiducial higher-mode analysis | \(m_1=32.6^{+6.1}_{-5.0}\,M_\odot\), \(m_2=22.8^{+5.0}_{-4.2}\,M_\odot\), \(q=0.70^{+0.13}_{-0.12}\), \(\mathcal{M}=26.4^{+3.0}_{-2.6}\,M_\odot\), \(\chi_{\rm eff}=0.12^{+0.10}_{-0.13}\), \(D_L=1.9^{+1.2}_{-0.8}\) Gpc, \(z=0.36^{+0.19}_{-0.14}\) | Higher-mode results adopted as fiducial [2010.14527] | | GWTC-2.1 | \(m_1=35.2^{+6.0}_{-5.5}\,M_\odot\), \(m_2=28.0^{+6.5}_{-7.4}\,M_\odot\), \(M=63.2^{+8.0}_{-9.0}\,M_\odot\), \(q=0.80^{+0.10}_{-0.13}\), \(\chi_{\rm eff}=0.03^{+0.17}_{-0.19}\), \(\chi_p=0.34^{+0.18}_{-0.16}\), \(D_L=375^{+150}_{-150}\) Mpc, \(z=0.08^{+0.03}_{-0.03}\) | Reprocessed strain data and updated PE [2108.01045] | | IMRPhenomXPHM spin reanalysis | \(\chi_1\approx0.83^{+0.14}_{-0.16}\), \(\chi_2\approx0.76^{+0.17}_{-0.19}\), with 90% highest-posterior-density intervals roughly \(\chi_1\in[0.60,0.98]\), \(\chi_2\in[0.58,0.95]\) | Only event with both component spins confidently large [2411.02252] | A central distinction in the published interpretation of GW190517 is between \(\chi_{\rm eff}\) and the individual spin magnitudes. GWTC-2 reported \[ \chi_{\rm eff}=\frac{m_1\vec\chi_1+m_2\vec\chi_2}{m_1+m_2}\cdot \hat L \] and found \(\chi_{\rm eff}=0.12^{+0.10}_{-0.13}\), while GWTC-2.1 quoted \(\chi_{\rm eff}=0.03^{+0.17}_{-0.19}\) and \(\chi_p=0.34^{+0.18}_{-0.16}\) [2010.14527] [2108.01045]. Later work instead emphasized the two-dimensional \((\chi_1,\chi_2)\) plane and found GW190517 far from the bulk of the catalog because both component spins are large and comparatively well measured [2411.02252]. This contrast is important for interpretation. In catalog-level summaries based on total mass, mass ratio, and \(\chi_{\rm eff}\), GW190517 was described as lying within the range of “ordinary” stellar-mass BBHs and as consistent with the broader O3a population [2108.01045]. In analyses that resolve individual spin magnitudes, it becomes a statistical outlier. ## 3. Hierarchical population inference and the rapidly spinning subpopulation A dedicated reanalysis of GWTC-3 modeled the joint spin-magnitude distribution as a two-component mixture of truncated Gaussians on \([0,1]\times[0,1]\), \[ p(\chi_1,\chi_2\mid\Lambda)=\eta\,\mathcal{N}_{[0,1]\times[0,1]}\!\left((\chi_1,\chi_2)\mid \mu^a,\Sigma^a\right) +(1-\eta)\,\mathcal{N}_{[0,1]\times[0,1]}\!\left((\chi_1,\chi_2)\mid \mu^b,\Sigma^b\right), \] where \(a\) denotes the dominant slowly spinning mode and \(b\) the subdominant rapidly spinning mode [2411.02252]. From the full 69-event fit, the inferred mixture fraction is \[ \eta=0.80^{+0.18}_{-0.18}, \] so \(20^{+18}_{-18}\%\) of binaries lie in the rapid-spin mode. The dominant mode has \(\mu^a \simeq (0.20\pm0.05,\,0.00^{+0.03}_{-0.00})\), diagonal dispersions \(\Sigma^a_{ii}\simeq(0.15^2,\,0.10^2)\), and a mild anticorrelation \(\rho^a\approx-0.3^{+0.2}_{-0.4}\). The rapid mode has \(\mu^b \simeq (0.75^{+0.10}_{-0.12},\,0.72^{+0.14}_{-0.16})\), diagonal dispersions \(\Sigma^b_{ii}\simeq(0.20^2,\,0.22^2)\), and \(\rho^b\) consistent with \(0\) [2411.02252]. Within this model, GW190517 is the event that anchors the high-spin component. Averaging over the hyper-posterior yields \(\langle \log_{10} {\rm BF}\rangle \approx 1.0\), corresponding to \(\approx 10{:}1\) odds in favor of the rapid-spin mode, and \[ P({\rm GW190517}\in{\rm rapid}\mid {\rm data})\simeq0.91. \] The same analysis reports support for two distinct subpopulations with greater than \(95\%\) credibility and states explicitly that the hint of a rapidly spinning subpopulation hinges on GW190517 [2411.02252]. The population result also refines the structure of the low-spin mode. In the dominant mode, the \(90\%\) highest-posterior-density interval for \(\rho^a\) is roughly \([-0.6,0.0]\), with Bayes factor against \(\rho^a=0\) of \(\sim2.9\). A single-component fit also hints at weak anticorrelation, with median \(\rho\approx-0.2\) and \( {\rm BF}_{\rho\neq0}\approx2.2\), but cannot capture the separate high-spin peak [2411.02252]. A common misconception is therefore that the result is merely a statement about broad spin variance; the actual claim is more specific, namely a bimodal structure in the \((\chi_1,\chi_2)\) plane. ## 4. Formation-channel interpretations Several formation-channel studies identify GW190517 as a candidate for channels that produce high, approximately aligned spins. In a Bayesian comparison among isolated-binary channels, the chemically homogeneous evolution (CHE) prior is strongly favored over both common-envelope evolution (CEE) and stable mass transfer (SMT). For GW190517\_055101, Qin et al. report \(\ln B_{{\rm CHE,CEE}}=3.4\) and \(\ln B_{{\rm CHE,SMT}}=4.2\). After applying prior-odds corrections based on local-rate ratios, the corresponding logarithmic odds are \(\ln \mathcal{O}_{{\rm CHE,CEE}}\simeq2.7\) and \(\ln \mathcal{O}_{{\rm CHE,SMT}}\simeq2.8\), implying \(p_{\rm CHE}\gtrsim0.9\) [2211.05945]. Under the CHE prior, the inferred source properties shift toward the expected channel morphology: chirp mass remains near \(\mathcal{M}_c\simeq28\,M_\odot\), the mass ratio peaks sharply at \(\sim0.9\), and \(\chi_{1z}\) and \(\chi_{2z}\) both peak near \(\sim0.5\), excluding zero and unity [2211.05945]. This is consistent with the later population-level statement that isolated chemically homogeneous evolution at low metallicity naturally accommodates a system in which both progenitors retain high angular momentum and collapse into rapidly spinning black holes with roughly aligned spins [2411.02252]. The same high-angular-momentum picture appears in long-gamma-ray-burst modeling. Bavera et al. define \[ p_{\rm LGRB}(x)=\iint f_{\rm LGRB}(\chi_{\rm eff},\mathcal{M})\,p(\chi_{\rm eff},\mathcal{M}\mid x)\,d\chi_{\rm eff}\,d\mathcal{M}, \] and find \(p_{\rm LGRB}\approx0.868\) for GW190517, corresponding to an \(\sim85\%\) probability that the binary formed in association with a luminous long gamma-ray burst [2106.15841]. Their broader model links tidal spin-up in close binaries to both spinning BBH mergers and luminous LGRBs. A distinct interpretation emerges in the 166-event three-component spin-mixture analysis of Bartos et al., which fixes component shapes for standard, hierarchical-merger, and accretion channels and infers only their mixing fractions. That study finds strong evidence, \(\ln B=5.7\), for an accretion-origin high-spin subpopulation with \(f_{\rm acc}=0.082\) and \(90\%\) credible interval \([0.014,0.137]\). GW190517 is among the top five accretion candidates and is color-coded at \(p_{\rm acc}\gtrsim0.5\), with \(m_1=39^{+7}_{-6}\,M_\odot\), \(q=0.77^{+0.10}_{-0.11}\), \(a_1=0.87^{+0.12}_{-0.15}\), and \(\chi_{\rm eff}=0.26^{+0.16}_{-0.21}\) [2605.09351]. Across these studies, hierarchical-merger explanations are comparatively disfavored for this event. The spin-population analysis states that GW190517’s pair of large, well-measured \(\chi\)’s is inconsistent with the tail of the slow-spin population and is also disfavored in a hierarchical \(1{\rm g}\)–\(2{\rm g}\) merger channel, which would produce a single large-spin component rather than two [2411.02252]. The accretion-mixture analysis similarly states that the hierarchical-merger prediction of \(a_1\approx0.7\) is decisively disfavored as the location of the high-spin subpopulation, whereas the accretion peak at \(a_1\approx0.9\) is preferred [2605.09351]. This suggests that, within current models, GW190517 is more naturally associated with channels that spin up both components. ## 5. Recoil kicks, remnant retention, and dynamical environments Fragione and Loeb use GW190517 as a case study for recoil kicks and remnant retention. In their summary of the event, the measured parameters are \(m_1\simeq38.4^{+11.5}_{-7.8}\,M_\odot\), \(m_2\simeq24.4^{+7.5}_{-5.0}\,M_\odot\), \(q\approx0.64\), \(\chi_{\rm eff}=0.17^{+0.16}_{-0.14}\), and final spin \(a_{\rm rem}\approx0.68^{+0.73}_{-0.08}\) [2011.08935]. Their recoil model adopts \[ \vec v_{\rm kick}=v_m\,\hat e_{\perp,1} +v_\perp\,[\cos\xi\,\hat e_{\perp,1}+\sin\xi\,\hat e_{\perp,2}] +v_\parallel\,\hat e_\parallel, \] with the coefficients and spin decompositions given explicitly in their implementation of Eqs. (1)–(3). For GW190517 they draw \(10^5\) realizations of \((\chi_1,\chi_2)\) under an isotropic-spin prior with \(|\chi_i|\) uniform in \([0,\chi_{\max}]\), discarding draws whose computed \(\chi_{\rm eff}\) or \(a_{\rm rem}\) lie outside the \(90\%\) LVC credible intervals [2011.08935]. The resulting recoil distribution depends strongly on \(\chi_{\max}\). For \(\chi_{\max}=0.1\), the kick distribution peaks at \(\simeq 80\,{\rm km\,s^{-1}}\) with median \(\simeq 75\,{\rm km\,s^{-1}}\); for \(\chi_{\max}=0.3\), peak \(\simeq 200\,{\rm km\,s^{-1}}\), median \(\simeq 180\,{\rm km\,s^{-1}}\); for \(\chi_{\max}=0.5\), peak \(\simeq 350\,{\rm km\,s^{-1}}\), median \(\simeq 340\,{\rm km\,s^{-1}}\); and for \(\chi_{\max}=0.8\), peak \(\simeq 550\,{\rm km\,s^{-1}}\), median \(\simeq 530\,{\rm km\,s^{-1}}\). Each \(P(v_{\rm kick})\) is well approximated by a log-normal with \(\sigma\simeq0.4\)–\(0.5\) [2011.08935]. The environmental implication is that only clusters with \(v_{\rm esc}\gtrsim100\,{\rm km\,s^{-1}}\) can retain GW190517’s remnant unless progenitor spins were very low, \(\chi_{\max}\lesssim0.1\); if \(\chi_{\max}\gtrsim0.5\), only the deepest potentials with \(v_{\rm esc}\gtrsim200\,{\rm km\,s^{-1}}\) suffice [2011.08935]. The same study argues that matching both \(\chi_{\rm eff}\approx0.17\) and \(a_{\rm rem}\approx0.68\) under an isotropic single-generation prior requires \(\chi_{\max}\gtrsim0.8\), while a second-generation progenitor with \(\chi\approx0.7\) offers an alternative route. This does not overturn the two-high-spin interpretation from later work, but it shows that dynamical scenarios with deep escape potentials remain relevant in recoil-based modeling. ## 6. Constraints on ultralight bosons and dark photons Because GW190517 contains a rapidly spinning black hole in the \(\sim30\)–\(40\,M_\odot\) range, it has been used to test superradiant spin-down by ultralight fields. For scalar bosons, the operative condition is \[ \omega_R < m\,\Omega_H, \] with \(\Omega_H\) the Kerr horizon angular velocity and \(\alpha \equiv G M \mu/\hbar c\) the gravitational fine-structure constant [2011.06010]. Ng et al. use GWTC-2 measurements \(m_1=39.4^{+6.0}_{-5.0}\,M_\odot\) and \(\chi_1=0.68^{+0.15}_{-0.36}\) and argue that, if a scalar boson of mass \(\mu\) in the range \(1.3\times10^{-13}\)–\(2.7\times10^{-13}\,{\rm eV}\) existed with negligible self-interaction and black-hole lifetime \(10\) Myr, the primary would have been spun down to \(\chi\lesssim a_{\rm crit}(\mu,M)\) well before merger. For GW190517, the entire \(95\%\) credible region of \((M_1,\chi_1)\) lies in the forbidden region above \(a_{\rm crit}\) for those \(\mu\), and the posterior probability of a boson in that band drops below \(0.01\%\) [2011.06010]. The abstract of that work states that the statistical evidence is mostly driven by GW190412 and GW190517. An analogous argument has been made for vector superradiance. Using the primary black hole in GW190517, Bhaumik et al. derive an excluded dark-photon mass window \[ 1.7\times10^{-14}\,{\rm eV}