---
title: 'GW190521: Evidence for an IMBH Merger'
url: https://www.emergentmind.com/topics/gw190521
type: topic
---

# GW190521: Evidence for an IMBH Merger

GW190521 is the gravitational-wave transient detected on **2019-05-21 at 03:02:29 UTC** by the **Advanced LIGO Hanford (H1), Advanced LIGO Livingston (L1), and Advanced Virgo (V1)** network during the first half of the O3 observing run. Under the standard quasi-circular binary-black-hole interpretation developed in the discovery analyses, it is a very short, very massive merger with source-frame component masses of about \(85^{+21}_{-14}\,M_\odot\) and \(66^{+17}_{-18}\,M_\odot\), producing a remnant of \(142^{+28}_{-16}\,M_\odot\). Because a remnant near \(142\,M_\odot\) lies in the conventional intermediate-mass-black-hole range \(10^2\!-\!10^5\,M_\odot\), GW190521 was presented as the first strong gravitational-wave evidence for an intermediate-mass black hole. The event is also unusual because the observed signal lasted only about \(0.1\) s in band and contained only about four cycles in the \(30\)–\(80\) Hz range, so much of the inference is driven by merger and ringdown rather than a long inspiral [2009.01075] [2009.11336] [2009.01190].

## 1. Observation and waveform morphology

GW190521 was publicly released in low latency within about 6 minutes as **S190521g**. Its morphology immediately set it apart from earlier LIGO/Virgo binary-black-hole events: the signal appeared as a short transient with fewer than 4 cycles in the \(30\)–\(80\) Hz band, peaking near \(58\)–\(60\) Hz, and showing an almost symmetric morphology rather than a textbook chirp [2009.11336] [2009.01075].

That morphology follows from the system’s large total mass. For very massive binaries, the merger occurs at low frequency, below or near the most contaminated low-frequency part of the detector band. The literature summarized here repeatedly emphasizes that intermediate-mass-black-hole binaries are expected to merge below \(100\) Hz, while detector sensitivity is limited below \(\sim 20\) Hz, so the instruments primarily observe the late inspiral, merger, and ringdown. A plausible implication is that GW190521 sits close to the regime in which generic burst morphology and compact-binary morphology become harder to separate cleanly.

The event’s mass scale was already suggested by its low-frequency content. In the coherent WaveBurst reconstruction, the peak signal frequency was about \(58\) Hz; under the rough equal-mass estimate quoted in that analysis, a binary merging at that frequency should have a detector-frame total mass exceeding \(300\,M_\odot\) [2009.11336]. This is consistent with the later source-frame inference of a total mass around \(150\,M_\odot\) once cosmological redshift is taken into account through the standard relation \(m_{\rm det}=(1+z)m_{\rm src}\) [2009.01075].

## 2. Detection pipelines and significance

GW190521 was established as a confident detection by several search pipelines with different assumptions. In low latency, **PyCBC Live** found the trigger with network SNR **14.5** and false-alarm rate **1 in 8 yr**, while **coherent WaveBurst (cWB)**, configured for intermediate-mass-black-hole searches, found network SNR **15.0** and FAR **< 1 in 28 yr**. Other low-latency matched-filter pipelines, **SPIIR** and **GstLAL**, also identified consistent candidates [2009.01075].

The decisive significance came from offline analyses. The offline **cWB** search, using improved calibration and data-quality information, found GW190521 with network SNR **14.7** and estimated the background by time-shifting the two LIGO detectors against each other. The effective background corresponded to **9800 yr** of observation and contained only two louder events, both consistent with accidental coincidence of glitches, yielding a **false-alarm rate of 1 in 4900 yr**. This is the key detection-significance result in both the Letter and the cWB-focused follow-up [2009.01075] [2009.11336].

The cWB analysis is methodologically important because it is a **coherent excess-power search** for short-duration transients and does **not** assume a compact-binary template bank. It operates on whitened data in a wavelet time-frequency representation, clusters excess-power pixels, and reconstructs coherent transients across the detector network with a constrained maximum-likelihood method. The event passed signal-independent coherence and morphology tests: cWB rejects events with \(c_{\rm c}<0.7\) or \(\chi^2>2.5\), rejects blip-like events with \(Q<0.1\), and GW190521 had \(Q=0.55\) [2009.11336].

This minimal-assumption detection mattered because the signal sat in a difficult regime for inspiral-dominated matched filtering. The cWB paper explicitly argues that the method does not depend on detailed waveform features such as **higher-order modes, high mass ratios, misaligned spins, eccentricity, or deviations from general relativity**, and is therefore especially useful for very massive binaries whose observable signal is short [2009.11336].

## 3. Baseline source inference under the quasi-circular binary-black-hole interpretation

Under the working assumption that GW190521 was a **quasi-circular binary black hole merger**, Bayesian parameter estimation was performed on **8 s of data** using **NRSur7dq4**, **SEOBNRv4PHM**, and **IMRPhenomPv3HM**. The Letter quotes the **NRSur7dq4** results because it was the only model calibrated to precessing numerical simulations, although some posterior support extended outside its formal calibration region [2009.01075].

The standard source-property summary is:

| Quantity | Inference |
|---|---|
| Component masses | \(85^{+21}_{-14}\,M_\odot\) and \(66^{+17}_{-18}\,M_\odot\) |
| Total source-frame mass | \(150^{+29}_{-17}\,M_\odot\) |
| Remnant | \(142^{+28}_{-16}\,M_\odot,\ \chi_f=0.72^{+0.09}_{-0.12}\) |
| Distance and redshift | \(5.3^{+2.4}_{-2.6}\ {\rm Gpc},\ 0.82^{+0.28}_{-0.34}\) |
| Merger rate of similar systems | \(0.13^{+0.30}_{-0.11}\ {\rm Gpc}^{-3}\,{\rm yr}^{-1}\) |

These values were accompanied by a source-frame mass ratio \(q = 0.79^{+0.19}_{-0.29}\), chirp mass \(\mathcal{M} = 64^{+13}_{-8}\,M_\odot\), and radiated energy \(E_{\rm rad} = 7.6^{+2.2}_{-2.1}\,M_\odot c^2\) in the broader properties paper [2009.01075] [2009.01190].

Spin information was informative but not decisive. The quoted effective aligned and precession parameters were \(\chi_{\rm eff} = 0.08^{+0.27}_{-0.36}\) and \(\chi_p = 0.68^{+0.25}_{-0.37}\), with \(\log_{10}{\cal B}_{\rm precessing/nonprecessing}=1.06^{+0.06}_{-0.06}\) and \(\log_{10}{\cal B}_{\rm spin/nonspin}=0.92^{+0.06}_{-0.06}\), which the discovery Letter explicitly describes as only mild preferences [2009.01075]. The properties paper similarly reports mild evidence for spin-induced orbital precession and notes that higher modes are slightly disfavored by \(\log_{10}{\cal B}_{\rm higher\ modes/no\ higher\ modes}=-0.38\), although higher-mode-capable models still improve distance and inclination constraints [2009.01190].

The astrophysical significance of the baseline inference is tied to pair instability. The Letter quantified \(P(m_1<65\,M_\odot)=0.32\%\), implying a **99.68%** probability that the primary lies above \(65\,M_\odot\), and the companion properties paper states that the probability that at least one component lies in the \(65\!-\!120\,M_\odot\) gap is **99.0%** with NRSur7dq4 [2009.01075] [2009.01190]. This is why GW190521 was presented not only as an IMBH-forming merger, but also as a challenge to ordinary isolated stellar-collapse channels.

## 4. Minimal-assumption reconstruction, ringdown, and waveform consistency

A central result of the cWB follow-up is that the **template-free reconstruction is consistent with a binary-black-hole interpretation**. Rather than comparing the observed cWB waveform directly to clean theoretical templates, the authors injected posterior samples from **LALInference** into nearby off-source data, passed them through the same cWB pipeline, and constructed confidence intervals that include posterior variability, detector noise, and cWB reconstruction error. Against a maximum-likelihood waveform, the network overlaps were **0.89** for **NRSur7dq4** and **0.85** for **SEOBNRv4**, with corresponding \(p\)-values **7.9%** and **1.0%**; the measured interpretation was that NRSur7dq4 is consistent with the cWB reconstruction, while SEOBNRv4 is less consistent but not ruled out [2009.11336].

The same paper also showed that template-free sky localization remained compatible with template-based localization for this unusual signal. In low latency, cWB estimated a **90% sky area of 1474 deg\(^2\)**, broadly consistent with the **1163 deg\(^2\)** **Bayestar** localization. In follow-up with calibrated data, the cWB and Bayestar areas became **1653 deg\(^2\)** and **765 deg\(^2\)**, respectively [2009.11336].

Ringdown-only analyses provided an additional consistency check. Modeling the late-time signal as a single damped sinusoid starting **12.7 ms** after the peak of the complex strain, the discovery Letter found \[ f = 66^{+13}_{-13}\ {\rm Hz}, \qquad \tau = 19^{+9}_{-9}\ {\rm ms}, \] with \(\log_{10}({\cal B}_{s/n}) = 25.45 \pm 0.02\). Imposing Kerr perturbation-theory relations for the \((l,m,n)=(2,2,0)\) mode gave \((1+z)M_f = 252^{+64}_{-64}\,M_\odot\) and \(\chi_f = 0.65^{+0.22}_{-0.48}\), consistent with the full inspiral-merger-ringdown inference [2009.01075].

Later ringdown work argued that the apparent tension between earlier ringdown analyses and full IMR analyses was largely a mode-content problem. In that reinterpretation, ringdown models including both the fundamental \(220\) and \(210\) quasinormal modes, and possibly a subdominant \(320\) mode, become fully consistent with **NRSur7dq4** over a range of start times. That study proposed a precessional interpretation in which misalignment between the orbital angular momentum and the remnant spin can rotate inspiral-merger perturbations into strong \(l\neq m\) ringdown content, but it explicitly did **not** rule out eccentricity or other explanations [2307.11975].

A complementary time-domain study of the quasi-circular interpretation reached a related conclusion by dissecting the observed cycles. It found that precession inference hinges on a quiet portion of the pre-merger data that is suppressed relative to the merger-ringdown, and that neither pre-merger nor post-merger data alone are the sole driver; rather, their combination is. In that picture, precession emerges as a mechanism to accommodate the lack of a stronger pre-merger signal in light of the observed post-merger [2310.01544].

## 5. Formation channels and astrophysical scenarios

The literature on GW190521 does not converge on a single formation channel. Instead, it uses the event as a stress test for several mechanisms capable of producing very massive black holes, black holes in or near the pair-instability gap, or both.

Hierarchical merger scenarios are among the most direct responses to the mass-gap problem. A backward-inference study that conditioned on the hierarchical-merger hypothesis found that the **heaviest parental black hole** of GW190521 would have mass \(62^{+21}_{-19}\,M_\odot\), with **78% probability** of lying in the \(50\text{--}120\,M_\odot\) gap and **43% probability** of lying above \(65\,M_\odot\). It also found that the heaviest great-grandparent would be roughly \(\sim 30\,M_\odot\), suggesting a possible ancestral chain from ordinary stellar-origin black holes to the observed system [2201.09943].

Dense stellar environments provide several concrete realizations of that idea. In **young massive star clusters**, \(2\times 10^5\) binary-single scattering experiments with post-Newtonian dynamics found that about **0.17%** of all simulated BHB mergers match simultaneously the component masses, \(\chi_{\rm eff}\), \(\chi_p\), remnant mass, and remnant spin of GW190521 within the \(90\%\) credible intervals under a fiducial low-spin assumption for first-generation black holes. Those matches consisted of **7 first-generation exchanged binaries** and **5 second-generation BHBs**, with an estimated merger-rate density \(\mathcal{R}_{\rm GW190521}\sim 0.03\,{\rm Gpc}^{-3}\,{\rm yr}^{-1}\) [2105.12757]. In **globular-cluster** \(N\)-body work, one object underwent a chain of **seven binary mergers within 6 Gyr**, reaching a final mass of \(97.8\,M_\odot\), and the authors estimated that IMBH formation through hierarchical mergers occurs with probability \(0.01 \lesssim p \lesssim 0.1\) over the lifetime of a median-mass globular cluster in the no-recoil limit [2010.06161].

Other studies asked whether direct stellar-origin formation could still work. One low- and zero-metallicity stellar-evolution analysis concluded that models with \(Z=0\) to \(0.0004\) have three properties favoring higher black-hole masses—lower post-main-sequence mass loss, more compact stars that disfavor binary stripping, and possible H–He shell interactions that reduce the CO core mass—and argued that direct-collapse/fallback black holes up to \(70\)–\(75\,M_\odot\) are supported, with uncertainties potentially pushing the limit to \(\sim 85\,M_\odot\) [2009.06585]. A different isolated-binary study focused on the **IMRI-like** interpretation rather than the near-equal-mass interpretation and found that GW190521-like systems with \(m_1\in[150,180]\,M_\odot\) and \(m_2\in[10,20]\,M_\odot\) can form in metal-poor environments with \(Z\le 0.0016\), with predicted merger-rate density \(\sim 4\times 10^{-5}-5\times 10^{-2}\,{\rm Gpc}^{-3}\,{\rm yr}^{-1}\) within \(z=1.1\) [2306.08441].

More exotic channels were also explored. In the **primordial black hole** scenario, GW190521 was found to be difficult to explain without PBH accretion, because the required abundance would conflict with current bounds; with efficient pre-reionization accretion, the event becomes viable and GW190521-like mergers can occur at roughly the observed rate [2009.01728]. Another study proposed that the components could be the **central black holes of two ultra-dwarf galaxies** of stellar mass \(\sim 10^5-10^6\,M_\odot\), arguing that the ultra-dwarf galaxy major-merger rate at \(1\lesssim z \lesssim 2\) can match the GW190521-like rate after a likely delay of \(\lesssim 4\) Gyr, though with major uncertainties in black-hole occupation fraction and low-mass-galaxy dynamics [2009.10688].

## 6. Competing waveform interpretations, multimessenger claims, and continuing controversy

Because the observed signal is so short, GW190521 has also become a case study in posterior multimodality and waveform dependence. A high-mass-ratio reinterpretation argued that if one uses priors uniform in mass ratio and source-frame total mass, the posterior shifts toward an **IMRI-like** solution with \[ m_1^{\rm src} = 168^{+15}_{-61}\,M_\odot,\qquad m_2^{\rm src} = 16^{+33}_{-3}\,M_\odot, \] while a prior uniform in source-frame component masses yields a bimodal posterior with a low-mass-ratio mode and a high-mass-ratio mode, the latter centered near \(m_1^{\rm src}=166^{+16}_{-35}\,M_\odot\), \(m_2^{\rm src}=16^{+14}_{-3}\,M_\odot\). That analysis argued that the maximum-likelihood point lies in the high-\(q\) mode and that earlier sampling and prior choices may have suppressed it [2010.12558].

A later detailed reanalysis with fourth-generation phenomenological waveform models confirmed that the posterior is genuinely multimodal, but substantially reduced the support for the most extreme unequal-mass solution. In that study, the robust structure was a near-equal-mass mode plus a secondary mode near \(Q\sim 5\), while the strong \(Q\sim 10\) interpretation was not retained as a significant mode. The same paper argued that support for both component masses being outside the pair-instability gap is drastically reduced relative to the earlier high-\(q\) claim [2105.06360].

Dynamical alternatives beyond quasicircular inspiral remain active. An eccentric analysis using **SEOBNRE** found that the data prefer a signal with \(e_{10}\ge 0.1\) over a non-precessing, quasi-circular signal, with \(\ln{\cal B}=5.0\), but also found that a moderately eccentric nonspinning GW190521-like binary can be mistaken for a quasi-circular precessing binary, and vice versa; the authors therefore concluded that one cannot confidently determine whether the source was precessing or eccentric, although either interpretation supports a dynamical origin [2009.04771]. A separate hyperbolic-capture study argued that a nonspinning dynamical-capture waveform is favored over a quasi-circular precessing-spin model by Bayes factors larger than **4300 to 1**, with preferred source-frame masses \(81^{+62}_{-25}\,M_\odot\) and \(52^{+32}_{-32}\,M_\odot\), though that claim depends on a restricted waveform family and uncertain astrophysical prior odds [2106.05575].

The proposed electromagnetic counterpart **ZTF19abanrhr** in **AGN J124942.3+344929** is similarly unsettled. One quasi-circular reanalysis using **NRSur7dq4** and a prior uniform in \(Q\in[1,4]\) inferred \(m_1=106^{+36}_{-28}\,M_\odot\), \(m_2=61^{+21}_{-19}\,M_\odot\), \(M_f = 162^{+29}_{-28}\,M_\odot\), and argued for a common-source odds ratio \({\cal O}_{C/R}=72{:}1\), which would imply a bright-siren estimate \(H_0 = 72.1^{+10.6}_{-6.4}\ {\rm km\,s^{-1}\,Mpc^{-1}}\) when combined with GW170817 [2112.12481]. A later GWTC-2.1-based analysis argued for even stronger association, with \(\log \mathcal{B}^A_C = 8.6 \pm 0.1\), corresponding to odds of about \(400{:}1\) under prior odds \(1/13\), and obtained \(H_0 = 102^{+27}_{-25}\ {\rm km\ s^{-1}\ Mpc^{-1}}\) from GW190521 alone or \(79.2^{+17.6}_{-9.6}\ {\rm km\ s^{-1}\ Mpc^{-1}}\) when using GW170817 as prior information [2310.16025]. By contrast, the phenomenological reanalysis reported only **mild** coincidence odds, \(\mathcal{O}_{\rm C/R}\sim 6\)–\(12\), not enough for a confident association [2105.06360]. The contrast across these studies directly reflects the event’s dependence on waveform choice, prior choice, and posterior release.

The enduring legacy of GW190521 is therefore not only its status as the heaviest binary-black-hole merger detected to date and the first strong gravitational-wave evidence for an intermediate-mass black hole, but also its role as a boundary case for gravitational-wave inference. Across matched filtering, weakly modeled reconstruction, ringdown spectroscopy, hierarchical-growth studies, eccentric and hyperbolic alternatives, AGN-counterpart analyses, and multiband forecasts, the event repeatedly appears as a system in which the shortness of the observed waveform makes modeling assumptions especially consequential [2009.01075] [2009.11336] [2101.06402].

Source: https://www.emergentmind.com/topics/gw190521