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Register a new userMinding the gap: GW190521 as a straddling binary
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Models for black hole (BH) formation from stellar evolution robustly predict the existence of a pair-instability supernova (PISN) mass gap in the range $sim50$ to $sim120$ solar masses. This theoretical prediction is supported by the binary black holes (BBHs) of LIGO/Virgos first two observing runs, whose component masses are well-fit by a power law with a maximum mass cutoff at $m_mathrm{max}=40.8^{+11.8}_{-4.4},M_odot$. Meanwhile, the BBH event GW190521 has a reported primary mass of $m_1=85^{+21}_{-14},M_odot$, firmly above the inferred $m_mathrm{max}$, and secondary mass $m_2=66^{+17}_{-18},M_odot$. Rather than concluding that both components of GW190521 belong to a new population of mass-gap BHs, we explore the conservative scenario in which GW190521s secondary mass belongs to the previously-observed population of BHs. We replace the default priors on $m_1$ and $m_2$, which assume that BH detector-frame masses are uniformly distributed, with this population-informed prior on $m_2$, finding $m_2<48,M_odot$ at 90% credibility. Moreover, because the total mass of the system is better constrained than the individual masses, the population prior on $m_2$ automatically increases the inferred $m_1$ to sit emph{above} the gap (39% for $m_1 > 120,M_odot$, or 25% probability for $m_1>130,M_odot$). As long as the prior odds for a double-mass-gap BBH are smaller than $sim 1:15$, it is more likely that GW190521 straddles the pair-instability gap. We argue that GW190521 may be the first example of a straddling binary black hole, composed of a conventional stellar mass BH and a BH from the ``far side of the PISN mass gap.
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The gravitational-wave signal GW190521 is consistent with a binary black hole merger source at redshift 0.8 with unusually high component masses, $85^{+21}_{-14},M_{odot}$ and $66^{+17}_{-18},M_{odot}$, compared to previously reported events, and shows mild evidence for spin-induced orbital precession. The primary falls in the mass gap predicted by (pulsational) pair-instability supernova theory, in the approximate range $65 - 120,M_{odot}$. The probability that at least one of the black holes in GW190521 is in that range is 99.0%. The final mass of the merger $(142^{+28}_{-16},M_{odot})$ classifies it as an intermediate-mass black hole. Under the assumption of a quasi-circular binary black hole coalescence, we detail the physical properties of GW190521s source binary and its post-merger remnant, including component masses and spin vectors. Three different waveform models, as well as direct comparison to numerical solutions of general relativity, yield consistent estimates of these properties. Tests of strong-field general relativity targeting the merger-ringdown stages of coalescence indicate consistency of the observed signal with theoretical predictions. We estimate the merger rate of similar systems to be $0.13^{+0.30}_{-0.11},{rm Gpc}^{-3},rm{yr}^{-1}$. We discuss the astrophysical implications of GW190521 for stellar collapse, and for the possible formation of black holes in the pair-instability mass gap through various channels: via (multiple) stellar coalescence, or via hierarchical merger of lower-mass black holes in star clusters or in active galactic nuclei. We find it to be unlikely that GW190521 is a strongly lensed signal of a lower-mass black hole binary merger. We also discuss more exotic possible sources for GW190521, including a highly eccentric black hole binary, or a primordial black hole binary.
The LIGO/Virgo Collaboration has recently observed GW190521, the first binary black hole merger with at least the primary component mass in the mass gap predicted by the pair-instability supernova theory. This observation disfavors the standard stellar-origin formation scenario for the heavier black hole, motivating alternative hypotheses. We show that GW190521 cannot be explained within the Primordial Black Hole (PBH) scenario if PBHs do not accrete during their cosmological evolution, since this would require an abundance which is already in tension with current constraints. On the other hand, GW190521 may have a primordial origin if PBHs accrete efficiently before the reionization epoch.
Two of the dominant channels to produce the black-hole binary mergers observed by LIGO and Virgo are believed to be the isolated evolution of stellar binaries in the field and dynamical formation in star clusters. Their relative efficiency can be characterized by a mixing fraction. Pair instabilities prevent stellar collapse from generating black holes more massive than about $45 M_odot$. This mass gap only applies to the field formation scenario, and it can be filled by repeated mergers in clusters. A similar reasoning applies to the binarys effective spin. If black holes are born slowly rotating, the high-spin portion of the parameter space (the spin gap) can only be populated by black hole binaries that were assembled dynamically. Using a semianalytical cluster model, we show that future gravitational-wave events in either the mass gap, the spin gap, or both can be leveraged to infer the mixing fraction between the field and cluster formation channels.
With the black hole mass function (BHMF; assuming an exponential cutoff at a mass of $sim 40,M_odot$) of coalescing binary black hole systems constructed with the events detected in the O1 run of the advanced LIGO/Virgo network, Liang et al.(2017) predicted that the birth of the lightest intermediate mass black holes (LIMBHs; with a final mass of $gtrsim 100,M_odot$) is very likely to be caught by the advanced LIGO/Virgo detectors in their O3 run. The O1 and O2 observation run data, however, strongly favor a cutoff of the BHMF much sharper than the exponential one. In this work we show that a power-law function followed by a sudden drop at $sim 40,M_odot$ by a factor of $sim $a few tens and then a new power-law component extending to $geq 100M_odot$ are consistent with the O1 and O2 observation run data. With this new BHMF, quite a few LIMBH events can be detected in the O3 observation run of advanced LIGO/Virgo. The first LIMBH born in GW190521, an event detected in the early stage of the O3 run of advanced LIGO/Virgo network, provides additional motivation for our hypothesis.
The recent gravitational wave transient GW190521 has been interpreted by the LIGO-Virgo collaboration (LVC) as sourced by a binary black hole (BH) merger. According to the LVC parameter estimation, at least one of these progenitors falls into the so-called pair-instability supernova mass gap. This raises the important question of how and when these progenitors formed. In this paper we use an accretion model with super-Eddington mass accretion rate obtained from General Relativity hydrodynamics simulations to analyse the scenario wherein the GW190521 original progenitors (OPs) formed at lower masses (and spins) and grew to their estimated LVC parameters by relativistic accretion. We consider that the environment wherein the binary is immersed has density gradients as well as a dependence on the Mach number of the gas. Taking the LVC parameter estimation at $z=0.82$ as the endpoint of the accretion evolution, we estimate the initial masses and spins of the OPs at three different red-shifts $z=100, 50$, and $20$. We found three distinct possible types of OPs: $(i)$ $10^{-4} M_{odot} - 3 M_{odot}$ almost non-rotating (with Kerr spin parameter $a_{star}< 10^{-2}$) primordial BHs; $(ii)$ $3 M_{odot} - 40M_{odot}$ slowly rotating ($ 10^{-2} < a_{star} < 0.5$) stellar mass BHs; $(iii)$ $40M_{odot} - 70M_{odot}$ BHs with a moderate spin parameter $a_{star}sim 0.5$, which could originate from the collapse of high mass Pop III stars. The mass spread is due to varying the density gradient and the relativistic Mach number of the cosmic plasma; the variation of the masses due to the origin at different red-shifts, on the other hand, is negligible, $sim 2%$ ...
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