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Register a new userDynamical Formation Scenarios for GW190521 and Prospects for Decihertz Gravitational-Wave Astronomy with GW190521-Like Binaries
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The gravitational-wave (GW) detection of GW190521 has provided new insights on the mass distribution of black holes and new constraints for astrophysical formation channels. With independent claims of GW190521 having significant pre-merger eccentricity, we investigate what this implies for GW190521-like binaries that form dynamically. The Laser Interferometer Space Antenna (LISA) will also be sensitive to GW190521-like binaries if they are circular from an isolated formation channel. We show, however, that GW190521-like binaries that form dynamically may skip the LISA band entirely. To this end, we simulate GW190521 analogues that dynamically form via post-Newtonian binary-single scattering. From these scattering experiments, we find that GW190521-like binaries may enter the LIGO-Virgo band with significant eccentricity as suggested by recent studies, though well below an eccentricity of $e_{rm 10Hz} lesssim 0.7$. Eccentric GW190521-like binaries further motivate the astrophysical science case for a decihertz GW observatory, such as the kilometer-scale version of the Midband Atomic Gravitational-wave Interferometric Sensor (MAGIS). Pre-merger observations of GW190521-like binaries with such a decihertz GW detector would be able to constrain the eccentricity of GW190521-like binaries to greater precision than with just LIGO-Virgo alone. These eccentricity constraints would also provide additional insights into the possible environments that GW190521-like binaries form in.
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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, $sim 2%$ ...
GW190521 is the compact binary with the largest masses observed to date, with at least one in the pair-instability gap. This event has also been claimed to be associated with an optical flare observed by the Zwicky Transient Facility in an Active Galactic Nucleus (AGN), possibly due to the post-merger motion of the merger remnant in the AGN gaseous disk. We show that the Laser Interferometer Space Antenna (LISA) will detect up to ten of such gas-rich black hole binaries months to years before their detection by LIGO/Virgo-like interferometers, localizing them in the sky within $approx1$ deg$^2$. LISA will also measure directly deviations from purely vacuum and stationary waveforms, arising from gas accretion, dynamical friction, and orbital motion around the AGNs massive black hole (acceleration, strong lensing, and Doppler modulation). LISA will therefore be crucial to alert and point electromagnetic telescopes ahead of time on this novel class of gas-rich sources, to gain direct insight on their physics, and to disentangle environmental effects from corrections to General Relativity that may also appear in the waveforms at low frequencies.
It is generally believed that Type Ia supernovae are thermonuclear explosions of carbon-oxygen white dwarfs (WDs). However, there is currently no consensus regarding the events leading to the explosion. A binary WD (WD-WD) merger is a possible progenitor of Type Ia supernovae. Space-based gravitational wave (GW) detectors with great sensitivity in the decihertz range like DECIGO can observe WD-WD mergers directly. Therefore, access to the deci-Hz band of GWs would enable multi-messenger observations of Type Ia supernovae to constrain their progenitor and explosion mechanism. In this paper, we consider the event rate of WD-WD mergers and minimum detection range to observe one WD-WD merger per year, using nearby galaxy catalog and the relation between the Ia supernova and the host galaxy. Furthermore, we calculate the DECIGOs ability to localize WD-WD mergers and to determine the masses of binary mergers. We estimate that if the deci-Hz GW observatory can detect the GW whose amplitude is $hsim10^{-20}[rm Hz^{-1/2}]$ at 0.1 Hz, 1000 times higher than the detection limit of DECIGO. In fact, DECIGO is expected to detect WD-WD ($1M_{odot}-1M_{odot}$) mergers within $z=0.115$,corresponding to the detection rate of $sim20000,rm yr^{-1}$, and identify the host galaxy of WD-WD mergers for $sim8000$ WD-WDs only by the GW detection.
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.
Soon after the observation of the first black hole binary (BHB) by advanced LIGO (aLIGO), GW150914, it was realised that such a massive system would have been observable in the milli-Hz (mHz) band few years prior to coalescence. Operating in the frequency range 0.1-100 mHz, the Laser Interferometer Space Antenna (LISA) can potentially detect up to thousands inspiralling BHBs, based on the coalescence rates inferred from the aLIGO first observing run (O1). The vast majority of them (those emitting at $f<10$ mHz) will experience only a minor frequency drift during LISA lifetime, resulting in signals similar to those emitted by galactic white dwarf binaries. At $f>10$ mHz however, several of them will sweep through the LISA band, eventually producing loud coalescences in the audio-band probed by aLIGO. This contribution reviews the scientific potential of these new class of LISA sources which, in the past few months, has been investigated in several contexts, including multi-messenger and multi-band gravitational wave astronomy, BHB astrophysics, tests of alternative theories of gravity and cosmography.
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