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Register a new userThe formation of heavy, radio-quiet neutron star binaries and the origin of GW190425
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Added by
Alejandro Vigna-Gomez
Publication date
2021
fields
Physics
and research's language is
English
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The detection of the unusually heavy binary neutron star merger GW190425 marked a stark contrast to the mass distribution from known Galactic millisecond pulsars in neutron star binaries and gravitational-wave source GW170817. We suggest here a formation channel for heavy binary neutron stars in which massive helium stars, assembled after common envelope, remain compact and avoid mass transfer onto the neutron star companion and thus evade pulsar recycling. In particular we present three-dimensional simulations of the supernova explosion of the massive stripped helium star and follow the mass fallback evolution and the subsequent accretion onto the neutron star companion. We find that fallback leads to significant mass growth in the newly formed neutron star and that the companion does not accrete sufficient mass to become a millisecond pulsar. This can explain the formation of heavy binary neutron star systems such as GW190425, as well as predict the assembly of neutron star - light black hole systems. Moreover, this hints to the existence of a sizable population of radio-quiet double compact objects in our Galaxy. Finally, this formation avenue is consistent with the observed mass-eccentricity correlation of binary neutron stars in the Milky Way.
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The accretion/ejection coupling in accreting black hole binaries has been described by empirical relations between the X-ray/radio and X-ray/optical-infrared luminosities. These correlations were initially supposed to be universal. However, recently many sources have been found to produce jets that, given certain accretion-powered luminosities, are fainter than expected from the correlations. This shows that black holes with similar accretion flows can produce a broad range of outflows in power. Here we discuss whether typical parameters of the binary system, as well as the properties of the outburst, produce any effect on the energy output in the jet. We also define a jet-toy model in which the bulk Lorentz factor becomes larger than ~1 above ~0.1% of the Eddington luminosity. We finally compare the radio quiet black holes with the neutron stars.
The accretion/ejection coupling in accreting black hole binaries has been described by empirical relations between the X-ray/radio and X-ray/optical-infrared luminosities. These correlations were initially thought to be universal. However, recently many sources have been found to produce jets that, given certain accretion-powered luminosities, are fainter than expected from the earlier correlations. This shows that black holes with similar accretion flows can produce a broad range of outflows in power, suggesting that some other parameters might be tuning the accretion/ejection coupling. Recent work has already shown that this jet power does not correlate with the reported black hole spin measurements. Here we discuss whether fixed parameters of the binary system, as well as the properties of the outburst, produce any effect on the energy output in the jet. No obvious dependence is found. We also show that there is no systematic variation of the slope of the radio:X-ray correlation with normalization. We define a jet-toy model in which the bulk Lorentz factor becomes larger than ~1 above ~0.1% of the Eddington luminosity. With this model, if we assume random inclination angles which result in highly variable boosting at large Eddington ratios, we are able to reproduce qualitatively the scatter of the X-ray/radio correlation and the radio quiet population. However the model seems to be at odds with some other observed properties of the systems. We also compare the radio quiet black holes with the neutron stars. We show that if a mass correction from the fundamental plane is applied, the possibility that they are statistically indistinguishable in the X-ray:radio plane can not be completely ruled out. This result suggests that some of the outliers could actually be neutron stars, or that the disc-jet coupling in the radio quiet black holes is more similar to the one in neutron stars.
Double neutron star (DNS) systems represent extreme physical objects and the endpoint of an exotic journey of stellar evolution and binary interactions. Large numbers of DNS systems and their mergers are anticipated to be discovered using the Square-Kilometre-Array searching for radio pulsars and high-frequency gravitational wave detectors (LIGO/VIRGO), respectively. Here we discuss all key properties of DNS systems, as well as selection effects, and combine the latest observational data with new theoretical progress on various physical processes with the aim of advancing our knowledge on their formation. We examine key interactions of their progenitor systems and evaluate their accretion history during the high-mass X-ray binary stage, the common envelope phase and the subsequent Case BB mass transfer, and argue that the first-formed NSs have accreted at most $sim 0.02;M_{odot}$. We investigate DNS masses, spins and velocities, and in particular correlations between spin period, orbital period and eccentricity. Numerous Monte Carlo simulations of the second supernova (SN) events are performed to extrapolate pre-SN stellar properties and probe the explosions. All known close-orbit DNS systems are consistent with ultra-stripped exploding stars. Although their resulting NS kicks are often small, we demonstrate a large spread in kick magnitudes which may, in general, depend on the past interaction history of the exploding star and thus correlate with the NS mass. We analyze and discuss NS kick directions based on our SN simulations. Finally, we discuss the terminal evolution of close-orbit DNS systems until they merge and possibly produce a short $gamma$-ray burst.
The first neutron star-neutron star (NS-NS) merger was discovered on August 17, 2017 through gravitational waves (GW170817) and followed with electromagnetic observations. This merger was detected in an old elliptical galaxy with no recent star formation. We perform a suite of numerical calculations to understand the formation mechanism of this merger. We probe three leading formation mechanisms of double compact objects: classical isolated binary star evolution, dynamical evolution in globular clusters and nuclear cluster formation to test whether they are likely to produce NS-NS mergers in old host galaxies. Our simulations with optimistic assumptions show current NS-NS merger rates at the level of 10^-2 yr^-1 from binary stars, 5 x 10^-5 yr^-1 from globular clusters and 10^-5 yr^-1 from nuclear clusters for all local elliptical galaxies (within 100 Mpc^3). These models are thus in tension with the detection of GW170817 with an observed rate 1.5 yr^-1 (per 100 Mpc^3; LIGO/Virgo estimate). Our results imply that either (i) the detection of GW170817 by LIGO/Virgo at their current sensitivity in an elliptical galaxy is a statistical coincidence; or that (ii) physics in at least one of our three models is incomplete in the context of the evolution of stars that can form NS-NS mergers; or that (iii) another very efficient (unknown) formation channel with a long delay time between star formation and merger is at play.
Using a Milky Way double neutron star (DNS) merger rate of 210 Myr$^{-1}$, as derived by the Laser Interferometer Gravitational-Wave Observatory (LIGO), we demonstrate that the Laser Interferometer Space Antenna (LISA) will detect on average 240 (330) DNSs within the Milky Way for a 4-year (8-year) mission with a signal-to-noise ratio greater than 7. Even adopting a more pessimistic rate of 42 Myr$^{-1}$, as derived by the population of Galactic DNSs, we find a significant detection of 46 (65) Milky Way DNSs. These DNSs can be leveraged to constrain formation scenarios. In particular, traditional NS-discovery methods using radio telescopes are unable to detect DNSs with $P_{rm orb}$ $lesssim$1 hour (merger times $lesssim$10 Myr). If a fast-merging channel exists that forms DNSs at these short orbital periods, LISA affords, perhaps, the only opportunity to observationally characterize these systems; we show that toy models for possible formation scenarios leave unique imprints on DNS orbital eccentricities, which may be measured by LISA for values as small as $sim$10$^{-2}$.
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