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Register a new userColliding neutron stars --- Gravitational waves, neutrino emission, and gamma-ray bursts
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Authors
Maximilian Ruffert
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Three-dimensional hydrodynamical simulations are presented for the direct head-on or off-center collision of two neutron stars, employing a basically Newtonian PPM code but including the emission of gravitational waves and their back-reaction on the hydrodynamical flow. A physical nuclear equation of state is used that allows us to follow the thermodynamical evolution of the stellar matter and to compute the emission of neutrinos. Predicted gravitational wave signals, luminosities and waveforms, are presented. The models are evaluated for their implications for gamma-ray burst scenarios. We find an extremely luminous outburst of neutrinos with a peak luminosity of more than 4E54 erg/s for several milliseconds. This leads to an efficiency of about 1% for the annihilation of neutrinos with antineutrinos, corresponding to an average energy deposition rate of more than 1E52 erg/s and a total energy of about 1E50 erg deposited in electron-positron pairs around the collision site within 10ms. Although these numbers seem very favorable for gamma-ray burst scenarios, the pollution of the $e^pm$ pair-plasma cloud with nearly 0.1$M_{odot}$ of dynamically ejected baryons is 5 orders of magnitude too large. Therefore the formation of a relativistically expanding fireball that leads to a gamma-ray burst powered by neutrino emission from colliding neutron stars is definitely ruled out.
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Three-dimensional hydrodynamical, Newtonian calculations of the coalescence of equal-mass binary neutron stars are performed, including a physical high-density equation of state and a treatment of the neutrino emission of the heated matter. The total neutrino luminosity climbs to a maximum value of 1--$1.5cdot 10^{53}$~erg/s of which 90--95% originate from the toroidal gas cloud surrounding the very dense core formed after the merging. When the neutrino luminosities are highest, $ ubar u$-annihilation deposits about 0.2--0.3% of the emitted neutrino energy in the immediate neighborhood of the merger, and the maximum integral energy deposition rate is 3--$4cdot 10^{50}$~erg/s. Since the $3,M_{odot}$ core of the merged object will most likely collapse into a black hole within milliseconds, the energy that can be pumped into a pair-photon fireball is insufficient by a factor of about 1000 to explain $gamma$-ray bursts at cosmological distances with an energy of the order of $10^{51}/(4pi)$~erg/steradian. Analytical estimates show that the additional energy provided by the annihilation of $ ubar u$ pairs emitted from a possible accretion torus of $sim 0.1,M_{odot}$ around the central black hole is still more than a factor of 10 too small, unless focussing of the fireball into a jet-like expansion plays an important role. About $10^{-4}$--$10^{-3}$~$M_odot$ of material lost during the neutron star merging and swept out from the system in a neutrino-driven wind might be a site for nucleosythesis. Aspects of a possible r-processing in these ejecta are discussed.
The cosmological origin of $gamma$-ray bursts (GRBs) is now commonly accepted and, according to several models for the central engine, GRB sources should also emit at the same time gravitational waves bursts (GWBs). We have performed two correlation searches between the data of the resonant gravitational wave detector AURIGA and GRB arrival times collected in the BATSE 4B catalog. No correlation was found and an upper limit bbox{$h_{text{RMS}} leq 1.5 times 10^{-18}$} on the averaged amplitude of gravitational waves associated with $gamma$-ray bursts has been set for the first time.
LOFAR, the Low Frequency Array, is an innovative new radio telescope currently under construction in the Netherlands. With its continuous monitoring of the radio sky we expect LOFAR will detect many new transient events, including GRB afterglows and pulsating/single-burst neutron stars. We here describe all-sky surveys ranging from a time resolution of microseconds to a cadence span of years.
We investigate prolonged engine activities of short gamma-ray bursts (SGRBs), such as extended and/or plateau emissions, as high-energy gamma-ray counterparts to gravitational waves (GWs). Binary neutron-star mergers lead to relativistic jets and merger ejecta with $r$-process nucleosynthesis, which are observed as SGRBs and kilonovae/macronovae, respectively. Long-term relativistic jets may be launched by the merger remnant as hinted in X-ray light curves of some SGRBs. The prolonged jets may dissipate their kinetic energy within the radius of the cocoon formed by the jet-ejecta interaction. Then the cocoon supplies seed photons to non-thermal electrons accelerated at the dissipation region, causing high-energy gamma-ray production through the inverse Compton scattering process. We numerically calculate high-energy gamma-ray spectra in such a system using a one-zone and steady-state approximation, and show that GeV--TeV gamma-rays are produced with a duration of $10^2-10^5$ seconds. They can be detected by {it Fermi}/LAT or CTA as gamma-ray counterparts to GWs.
We infer the collapse times of long-lived neutron stars into black holes using the X-ray afterglows of 18 short gamma-ray bursts. We then apply hierarchical inference to infer properties of the neutron star equation of state and dominant spin-down mechanism. We measure the maximum non-rotating neutron star mass $M_mathrm{TOV} = 2.31 ^{+0.36}_{-0.21} M_{odot}$ and constrain the fraction of remnants spinning down predominantly through gravitational-wave emission to $eta = 0.69 ^{+0.21}_{-0.39}$ with $68 %$ uncertainties. In principle, this method can determine the difference between hadronic and quark equation of states. In practice, however, the data is not yet informative with indications that these neutron stars do not have hadronic equation of states at the $1sigma$ level. These inferences all depend on the underlying progenitor mass distribution for short gamma-ray bursts produced by binary neutron star mergers. The recently announced gravitational-wave detection of GW190425 suggests this underlying distribution is different from the locally-measured population of double neutron stars. We show that $M_mathrm{TOV}$ and $eta$ constraints depend on the fraction of binary mergers that form through a distribution consistent with the locally-measured population and a distribution that can explain GW190425. The more binaries that form from the latter distribution, the larger $M_mathrm{TOV}$ needs to be to satisfy the X-ray observations. Our measurements above are marginalised over this unknown fraction. If instead, we assume GW190425 is not a binary neutron star merger, i.e the underlying mass distribution of double neutron stars is the same as observed locally, we measure $M_mathrm{TOV} = 2.26 ^{+0.31}_{-0.17} M_{odot}$.
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