PHASE TRANSITIONS AND He-SYNTHESIS-DRIVEN WINDS IN NEUTRINO COOLED ACCRETION DISKS: PROSPECTS FOR LATE FLARES IN SHORT GAMMA-RAY BURSTS
TL;DR: In this article, the authors consider the long-term evolution of debris following the tidal disruption of compact stars in the context of short gamma ray bursts and find that powerful winds are launched from the surface of the disk, driven by the recombination of free nucleons into α-particles.
Abstract: We consider the long-term evolution of debris following the tidal disruption of compact stars in the context of short gamma ray bursts. The initial encounter impulsively creates a hot, dense, neutrino-cooled disk capable of powering the prompt emission. After a long delay, we find that powerful winds are launched from the surface of the disk, driven by the recombination of free nucleons into α-particles. The associated energy release depletes the mass supply and eventually shuts off activity of the central engine. As a result, the luminosity and mass accretion rate deviate from the earlier self-similar behavior expected for an isolated ring with efficient cooling. This then enables a secondary episode of delayed activity to become prominent as an observable signature, when material in the tidal tails produced by the initial encounter returns to the vicinity of the central object. The timescale of the new accretion event can reach tens of seconds to minutes, depending on the details of the system. The associated energies and timescales are consistent with those occurring in X-ray flares.
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TL;DR: A review of nearly a decade of short gamma-ray bursts and their afterglow and host-galaxy observations is presented in this article, where the authors use this information to shed light on the nature and properties of their progenitors, the energy scale and collimation of the relativistic outflow, and the properties of the circumburst environments.
Abstract: Gamma-ray bursts (GRBs) display a bimodal duration distribution with a separation between the short- and long-duration bursts at about 2 s. The progenitors of long GRBs have been identified as massive stars based on their association with Type Ic core-collapse supernovae (SNe), their exclusive location in star-forming galaxies, and their strong correlation with bright UV regions within their host galaxies. Short GRBs have long been suspected on theoretical grounds to arise from compact object binary mergers (neutron star–neutron star or neutron star–black hole). The discovery of short GRB afterglows in 2005 provided the first insight into their energy scale and environments, as well as established a cosmological origin, a mix of host-galaxy types, and an absence of associated SNe. In this review, I summarize nearly a decade of short GRB afterglow and host-galaxy observations and use this information to shed light on the nature and properties of their progenitors, the energy scale and collimation of the relativistic outflow, and the properties of the circumburst environments. The preponderance of the evidence points to compact object binary progenitors, although some open questions remain. On the basis of this association, observations of short GRBs and their afterglows can shed light on the on- and off-axis electromagnetic counterparts of gravitational wave sources from the Advanced LIGO/Virgo experiments.
1,061 citations
TL;DR: In this paper, the authors present the first calculations of the optical transients from compact object mergers that self-consistently determine the radioactive heating by means of a nuclear reaction network; using this heating rate, they model the light curve with a one-dimensional Monte Carlo radiation transfer calculation.
Abstract: The most promising astrophysical sources of kHz gravitational waves (GWs) are the inspiral and merger of binary neutron star(NS)/black hole systems. Maximizing the scientific return of a GW detection will require identifying a coincident electromagnetic (EM) counterpart. One of the most likely sources of isotropic EM emission from compact object mergers is a supernova-like transient powered by the radioactive decay of heavy elements synthesized in ejecta from the merger. We present the first calculations of the optical transients from compact object mergers that self-consistently determine the radioactive heating by means of a nuclear reaction network; using this heating rate, we model the light curve with a one-dimensional Monte Carlo radiation transfer calculation. For an ejecta mass ~ 10 -2 M ⊙ (10 -3 M ⊙ ) the resulting light-curve peaks on a time-scale ~1 d at a V-band luminosity vL v ~ 3 x 10 41 (10 41 )erg s -1 [M V = -15(-14)]; this corresponds to an effective 'f' parameter ~3 × 10- 6 in the Li-Paczynski toy model. We argue that these results are relatively insensitive to uncertainties in the relevant nuclear physics and to the precise early-time dynamics and ejecta composition. Since NS merger transients peak at a luminosity that is a factor of ~10 3 higher than a typical nova, we propose naming these events 'kilo-novae'. Because of the rapid evolution and low luminosity of NS merger transients, EM counterpart searches triggered by GW detections will require close collaboration between the GW and astronomical communities. NS merger transients may also be detectable following a short-duration gamma-ray burst or 'blindly' with present or upcoming optical transient surveys. Because the emission produced by NS merger ejecta is powered by the formation of rare r-process elements, current optical transient surveys can directly constrain the unknown origin of the heaviest elements in the Universe.
1,021 citations
TL;DR: The ejected mass and a merger rate inferred from GW170817 imply that such mergers are a dominant mode of r-process production in the Universe.
Abstract: Modelling the electromagnetic emission of kilonovae enables the mass, velocity and composition (with some heavy elements) of the ejecta from a neutron-star merger to be derived from the observations. Merging neutron stars are potential sources of gravitational waves and have long been predicted to produce jets of material as part of a low-luminosity transient known as a 'kilonova'. There is growing evidence that neutron-star mergers also give rise to short, hard gamma-ray bursts. A group of papers in this issue report observations of a transient associated with the gravitational-wave event GW170817—a signature of two neutron stars merging and a gamma-ray flash—that was detected in August 2017. The observed gamma-ray, X-ray, optical and infrared radiation signatures support the predictions of an outflow of matter from double neutron-star mergers and present a clear origin for gamma-ray bursts. Previous predictions differ over whether the jet material would combine to form light or heavy elements. These papers now show that the early part of the outflow was associated with lighter elements whereas the later observations can be explained by heavier elements, the origins of which have been uncertain. However, one paper (by Stephen Smartt and colleagues) argues that only light elements are needed for the entire event. Additionally, Eleonora Troja and colleagues report X-ray observations and radio emissions that suggest that the 'kilonova' jet was observed off-axis, which could explain why gamma-ray-burst detections are seen as dim. The cosmic origin of elements heavier than iron has long been uncertain. Theoretical modelling1,2,3,4,5,6,7 shows that the matter that is expelled in the violent merger of two neutron stars can assemble into heavy elements such as gold and platinum in a process known as rapid neutron capture (r-process) nucleosynthesis. The radioactive decay of isotopes of the heavy elements is predicted8,9,10,11,12 to power a distinctive thermal glow (a ‘kilonova’). The discovery of an electromagnetic counterpart to the gravitational-wave source13 GW170817 represents the first opportunity to detect and scrutinize a sample of freshly synthesized r-process elements14,15,16,17,18. Here we report models that predict the electromagnetic emission of kilonovae in detail and enable the mass, velocity and composition of ejecta to be derived from observations. We compare the models to the optical and infrared radiation associated with the GW170817 event to argue that the observed source is a kilonova. We infer the presence of two distinct components of ejecta, one composed primarily of light (atomic mass number less than 140) and one of heavy (atomic mass number greater than 140) r-process elements. The ejected mass and a merger rate inferred from GW170817 imply that such mergers are a dominant mode of r-process production in the Universe.
932 citations
TL;DR: A comprehensive review of major developments in our understanding of gamma-ray bursts, with particular focus on the discoveries made within the last fifteen years when their true nature was uncovered, can be found in this paper.
864 citations
Cites background from "PHASE TRANSITIONS AND He-SYNTHESIS-..."
...[751] suggested that post merger accretion disk may undergo ‘‘phase transition’’ triggered by He-synthesis, which would temporarily launch a powerful wind to shut down accretion....
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TL;DR: A comprehensive review of major developments in the understanding of gamma-ray bursts can be found in this article, with particular focus on the discoveries made within the last fifteen years when their true nature was uncovered.
Abstract: We provide a comprehensive review of major developments in our understanding of gamma-ray bursts, with particular focus on the discoveries made within the last fifteen years when their true nature was uncovered. We describe the observational properties of photons from the radio to multi-GeV bands, both in the prompt emission and the afterglow phases. Mechanisms for the generation of these photons in GRBs are discussed and confronted with observations to shed light on the physical properties of these explosions, their progenitor stars and the surrounding medium. After presenting observational evidence that a powerful, collimated, jet moving at close to the speed of light is produced in these explosions, we describe our current understanding regarding the generation, acceleration, and dissipation of the jet and compare these properties with jets associated with AGNs and pulsars. We discuss mounting observational evidence that long duration GRBs are produced when massive stars die, and that at least some short duration bursts are associated with old, roughly solar mass, compact stars. The question of whether a black-hole or a strongly magnetized, rapidly rotating neutron star is produced in these explosions is also discussed. We provide a brief summary of what we have learned about relativistic collisionless shocks and particle acceleration from GRB afterglow studies, and discuss the current understanding of radiation mechanism during the prompt emission phase. We discuss theoretical predictions of possible high-energy neutrino emission from GRBs and the current observational constraints. Finally, we discuss how these explosions may be used to study cosmology, e.g. star formation, metal enrichment, reionization history, as well as the formation of first stars and galaxies in the universe.
814 citations
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3,832 citations
"PHASE TRANSITIONS AND He-SYNTHESIS-..." refers background in this paper
...In such a scenario, fields anchored in the disk can drive outflows with corresponding luminosities reaching LB ≃ 10 48erg s−1 (Blandford & Payne 1982), which are high enough to account for the observed power outputs....
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TL;DR: In this paper, the theory and application of Smoothed particle hydrodynamics (SPH) since its inception in 1977 are discussed, focusing on the strengths and weaknesses, the analogy with particle dynamics and the numerous areas where SPH has been successfully applied.
Abstract: In this review the theory and application of Smoothed particle hydrodynamics (SPH) since its inception in 1977 are discussed. Emphasis is placed on the strengths and weaknesses, the analogy with particle dynamics and the numerous areas where SPH has been successfully applied.
3,623 citations
"PHASE TRANSITIONS AND He-SYNTHESIS-..." refers methods in this paper
...The evolution of the disk is followed with a two dimensional Lagrangian Smooth Particle Hydrodynamics (SPH) code (Monaghan 1992) in azimuthal symmetry....
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TL;DR: In this paper, a cosmological model for gamma-ray bursts is explored in which the radiation is produced as a broadly beamed pair fireball along the rotation axis of an accreting black hole.
Abstract: A cosmological model for gamma-ray bursts is explored in which the radiation is produced as a broadly beamed pair fireball along the rotation axis of an accreting black hole. The black hole may be a consequence of neutron star merger or neutron star-black hole merger, but for long complex bursts, it is more likely to come from the collapse of a single Wolf-Rayet star endowed with rotation ('failed' Type Ib supernova). The disk is geometrically thick and typically has a mass inside 100 km of several tenths of a solar mass. In the failed supernova case, the disk is fed for a longer period of time by the collapsing star. At its inner edge the disk is thick to its own neutrino emission and evolves on a viscous time scale of several seconds. In a region roughly 30 km across, interior to the accretion disk and along its axis of rotation, a pair fireball is generated by neutrino annihilation and electron-neutrino scattering which deposit approximately 10 exp 50 ergs/s.
2,399 citations
"PHASE TRANSITIONS AND He-SYNTHESIS-..." refers background in this paper
...Previous work on ν–cooled disks has shown the importance of the proper consideration of thermodynamics (Woosley 1993; MacFadyen & Woosley 1999; Popham, Woosley & Fryer 1999; Narayan, Piran & Kumar 2001; Di Matteo, Perna & Narayan 2002; Kohri & Mineshige 2002; Beloborodov 2003; Setiawan, Ruffert &…...
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TL;DR: In this paper, the authors explore the continued evolution of rotating helium stars, Mα 10 M☉, in which iron-core collapse does not produce a successful outgoing shock but instead forms a black hole of 2-3 Mˉ.
Abstract: Using a two-dimensional hydrodynamics code (PROMETHEUS), we explore the continued evolution of rotating helium stars, Mα 10 M☉, in which iron-core collapse does not produce a successful outgoing shock but instead forms a black hole of 2-3 M☉. The model explored in greatest detail is the 14 M☉ helium core of a 35 M☉ main-sequence star. The outcome is sensitive to the angular momentum. For j16 ≡ j/(1016 cm2 s-1) 3, material falls into the black hole almost uninhibited. No outflows are expected. For j16 20, the infalling matter is halted by centrifugal force outside 1000 km where neutrino losses are negligible. The equatorial accretion rate is very low, and explosive oxygen burning may power a weak equatorial explosion. For 3 j16 20, however, a reasonable value for such stars, a compact disk forms at a radius at which the gravitational binding energy can be efficiently radiated as neutrinos or converted to beamed outflow by magnetohydrodynamical (MHD) processes. These are the best candidates for producing gamma-ray bursts (GRBs). Here we study the formation of such a disk, the associated flow patterns, and the accretion rate for disk viscosity parameter α ≈ 0.001 and 0.1. Infall along the rotational axis is initially uninhibited, and an evacuated channel opens during the first few seconds. Meanwhile the black hole is spun up by the accretion (to a ≈ 0.9), and energy is dissipated in the disk by MHD processes and radiated by neutrinos. For the α = 0.1 model, appreciable energetic outflows develop between polar angles of 30° and 45°. These outflows, powered by viscous dissipation in the disk, have an energy of up to a few times 1051 ergs and a mass ~1 M☉ and are rich in 56Ni. They constitute a supernova-like explosion by themselves. Meanwhile accretion through the disk is maintained for approximately 10-20 s but is time variable (±30%) because of hydrodynamical instabilities at the outer edge in a region where nuclei are experiencing photodisintegration. Because the efficiency of neutrino energy deposition is sensitive to the accretion rate, this instability leads to highly variable energy deposition in the polar regions. Some of this variability, which has significant power at 50 ms and overtones, may persist in the time structure of the burst. During the time followed, the average accretion rate for the standard α = 0.1 and j16 = 10 model is 0.07 M☉ s-1. The total energy deposited along the rotational axes by neutrino annihilation is (1-14) × 1051 ergs, depending upon the evolution of the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of energy in the polar regions, at a constant rate of 5 × 1050 ergs s-1 per pole, results in strong relativistic outflow jets beamed to about 1% of the sky. These jets may be additionally modulated by instabilities in the sides of the "nozzle" through which they flow. The jets blow aside the accreting material, remain highly focused, and are capable of penetrating the star in ~10 s. After the jet breaks through the surface of the star, highly relativistic flow can emerge. Because of the sensitivity of the mass ejection and jets to accretion rate, angular momentum, and disk viscosity, and the variation of observational consequences with viewing angle, a large range of outcomes is possible, ranging from bright GRBs like GRB 971214 to faint GRB-supernovae like SN 1998bw. X-ray precursors are also possible as the jet first breaks out of the star. While only a small fraction of supernovae make GRBs, we predict that collapsars will always make supernovae similar to SN 1998bw. However, hard, energetic GRBs shorter than a few seconds will be difficult to produce in this model and may require merging neutron stars and black holes for their explanation.
2,209 citations
"PHASE TRANSITIONS AND He-SYNTHESIS-..." refers background in this paper
...Previous work on ν–cooled disks has shown the importance of the proper consideration of thermodynamics (Woosley 1993; MacFadyen & Woosley 1999; Popham, Woosley & Fryer 1999; Narayan, Piran & Kumar 2001; Di Matteo, Perna & Narayan 2002; Kohri & Mineshige 2002; Beloborodov 2003; Setiawan, Ruffert &…...
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TL;DR: In this article, it was shown that most long-duration soft-spectrum gamma-ray bursts are accompanied by massive stellar explosions (GRB-SNe) and that most of the energy in the explosion is contained in nonrelativistic ejecta (producing the supernova) rather than in the relativistic jets responsible for making the burst and its afterglow.
Abstract: Observations show that at least some gamma-ray bursts (GRBs) happen simultaneously with core-collapse supernovae (SNe), thus linking by a common thread nature's two grandest explosions. We review here the growing evidence for and theoretical implications of this association, and conclude that most long-duration soft-spectrum GRBs are accompanied by massive stellar explosions (GRB-SNe). The kinetic energy and luminosity of well-studied GRB-SNe appear to be greater than those of ordinary SNe, but evidence exists, even in a limited sample, for considerable diversity. The existing sample also suggests that most of the energy in the explosion is contained in nonrelativistic ejecta (producing the supernova) rather than in the relativistic jets responsible for making the burst and its afterglow. Neither all SNe, nor even all SNe of Type Ibc produce GRBs. The degree of differential rotation in the collapsing iron core of massive stars when they die may be what makes the difference.
1,389 citations