Long-term evolution of a collapsar

Core-collapse supernovae (CCSNe) are among the most energetic explosions in the universe, liberating the prodigious amount of ~ 1053 erg, the binding energy of their compact remnants, neutron stars or stellar mass black holes. While 99% of this energy is emitted in neutrinos, 1% goes into the internal and asymptotic kinetic energy of the ejecta, and it is reasonable to assume that a tiny fraction is radiated in gravitational waves (GWs). Ever since the first experimental efforts to detect GWs, CCSNe have been considered prime sources of gravitational waves for interferometric detectors. Besides neutrinos, which have already been observed in the context of stellar core collapse of SN1987A, GWs could provide us access to the electromagnetically hidden compact inner core of some such cataclysmic events, supplying us for example with valuable information about the angular momentum distribution and the baryonic equation of state, both of which are uncertain. Furthermore, they might even help to constrain theoretically predicted SN mechanisms. However, GW astronomy strongly depends on the extensive data processing of the detector output on the basis of reliable GW estimates, which only recently have become feasible with the emerging power of supercomputers. The work presented in this thesis is concerned with numerical CCSN models and their imprints in GWs. I performed an extensive series of more than 30 three-dimensional magnetohydrodynamical (MHD) core-collapse simulations. My models are based on a 15M [...] progenitor stemming from stellar evolution calculations, an effective general relativistic potential and either the Lattimer-Swesty (with three possible compressibilities) or the Shen equation of state (EoS) for hot, dense matter. Furthermore, the neutrino transport is tracked by computationally efficient algorithms for the radiative transfer of massless fermions. I systematically investigated the effects of the microphysical finite-temperature nuclear EoS, the initial rotation rate, both the toroidal and the poloidal magnetic fields, and multidimensional gravitational potentials on the GW signature. Based on the results of these calculations, I obtained the largest – and also one of the most realistic – catalogue of GW signatures from 3D MHD stellar core collapse simulations at present. I stress the importance of including postbounce neutrino physics, since it quantitatively alters the GW signature. Non- and slowly-rotating models show GW emission caused by prompt and protoneutron star (PNS) convection. Moreover, the signal stemming from prompt convection allows for the distinction between the two different nuclear EoS indirectly by different properties of the fluid instabilities. For simulations with moderate or even fast rotation rates, I only find the axisymmetric type I wave signature at core bounce. In line with recent results, I could confirm that the maximum GW amplitude scales roughly linearly with the ratio of rotational to gravitational energy (T/|W|) at core bounce below a threshold value of about 10%. Furthermore, I point out that PNS can become dynamically unstable to rotational instabilities at T/|W| values as low as ~ 2% at core bounce. Apart from these two points, I show that it is generally very difficult to discern the effects of the individual features of the input physics in a GW signal from a rotating CCSN that can be attributed unambiguously to a specific model. Weak magnetic fields do not notably influence the dynamical evolution of the core and thus the GW emission. However, for strong initial poloidal magnetic fields ≥ 1012G, the combined action of flux-freezing and field winding leads to conditions where the ratio of magnetic field pressure to matter pressure reaches about unity which leads to the onset of a jet-like supernova explosion. The collimated bipolar out-stream of matter is then reflected in the emission of a type IV GW signal. In contradiction to axisymmetric simulations, I find evidence that nonaxisymmetric fluid modes can counteract or even suppress jet formation for models with strong initial toroidal magnetic fields. I emphasize the importance of including multidimensional gravitational potentials in rapidly rotating 3D CCSN simulations: taking them into account can alter the resulting GW amplitudes up to a factor of 2 compared to simulations which encounter gravity only by a monopolar approximation. Moreover, I show that the postbounce dynamics occuring in the outer layers (at radii R ≥ 200km) of models run with 3D gravity deviates vastly from the ones run with a 1D or 2D gravitational potential. The latter finding implies that both spherically symmetric and axisymmetric treatments of gravity are too restrictive for a quantitative description of the overall postbounce evolution of rapidly rotating CCSN models. The results of models with continued neutrino emission show that including deleptonization during the postbounce phase is an indispensable issue for the quantitative prediction of GWs from core-collapse supernovae, because it can alter the GW amplitude up to a factor of 10 compared to a pure hydrodynamical treatment. My collapse simulations indicate that corresponding events in our Galaxy would be detectable either by LIGO, if the source is rotating, or at least by the advanced LIGO detector, if it is not or only slowly rotating.

We construct a simple but self-consistent collapsar model for gamma-ray bursts (GRBs) and SNe associated with GRBs (GRB-SNe).Our model includes a black hole, an accretion disk, and the envelope surrounding the central system.The evolutions of the different components are connected by the transfer of the mass and angular momentum.To address properties of the jet and the wind-driven SNe, we consider competition of the ram pressure from the infalling envelope and those from the jet and wind.The expected properties of the GRB jet and the wind-driven SN are investigated as a function of the progenitor mass and angular momentum.We find two conditions which should be satisfied if the wind-driven explosion is to explain the properties of the observed GRB-SNe. (1) The wind should be collimated at its base, and (2) it should not prevent further accretion even after the launch of the SN explosion. Under these conditions, some relations seen in the properties of the GRB-SNe could be reproduced by a sequence of different angular momentum in the progenitors.Only the model with the largest angular momentum could explain the observed (energetic) GRB-SNe, and we expect that the collapsar model can result in a wide variety of observational counterparts mainly depending on the angular momentum of the progenitor star.

The advent of gravitational wave (GW) astronomy has provided a new window through which to view and understand the Universe. To fully exploit the potential of GW astronomy, an understanding of all the potential electromagnetic counterparts to a gravitational wave detected source will help maximise the science returns. Here I present a study of the electromagnetic emission from relativistic jets that accompany the merger of binary neutron stars or black hole-neutron star systems. These counterparts provide a probe for the structure and dynamics of these relativistic outflows. Binary neutron star, or neutron star-black hole, mergers are thought to be the dominant progenitor of short gamma-ray bursts (GRBs). Here we investigate the possibility that there is a hidden population of low-Lorentz factor jets resulting in failed GRBs, on-axis orphan afterglows, and what kind of counterparts can be expected given a merger-jet population dominated by these failed-GRB jets. I find that for GW detected mergers, ∼ 80% of the population of on-axis events may result in a failed GRB afterglow. The afterglow of a failed GRB is characterised by the lack of any prompt emission; where the γ-rays are emitted within an optically thick region of the low-Lorentz factor (Γ) outflow and significant suppression via pair production and a high opacity results in the photons coupled to the pair plasma. This plasma will undergo adiabatic expansion, and the photons will decouple at the photospheric radius. The energy in the prompt photons, for a sufficiently low-Γ outflow, will have been significantly suppressed. GW detected mergers have a Malmquist bias towards on-axis events (i.e. the rotational axis of the system), where the peak of the probability distribution is an inclination ∼ 30⁰. If the jets from these mergers have an intrinsic structure out to wider angles, then the majority of mergers will be accompanied by electromagnetic counterparts from these various jet structures. By making some simple assumptions about the energetic structure of a jet outside of a bright core region, the various temporal features that result from a given jet structure can be predicted. Where the population of merger jets is dominated by a single structure model, I show the expected fractions of optical counterparts brighter than m_AB = 21. On 17 August 2017, the Light Interferometer Gravitational Wave Observatory (LIGO) in collaboration with Virgo detected the merger of a binary neutron star system. Various electromagnetic counterparts were detected: the GRB 170817A by Fermi/GBM and INTEGRAL; an optical, blue to red, macro/kilo-nova from ∼ 1/2 day post merger to ∼ 5 − 10 days; and a brightening radio, and X-ray counterpart from ∼ 10 days. Optical detection of this counterpart at a magnitude ∼ 26 was made at ∼ 100 days post-merger. Analysis of this counterpart is consistent with the afterglow of a Gaussian structured jet viewed at the system inclination, ∼ 18 ± 8⁰. If all short GRB jets have a similar jet structure, then the rates of orphan afterglows in deep drilling blind surveys e.g. the Large Synoptic Survey Telescope (LSST), will be higher than those expected from a homogeneous, or ‘top-hat’ jet, population. The rates for the various jet structures for orphan afterglows from mergers are discussed, showing that for a population of failed GRBs, or an intrinsic Gaussian structure, an excess in the orphan rate may be apparent. Understanding the dynamics and structure for the jets from black-hole systems born at the merger of a compact binary can help give clues as to the nature of jets from black holes on all scales. As an aside, I show empirically that regardless of black hole mass or system phenomenology, the relativistic jets from such systems share a universal scaling for the jet power and emitted γ-ray luminosity. This scaling could be due to the similar efficiencies of various processes, or alternatively, the scaling may be able to give insights into the emission and physical processes that are responsible for high-energy photons from these outflows. GW astronomy offers a probe of the most extreme relativistic outflows in the Universe, GRBs. The predicted electromagnetic counterparts from these outflows, in association with GW detections, provides a way to probe the Lorentz-factor distribution for merger-jets. Additionally, the phenomenological shape of the afterglows, at various inclinations, gives an indication of the intrinsic structure of these jets. An understanding of these dynamical and structural qualities can be used to constrain the parent population, merger rates, and binary evolution models for compact binary systems.

Context. We explore the formation, energetics, and geometry of relativistic jets along with the variability of their central engine. We study both fast and slowly rotating black holes and address our simulations to active galaxy centers as well as gamma ray burst engines. Aims. The structured jets are postulated to account for emission properties of high energy sources across the mass scale, launched from stellar mass black holes in gamma ray bursts (GRBs) and from supermassive black holes in active galactic nuclei (AGNs). Their active cores contain magnetized accretion disks and the rotation of the Kerr black hole provides a mechanism for launching relativistic jets. This process works most effectively if the mode of accretion turns out to be magnetically arrested. In this mode, the modulation of jets launched from the engine is related to internal instabilities in the accretion flow that operate on smallest time and spatial scales. As these scales are related to the light-crossing time and the black hole gravitational radius, the universal model of jet-disk connection is expected to scale with the mass of the black hole. Methods. We investigated the jet-disk connection by means of 3D general relativistic magneto-hydrodynamical simulations of the magnetically arrested disk accretion in Kerr geometry. We also quantified the variability of the disk by means of a Fourier analysis. Results. We found that the system evolution is governed by the physical parameters of the engine, such as the black hole spin and disk size, as well as disk magnetization, and we applied our scenarios to typical types of sources in AGN and GRB classes. We found that the magnetically arrested disk (MAD) scenario is applicable to AGN engines and supports persistent jet emissions. It can also be applied to GRBs, as it gives the variability pattern roughly consistent with observations. However, in some cases, strong magnetic fields may lead to jet quenching, and this effect is found to be important mainly for GRB jets. We speculate that it may be related to the strength of magnetically driven winds from the GRB engines.

We discuss the problem of the formation of a large-scale magnetic field in the accretion disks around black holes, taking into account the non-uniform vertical structure of the disk. The high electrical conductivity of the outer layers of the disk prevents the outward diffusion of the magnetic field. This implies a stationary state with a strong magnetic field in the inner parts of the accretion disk close to the black hole, and zero radial velocity at the surface of the disk. Structure of advective accretion disks is investigated, and conditions for formation of optically thin regions in central parts of the accretion disk are found. The problem of jet collimation by magneto-torsion oscillations is considered.

The collapsar model for gamma-ray bursts requires three essential ingredients: a massive core, removal of the hydrogen envelope, and enough angular momentum in the core. We study current massive star evolution models of solar metallicity to determine which massive star physics is capable of producing these ingredients. In particular, we investigate the role of hydrodynamic and magnetic internal angular momentum transport and binary mass and angular momentum transfer. We follow the evolution of rotating single stars and of binary systems that include rotational processes for both stars. Neglecting magnetic fields, we show that the cores of massive single stars can maintain a high specific angular momentum when evolved with the assumption that mean molecular weight gradients suppress rotational mixing processes. In binary systems that undergo mass transfer during core hydrogen burning the mass receiving star accretes large amounts of high angular momentum material, leading to a spin-up of the core. We find, however, that this merely compensates for the tidal angular momentum loss due to spin-orbit coupling, which leads to synchronous rotation before the mass transfer event. Therefore the resulting cores do not rotate faster than in single stars. We also present models that include magnetic fields generated by differential rotation and we consider the internal angular momentum transport by magnetic torques. We investigate the capability of magnetic torques to efficiently pump angular momentum into the cores of accreting stars. Despite our finding that this mechanism works, the magnetic coupling of core and envelope after the accreting star ends core hydrogen burning leads to slower rotation than in the non-magnetic case.

We explore the effect of the magnetic Prandtl number Pr_M on energy and dissipation in fully-resolved direct numerical simulations of steady-state, mechanically-forced homogeneous magnetohydrodynamic turbulence in the range Pr_M = 1/32 to 32. We compare the spectra and show that if the simulations are not fully resolved, the steepness of the scaling of the kinetic-to-magnetic dissipation ratio with Pr_M is overestimated. We also present results of decaying turbulence with helical and nonhelical magnetic fields, where we find nonhelical reverse spectral transfer for Pr_M < 1 for the first time. The results of this systematic analysis have applications ranging from tokamak reactors to black hole accretion disks.

We present results from axisymmetric, time-dependent magnetohydrodynamic (MHD) simulations of the collapsar model for gamma-ray bursts. Our main conclusion is that, within the collapsar model, MHD effects alone are able to launch, accelerate and sustain a strong polar outflow. We also find that the outflow is Poynting flux-dominated, and note that this provides favorable initial conditions for the subsequent production of a baryon-poor fireball.

In recent work we presented the first results of global general relativistic magnetohydrodynamic (GRMHD) simulations of tilted (or misaligned) accretion disks around rotating black holes. The simulated tilted disks showed dramatic differences from comparable untilted disks, such as asymmetrical accretion onto the hole through opposing 'plunging streams' and global precession of the disk powered by a torque provided by the black hole. However, those simulations used a traditional spherical-polar grid that was purposefully underresolved along the pole, which prevented us from assessing the behavior of any jets that may have been associated with the tilted disks. To address this shortcoming we have added a block-structured 'cubed-sphere' grid option to the Cosmos++ GRMHD code, which will allow us to simultaneously resolve the disk and polar regions. Here we present our implementation of this grid and the results of a small suite of validation tests intended to demonstrate that the new grid performs as expected. The most important test in this work is a comparison of identical tilted disks, one evolved using our spherical-polar grid and the other with the cubed-sphere grid. We also demonstrate an interesting dependence of the early-time evolution of our disks on their orientation with respect to the more » grid alignment. This dependence arises from the differing treatment of current sheets within the disks, especially whether they are aligned with symmetry planes of the grid or not. « less

Despite a rich phenomenology, gamma-ray bursts (GRBs) are divided into two classes based on their duration and spectral hardness--the long-soft and the short-hard bursts. The discovery of afterglow emission from long GRBs was a watershed event, pinpointing their origin to star-forming galaxies, and hence the death of massive stars, and indicating an energy release of about 10(51) erg. While theoretical arguments suggest that short GRBs are produced in the coalescence of binary compact objects (neutron stars or black holes), the progenitors, energetics and environments of these events remain elusive despite recent localizations. Here we report the discovery of the first radio afterglow from the short burst GRB 050724, which unambiguously associates it with an elliptical galaxy at a redshift z = 0.257. We show that the burst is powered by the same relativistic fireball mechanism as long GRBs, with the ejecta possibly collimated in jets, but that the total energy release is 10-1,000 times smaller. More importantly, the nature of the host galaxy demonstrates that short GRBs arise from an old (> 1 Gyr) stellar population, strengthening earlier suggestions and providing support for coalescing compact object binaries as the progenitors.

Electron–positron pairs in physics and astrophysics: From heavy nuclei to black holes

The coalesence of compact binary systems might be the plausible candidate to power short Gamma-Ray Bursts (GRBs), since in this scenario a low mass disk can be formed, and the lifetime of the disk is comparable to the duration of the short GRBs. After merger, a neutrino-dominated accretion flow (NDAF) surrounding a stellar-mass black hole (BH) will be formed and neutrino-antineutrino annihilation above the disk may power short GRBs. The results of the simulation show that the disk mass of the NDAF has a limit after merger events. So the question is: can annihilations of neutrinos from NDAFs owning such disk masses power all the observed short GRBs? Using the current short GRB observational data and fireball model, we estimate the disk mass of the NDAF. The neutrino annihilation luminosity as a function of BH spin and accretion rate is fitted from the global solutions of NDAFs. The results show that the estimation of the disk mass mainly depends on its output energy, jet opening angle, and central BH characteristic of GRBs. And some short GRBs require massive disks, which approach or exceed the limits in simulations. If the precise value of jet opening angle is considered, the real requirements of the disk masses will be much larger than the present results, since we use only the lower limit of them in the calculation. These results reflect that the neutrino annihilation process is deficient to power short GRBs. We suggest that there may exit other magnetohydrodynamic processes or some mechanisms to increase the efficiency of the neutrino emission, and then provide energy to power short GRBs.

We study the hydrodynamical evolution of massive accretion disks around black holes, formed when a neutron star is disrupted by a black hole in a binary system. Initial conditions are taken from 3D calculations of coalescing binaries. Assuming azimuthal symmetry, we follow the time dependence of the disk structure for 0.2 seconds. We use an ideal gas e.o.s., and assume that all the dissipated energy is radiated away. The disks evolve due to viscous stresses, modeled with an alpha law. We study the disk structure, and the strong meridional circulations that are established and persist throughout our calculations. These consist of strong outflows along the equatorial plane that reverse direction close to the surface of the disk and converge on the accretor. In the context of GRBs, we estimate the energy released from the system in neutrinos and through magnetic-dominated mechanisms, and find it can be as high as 10^52 erg and 10^51 erg respectively, during an estimated timescale of 0.1-0.2 seconds. neutrino-anti neutrino annihilation is likely to produce bursts from only an impulsive energy input (the annihilation luminosity scales as t^(-5/2)) and so would be unable to account for a large fraction of bursts with complicated light curves. However a gas mass ~0.1-0.25 Msun survives in the orbiting debris, enabling strong magnetic fields (~10^16 Gauss) to be anchored in the dense matter long enough to power short GRBs. We also investigate the continuous energy injection that arises as the black hole slowly swallows the rest of the disk and discuss its consequences on the GRB afterglow emission.

High-energy astrophysics focuses on understanding the origins of the most energetic radiation and particles observed or theoretically predicted in the universe. This area of research probes the most mysterious and most difficult-to-observe astrophysical phenomena. Understanding these phenomena is worth the effort, however, because they may be key to understanding the origin of the universe and the fundamentals of particle physics. Recent observations of extreme events such as ultrahigh-energy cosmic rays and gamma ray bursts have led to significant progress in determining their sources and modeling the mechanisms that produced them. Relativistic jets accelerated from massive compact objects, such as neutron stars or black holes, have been implicated as the primary culprits for producing high- energy events. Here we present three Reviews of the more rapidly advancing areas of research, ultrahigh-energy cosmic rays, gamma ray bursts, and relativistic jets. The highest energy cosmic rays are thought to be extragalactic. However, their source remains a mystery, as discussed in a Review by Guenter Sigl (p. [73][1]). About 12 events have been detected by large-area, ground-based detectors that sample the secondary particles of the cosmic rays. These observations, combined with theoretical models, suggest that ultrahigh-energy cosmic rays come from the earliest processes in the formation of the universe, nonstandard particle physics, or acceleration of particles by energetic processes such as gamma ray bursts. Gamma ray bursts—which last for only a few seconds and generate about 1053 ergs per second of energy output, greater than any other astrophysical phenomenon—are now being detected at a rate of about one per day. Peter Meszaros (p. [79][2]) reviews our current understanding of these curious events. The Italian-Dutch satellite BeppoSAX in particular has helped to locate gamma ray bursts, and accurate locations make it much easier to observe the afterglows with ground-based telescopes and satellites. These observations have led to a model that favors the collapse of a massive object into a neutron star or black hole, generating jets that create gamma ray bursts. Relativistic jets, collimated flows of plasma that travel at almost the speed of light, may produce gamma ray bursts, ultrahigh-energy cosmic rays, and many other high-energy phenomena. David Meier, Shinji Koide, and Yutaka Uchida (p. [84][3]) review the latest observations of jet structures, mainly from radio and x-ray wavelength observations, and focus on magnetohydrodynamic simulations of relativistic jets that reproduce these structures. A neutron star or black hole, in a binary system or in the act of formation, will accrete mass and angular momentum. Differential rotation of this mass and momentum may twist the magnetic field lines of the system, producing bipolar jets, and simulations can provide the appropriate conditions to accelerate particles to produce ultra high-energy cosmic rays or create gamma ray bursts in the jet stream. Relativistic jets also make up the centerpiece of the first of three News stories. Mark Sincell (p. [66][4]) describes how the jets of recently discovered objects known as microquasars may shed light on the mechanisms behind quasars—galaxy-spanning structures powered by supermassive black holes. Next, Govert Schilling (p. [68][5]) tells of another attempt to link two similar-looking phenomena: faint high- energy blips known as soft gamma repeaters and anomalous x-ray pulsars. Finally, Richard Stone (p. [70][6]) travels to Tatarstan to report on a project intended to rival much larger Western ground- and space-based attempts to detect gravitational waves from binary pulsars Thus, the combination of numerical simulations, observations, and theory is improving our understanding of the origins of high-energy astrophysical phenomena. The future looks bright for resolving some of these great mysteries in the early 21st century. [1]: /lookup/doi/10.1126/science.291.5501.73 [2]: /lookup/doi/10.1126/science.291.5501.79 [3]: /lookup/doi/10.1126/science.291.5501.84 [4]: /lookup/doi/10.1126/science.10.1126/SCIENCE.291.5501.66 [5]: /lookup/doi/10.1126/science.10.1126/SCIENCE.291.5501.68 [6]: /lookup/doi/10.1126/science.10.1126/SCIENCE.291.5501.70

We study a cosmological scenario for gamma ray bursts (GRBs) where relativistic flows interact with dense radiation fields. It is shown that this scenario is plausible in very dense stellar regions which are known to exist in collapsed cores of globular clusters or dense nuclei of galaxies. It yields a correct quantitative description of the temporal behavior of GRBs. Several other properties of GRBs are easily explained.