ABSTRACT
Long gamma-ray bursts (LGRBs) are associated to the collapse of a massive star and the formation of a relativistic jet. As the jet propagates through the star, it forms an extended, hot ...cocoon. The dynamical evolution of the jet/cocoon system and its interaction with the environment has been studied extensively both analytically and numerically. On the other hand, the role played by the supernova (SN) explosion associated with LGRBs in determining the outcome of the system has been barely considered. In this paper, we discuss the large landscape of outcomes resulting from the interaction of the SN, jet, and cocoon. We show that the outcome depends mainly on three time-scales: the times for the cocoon and SN shock wave to break through the surface of the progenitor star, and the time needed for the cocoon to engulf completely the progenitor star. The delay between the launch of the SN shock moving through the progenitor star and the jet can be related to these three time-scales. Depending on the ordering of these time-scales, the jet-cocoon might propagate inside the SN ejecta or the other way around, and the outcome for the properties of the explosion would be different. We discuss the imprint of the complex interaction between the jet-cocoon and the SN shock on the emergent thermal and non-thermal radiation.
ABSTRACT
The discovery of GRB 170817A, the first unambiguous off-axis short gamma-ray burst (sGRB) arising from a neutron star merger, has challenged our understanding of the angular structure of ...relativistic jets. Studies of the jet propagation usually assume that the jet is ejected from the central engine with a top-hat structure and its final structure, which determines the observed light curve and spectra, is primarily regulated by the interaction with the nearby environment. However, jets are expected to be produced with a structure that is more complex than a simple top-hat, as shown by global accretion simulations. We present numerical simulations of sGRBs launched with a wide range of initial structures, durations, and luminosities. We follow the jet interaction with the merger remnant wind and compute its final structure at distances ≳1011 cm from the central engine. We show that the final jet structure, as well as the resulting afterglow emission, depends strongly on the initial structure of the jet, its luminosity, and duration. While the initial structure of the jet is preserved for long-lasting sGRBs, it is strongly modified for jets barely making their way through the wind. This illustrates the importance of combining the results of global simulations with propagation studies in order to better predict the expected afterglow signatures from neutron star mergers. Structured jets provide a reasonable description of the GRB 170817A afterglow emission with an off-axis angle θobs ≈ 22.5°.
Abstract
The first neutron star binary merger detected in gravitational waves, GW170817, and the subsequent detection of its emission across the electromagnetic spectrum showed that these systems are ...viable progenitors of short
γ
-ray bursts (sGRB). The afterglow signal of GW170817 has been found to be consistent with a structured GRB jet seen off-axis, requiring significant amounts of relativistic material at large angles. This trait can be attributed to the interaction of the relativistic jet with the external wind medium. Here we perform numerical simulations of relativistic jets interacting with realistic wind environments in order to explore how the properties of the wind and central engine affect the structure of successful jets. We find that the angular energy distribution of the jet depends primarily on the ratio between the lifetime of the jet and the time it takes the merger remnant to collapse. We make use of these simulations to constrain the time it took for the merger remnant in GW170817 to collapse into a black hole based on the angular structure of the jet as inferred from afterglow observations. We conclude that the lifetime of the merger remnant in GW170817 was ≈1–1.7 s, which, after collapse, triggered the formation of the jet.
The afterglow of GRB 170817A/GW170817 was very unusual, slowly rising as , peaking at days, and sharply decaying as . Very-long-baseline interferometry observations revealed an unresolved radio ...afterglow image whose flux centroid apparently moved superluminally with vapp 4c between 75 and 230 days, clearly indicating that the afterglow was dominated by a relativistic jet's compact core. Different jet angular structures successfully explained the afterglow light curves: Gaussian and steep power-law profiles with narrow core angles θc 5° and significantly larger viewing angles θobs/θc ∼ 3−5. However, a top-hat jet (THJ; conical with sharp edges at θ = θ0) was ruled out because it appeared to produce an early flux rise much steeper ( with a 3) than observed. Using 2D relativistic hydrodynamic simulations of an initially THJ, we show that the initial steep flux rise is an artifact caused by the simulation's finite start time, t0, missing its flux contributions from t < t0 and sometimes "compensated" using an analytic THJ. While an initially THJ is not very physical, such simulations are particularly useful at when the afterglow emission is dominated by the jet's core and becomes insensitive to its exact initial angular profile if it drops off sharply outside of the core. We demonstrate that an initially THJ fits GW170817/GRB 170817A's afterglow light curves and flux centroid motion at , for θobs/θ0 3 and may also fit the earlier light curves for Γ0 = Γ(t0) 102.5. We analytically express the degeneracies between the model parameters, and find a minimal jet energy of erg and circumburst medium density of .
ABSTRACT
We present hydrodynamic simulations of the hot cocoon produced when a relativistic jet passes through the gamma-ray burst (GRB) progenitor star and its environment, and we compute the light ...curve and spectrum of the radiation emitted by the cocoon. The radiation from the cocoon has a nearly thermal spectrum with a peak in the X-ray band, and it lasts for a few minutes in the observer frame; the cocoon radiation starts at roughly the same time as when γ-rays from a burst trigger detectors aboard GRB satellites. The isotropic cocoon luminosity (∼1047 erg s−1) is a few times smaller than the X-ray luminosity of a typical long-GRB afterglow during the plateau phase. This radiation should be identifiable in the Swift data because of its nearly thermal spectrum that is distinct from the somewhat brighter power-law component. The detection of this thermal component would provide information regarding the size and density stratification of the GRB progenitor star. Photons from the cocoon are also inverse-Compton (IC) scattered by electrons in a delayed jet. We present the IC light curve and spectrum by post-processing the results of the numerical simulations. The IC spectrum lies in 10 keV–MeV band for typical GRB parameters. The detection of this IC component would provide an independent measurement of GRB jet Lorentz factor, and it would also help to determine the jet magnetization parameter.
ABSTRACT
After the detection of GRB 170817A, the first unambiguous off-axis gamma-ray burst (GRB), several studies tried to understand the structure of GRB jets. The initial jet structure (directly ...produced by the central engine) can be partially preserved, or can be completely modified by the interaction with the environment. In this study, we perform three-dimensional, special relativistic hydrodynamics simulations of long GRB jets evolving through a massive progenitor star. Different jet scenarios were considered: Top-hat, Gaussian jets dominated by pressure or by kinetic energy, as well as a model of a supernova (SN) plus a jet both propagating through the progenitor. We found that, while propagating inside the progenitor star, jets with different initial structures are nearly indistinguishable. Kinetic-dominated jets are faster and more collimated than pressure-dominated jets. The dynamics of jets inside the progenitor star strongly depends on the presence of an associated SN, which can substantially decelerate the jet propagation. We show that the initial structure of GRB jets is preserved, or not, mainly depending on the jet collimation. The initial structure is preserved in uncollimated jets, i.e. jets which move through low-density environments. Meanwhile, jets which move through dense environments are shaped by the interaction with the medium and remain collimated.
ABSTRACT
The evolution and physics of the common envelope (CE) phase are still not well understood. Jets launched from a compact object during this stage may define the evolutionary outcome of the ...binary system. We focus on the case in which jets are launched from a neutron star (NS) engulfed in the outer layers of a red giant (RG). We run a set of three-dimensional hydrodynamical simulations of jets with different luminosities and inclinations. The luminosity of the jet is self-regulated by the mass accretion rate and an efficiency η. Depending on the value of η the jet can break out of the previously formed bulge (‘successful jet’) and aligns against the incoming wind, in turn, it will realign in favour of the direction of the wind. The jet varies in size and orientation and may present quiescent and active epochs. The inclination of the jet and the Coriolis and centrifugal forces, only slightly affect the global evolution. As the accretion is hypercritical, and the specific angular momentum is above the critical value for the formation of a disc, we infer the formation of a disc and launching of jets. The discs’ mass and size would be ∼10−2 M⊙ and ≳1010 cm, and it may have rings with different rotation directions. In order to have a successful jet from a white dwarf, the ejection process needs to be very efficient (η ∼ 0.5). For main-sequence stars, there is not enough energy reservoir to launch a successful jet.
Relativistic supernovae constitute a subclass of Type Ic supernovae (SNe). Their nonthermal, radio emission differs notably from that of regular Type Ic supernovae as they have a fast expansion speed ...(with velocities ∼0.6-0.8 c) which cannot be explained by a "standard" spherical SN explosion, but advocates for a quickly evolving, mildly relativistic ejecta associated with the SN. In this paper, we compute the synchrotron radiation emitted by the cocoon of a long gamma-ray burst jet (GRB). We show that the energy and velocity of the expanding cocoon, and the radio nonthermal light curves and spectra are consistent with those observed in relativistic SNe. Thus, the radio emission from this events is not coming from the SN shock front, but from the mildly relativistic cocoon produced by the passage of a GRB jet through the progenitor star. We also show that the cocoon radio emission dominates the GRB emission at early times for GRBs seen off-axis, and the flux can be larger at late times compared with on-axis GRBs if the cocoon energy is at least comparable with respect to the GRB energy.
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