The main hard pulse of prompt gamma-ray emission in GRB 170817A had a duration of ∼0.5 s, and its onset was delayed with respect to the gravitational-wave chirp signal by . Detailed follow-up of the ...subsequent broadband kilonova emission revealed a two-component ejecta-a lanthanide-poor ejecta with mass Mej,blue 0.025 M that powered the early but rapidly fading blue emission and a lanthanide-rich ejecta with mass that powered the longer-lasting redder emission. Both the prompt gamma-ray onset delay and the existence of the blue ejecta with a modest electron fraction, 0.2 Ye 0.3, can be explained if the collapse to a black hole (BH) was delayed by the formation of a hypermassive neutron star. Here we determine the survival time of the merger remnant by combining two different constraints, namely, the time needed to produce the requisite blue-ejecta mass and that necessary for the relativistic jet to bore its way out of the expanding ejecta. In this way, we determine that the remnant of GW170817 must have collapsed to a BH after . We also discuss how future detections and the delays between the gravitational and electromagnetic emissions can be used to constrain the properties of the merged object.
ABSTRACT
GRB 170817A/GW 170817 is the first gamma-ray burst (GRB) clearly viewed far from the GRB jet’s symmetry axis. Its afterglow was densely monitored over a wide range of frequencies and times. ...It has been modelled extensively, primarily numerically, and although this endeavour was very fruitful, many of the underlying model parameters remain undetermined. We provide analytic modelling of GRB afterglows observed off-axis, considering jets with a narrow core (of half-opening angle θc) and power-law wings in energy per unit solid angle (ϵ = ϵcΘ−a where Θ = 1 + (θ/θc)21/2) and initial specific kinetic energy (Γ0 − 1 = Γc, 0 − 1Θ−b), as well as briefly discuss Gaussian jets. Our study reveals qualitatively different types of light curves that can be viewed in future off-axis GRBs, with either single or double peaks, depending on the jet structure and the viewing angle. Considering the light-curve shape rather than the absolute normalizations of times and/or fluxes, removes the dependence of the light curve on many of the highly degenerate burst parameters. This study can be easily used to determine the underlying jet structure, significantly reduce the effective parameter space for numerical fitting attempts and provide physical insights. As an illustration, we show that for GRB 170817A, there is a strong correlation between the allowed values of Γc, 0 and b, leading to a narrow strip of allowed solutions in the Γc, 0–b plane above some minimal values Γc, 0 ≳ 40, b ≳ 1.2. Furthermore, the Lorentz factor of the material dominating the early light curve can be constrained by three independent techniques to be Γ0(θmin, 0) ≈ 5–7.
The first, long-awaited, detection of a gravitational-wave (GW) signal from the merger of a binary neutron star (NS-NS) system was finally achieved (GW170817) and was also accompanied by an ...electromagnetic counterpart-the short-duration gamma-ray burst (GRB) 170817A. It occurred in the nearby ( Mpc) elliptical galaxy NGC 4993 and showed optical, IR, and UV emission from half a day up to weeks after the event, as well as late-time X-ray (at days) and radio (at days) emission. There was a delay of between the GW merger chirp signal and the prompt GRB emission onset, and an upper limit of was set on the viewing angle w.r.t the jet's symmetry axis from the GW signal. In this letter we examine some of the implications of these groundbreaking observations. The delay sets an upper limit on the prompt GRB emission radius, , for a jet with sharp edges at an angle . GRB 170817A's relatively low isotropic equivalent γ-ray energy output may suggest a viewing angle slightly outside the jet's sharp edge, , but its peak photon energy and afterglow emission suggest instead that the jet does not have sharp edges and the prompt emission was dominated by less energetic material along our line of sight, at . Finally, we consider the type of remnant that is produced by the NS-NS merger and find that a relatively long-lived ( s) massive NS is strongly disfavored, while a hyper-massive NS of lifetime appears to be somewhat favored over the direct formation of a black hole.
Abstract
Despite being hard to measure, GRB prompt γ-ray emission polarization is a valuable probe of the dominant emission mechanism and the GRB outflow’s composition and angular structure. During ...the prompt emission the GRB outflow is ultra-relativistic with Lorentz factors Γ ≫ 1. We describe in detail the linear polarization properties of various emission mechanisms: synchrotron radiation from different magnetic field structures (ordered: toroidal Btor or radial B∥, and random: normal to the radial direction B⊥), Compton drag, and photospheric emission. We calculate the polarization for different GRB jet angular structures (e.g. top-hat, Gaussian, power-law) and viewing angles θobs. Synchrotron with B⊥ can produce large polarizations, up to $25\%\lesssim \Pi \lesssim 45\%$, for a top-hat jet but only for lines of sight just outside (θobs − θj ∼ 1/Γ) the jet’s sharp edge at θ = θj. The same also holds for Compton drag, albeit with a slightly higher overall Π. Moreover, we demonstrate how Γ-variations during the GRB or smoother jet edges (on angular scales ≳ 0.5/Γ) would significantly reduce Π. We construct a semi-analytic model for non-dissipative photospheric emission from structured jets. Such emission can produce up to $\Pi \lesssim 15\%$ with reasonably high fluences, but this requires steep gradients in Γ(θ). A polarization of $50\%\lesssim \Pi \lesssim 65\%$ can robustly be produced only by synchrotron emission from a transverse magnetic field ordered on angles ≳ 1/Γ around our line of sight (like a global toroidal field, Btor, for 1/Γ < θobs < θj). Therefore, such a model would be strongly favored even by a single secure measurement within this range. We find that such a model would also be favored if $\Pi \gtrsim 20\%$ is measured in most GRBs within a large enough sample, by deriving the polarization distribution for our different emission and jet models.
Abstract
The highly luminous and variable prompt emission in gamma-ray bursts (GRBs) arises in an ultra-relativistic outflow. The exact underlying radiative mechanism shaping its non-thermal spectrum ...is still uncertain, making it hard to determine the outflow's bulk Lorentz factor Γ. GRBs with spectral cut-off due to pair production (γγ → e+e−) at energies Ec ≳ 10 MeV are extremely useful for inferring Γ. We find that when the emission region has a high enough compactness, then as it becomes optically thick to scattering, Compton downscattering by non-relativistic e±-pairs can shift the spectral cut-off energy well below the self-annihilation threshold, Esa = Γmec2/(1 + z). We treat this effect numerically and show that Γ obtained assuming Ec = Esa can underpredict its true value by as much as an order of magnitude.
ABSTRACT
We perform 3D general-relativistic magnetohydrodynamic simulations to model the jet break-out from the ejecta expected to be produced in a binary neutron-star merger. The structure of the ...relativistic outflow from the 3D simulation confirms our previous results from 2D simulations, namely, that a relativistic magnetized outflow breaking out from the merger ejecta exhibits a hollow core of θcore ≈ 4°, an opening angle of θjet ≳ 10°, and is accompanied by a wind of ejected matter that will contribute to the kilonova emission. We also compute the non-thermal afterglow emission of the relativistic outflow and fit it to the panchromatic afterglow from GRB170817A, together with the superluminal motion reported from VLBI observations. In this way, we deduce an observer angle of $\theta _{\rm obs}= 35.7^{\circ \, \, +1.8}_{\phantom{\circ \, \, }-2.2}$. We further compute the afterglow emission from the ejected matter and constrain the parameter space for a scenario in which the matter responsible for the thermal kilonova emission will also lead to a non-thermal emission yet to be observed.
ABSTRACT
The observations of GW170817/GRB170817A have confirmed that the coalescence of a neutron-star binary is the progenitor of a short gamma-ray burst (GRB). In the standard picture of a short ...GRB, a collimated highly relativistic outflow is launched after merger and it successfully breaks out from the surrounding ejected matter. Using initial conditions inspired from numerical-relativity binary neutron-star merger simulations, we have performed general-relativistic hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations in which the jet is launched and propagates self-consistently. The complete set of simulations suggests that: (i) MHD jets have an intrinsic energy and velocity polar structure with a ‘hollow core’ subtending an angle θcore ≈ 4°–5° and an opening angle of θjet > ≳ 10°; (ii) MHD jets eject significant amounts of matter and two orders of magnitude more than HD jets; (iii) the energy stratification in MHD jets naturally yields the power-law energy scaling E(> Γβ) ∝ (Γβ)−4.5; (iv) MHD jets provide fits to the afterglow data from GRB170817A that are comparatively better than those of the HD jets and without free parameters; and (v) finally, both of the best-fitting HD/MHD models suggest an observation angle θobs ≃ 21° for GRB170817A.
Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe and are powered by ultra-relativistic jets. Their prompt γ-ray emission briefly outshines the rest of the γ-ray sky, making ...them detectable from cosmological distances. A burst is followed by, and sometimes partially overlaps with, a similarly energetic but very broadband and longer-lasting afterglow emission. While most GRBs are detected below a few MeV, over 100 have been detected at high (≳0.1 GeV) energies, and several have now been observed up to tens of GeV with the Fermi Large Area Telescope (LAT). A new electromagnetic window in the very-high-energy (VHE) domain (≳0.1 TeV) was recently opened with the detection of an afterglow emission in the (0.1–1)TeV energy band by ground-based imaging atmospheric Cherenkov telescopes. The emission mechanism for the VHE spectral component is not fully understood, and its detection offers important constraints for GRB physics. This review provides a brief overview of the different leptonic and hadronic mechanisms capable of producing a VHE emission in GRBs. The same mechanisms possibly give rise to the high-energy spectral component seen during the prompt emission of many Fermi-LAT GRBs. Possible origins of its delayed onset and long duration well into the afterglow phase, with implications for the emission region and relativistic collisionless shock physics, are discussed. Key results for using GRBs as ideal probes for constraining models of extra-galactic background light and intergalactic magnetic fields, as well as for testing Lorentz invariance violation, are presented.
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