Late Afterglow Research Articles

The Detection Prospect of the Counter Jet Radiation in the Late Afterglow of GRB 170817A

The central engine of a gamma-ray burst (GRB) is widely believed to launch a pair of oppositely moving jets, i.e., the forward jet moving toward us and the counter jet regressing away. The forward jet generates the radiation typically observed in GRBs, while the counter jet has not been detected yet due to its dimness. GRB 170817A, a short burst associated with a binary neutron star merger event, is a nearby event (z = 0.0097) with an off-axis structured energetic forward jet and hence probably the most suitable target for searching the counter jet radiation. Assuming the same properties for the forward and counter jet components as well as the shock parameters, the fit to the multiwavelength afterglow emission of GRB 170817A suggests a peak time ∼quite a few ×103 day of the counter jet radiation, but the detection prospect of this new component is not promising. Anyhow, if the shock parameters (ϵ e and ϵ B) of the counter jet component are (a few times) higher than that of the forward shock, e.g., the posterior results of the magnetic energy fraction of the forward shock and the counter jet are log10ϵB =−4.23−0.69+1.42 and log10ϵB,cj =−3.65−2.11+3.06 , respectively, as still allowed by the current data, the counter jet afterglow emission will be enhanced and hence may be detected. A few hours of exposure by JWST in the F356W band will stringently test such a scenario.

Scaling relations for gamma-ray burst afterglow light curves and centroid motion independent of jet structure and dynamics

ABSTRACT Models for gamma-ray burst afterglow dynamics and synchrotron spectra are known to exhibit various scale invariances, owing to the scale-free nature of fluid dynamics and the power-law shape of synchrotron spectra. Since GRB 170817A, off-axis jet models including a lateral energy structure in the initial outflow geometry have gained in prominence. Here, we demonstrate how the scale invariance for arbitrary jet structure and dynamical stage can be expressed locally as a function of jet temporal light-curve slope. We provide afterglow flux expressions and demonstrate their use to quickly assess the physical implications of observations. We apply the scaling expressions to the Swift X-ray Telescope sample, which shows a spread in observed fluxes, binned by light-curve slope at time of observation, that increases with increasing light-curve slope. According to the scaling relations, this pattern is inconsistent with a large spread in environment densities if these were the dominant factor determining the variability of light curves. We further show how the late deep Newtonian afterglow stage remains scale-invariant but adds distinct spectral scaling regimes. Finally, we show that for given jet structure a universal curve can be constructed of the centroid offset, image size, and ellipticity (that can be measured using very large baseline interferometry) versus observer angle, in a manner independent of explosion energy and circumburst density. Our results apply to any synchrotron transient characterized by a release of energy in an external medium, including supernova remnants, kilonova afterglows, and soft gamma-repeater flares.

Plasma properties and discharging of dust particles in an Ar/C2H2 plasma afterglow

A global (volume averaged) model is developed for an argon-acetylene plasma afterglow. The model is used to study the electron and ion densities, electron temperature and densities of argon metastable atoms in the afterglow plasma. The calculated time dependence for the electron density is found to be in agreement with the existing experimental data. These calculated plasma characteristics are used to investigate the dust charge distribution function (DCDF) for particles with radii of 10–200 nm. The DCDF is found by solving numerically the master equation describing dust discharging as a one-step stochastic process and is also calculated as a Gaussian distribution with mean dust charge and variance, which are functions of time. The time dependences for mean dust charge, variance and dust charging time are obtained and analysed. If the electronegativity of the plasma in the steady-state is low, negative ions do not affect much discharging of dust particles in the afterglow, while at large electronegativity their role is essential. In the case of low electronegativity, discharging of dust particles is mainly due to deposition of positive ions with small and moderate masses (less than the mass of C10H6 + ions). Increasing electronegativity, the effect of heavy positive ions on dust discharging in the late afterglow is important. Secondary electron emission from dust surface at collisions of metastable atoms appears to be negligible.

The Late Afterglow of GW170817/GRB 170817A: A Large Viewing Angle and the Shift of the Hubble Constant to a Value More Consistent with the Local Measurements

The multimessenger data of neutron star merger events are promising for constraining the Hubble constant. So far, GW170817 is still the unique gravitational wave event with multiwavelength electromagnetic counterparts. In particular, its radio and X-ray emission have been measured in the past ∼3–5 yr. In this work, we fit the long-lasting X-ray, optical, and radio afterglow light curves of GW170817/GRB 170817A, including the forward shock radiation from both the decelerating relativistic gamma-ray burst outflow and the subrelativistic kilonova outflow (though whether the second component contributes significantly is still uncertain), and find out a relatively large viewing angle (∼0.5 rad). Such a viewing angle has been taken as a prior in the gravitational wave data analysis, and the degeneracy between the viewing angle and the luminosity distance is broken. Finally, we have a Hubble constant , which is more consistent with that obtained by other local measurements. If rather similar values are inferred from multimessenger data of future neutron star merger events, it will provide critical support to the existence of the Hubble tension.

Formation of N2(B, v′) levels in N2, N2/Ar and N2/He reduced pressure late afterglows

Formation of N2(B, v′) levels in N2, N2/Ar and N2/He reduced pressure late afterglows

Temporal dusty plasma afterglow: A review

In complex plasmas, dust particles are charged through their interactions with the electrons and ions of the surrounding plasma. In low-temperature laboratory plasmas, dust particles most commonly acquire a negative charge. In particular, in a laboratory glow-discharge plasma, the typical charge for a micrometer-size grain generally attains a few thousands of electronic charges. Under stable discharge conditions, this large negative charge is relatively well-characterized. However, for unsteady discharge conditions, the charge can differ and even fluctuate. In particular, when the power source of the discharge is turned off, the charged species of the plasma diffuse away and recombine into neutral species: this is a temporal afterglow. When dust particles are present inside a temporal plasma afterglow, the diffusion of charged species and the plasma decay dynamics are affected. Moreover, the dust particle charges also evolve during the afterglow period. In the late afterglow, dust particles are known to keep residual charges. The value of these residual charges strongly depends on the ambipolar-to-free diffusion transition. In addition, the presence of a constant electric field, causing ions to drift through the neutral gas, has a strong influence on the final dust particle residual charges, eventually leading to large positive residual charges. In this review article, the dynamics of temporal complex plasma afterglow are discussed. Experimental and theoretical results are presented. The basics of temporal afterglow modeling are also given.

Distinct Thermal Emission from GRB 190109A

The gamma-ray bursts (GRBs) with distinct thermal components are rarely detected, especially in cases with thermal components throughout the prompt phase. Recently, Fermi/GBM, Swift/BAT, and Swift/XRT detected the special long-duration GRB 190109A, which has four pulses in the prompt gamma-ray emission, i.e, Pulse I (−4 to 20 s), Pulse II (20–50 s), Pulse III (50–90 s), and Pulse IV (90–120 s). GRB 190109A exhibits a very hard low-energy index (α ∼ 1) in the Band function relative to the typical GRBs (α ∼ − 1). In the whole burst prompt emission, we find distinct thermal emissions in the time-resolved spectra throughout four pulses. The blackbody (BB) temperature kT varies from 24.7 to 8.2 keV for Pulse I to Pulse IV. We also obtain the relation of F ∝ kT −0.40 for the early phase (Pulse I) and F ∝ kT 3.33±0.76 for the late phase (Pulses II–IV), respectively. The significant deviation of the kT − F relation in the early epochs from that in the late epochs likely suggests that the BB spectra origin of the early phase (Pulse I) may have disparate physical processes from those of the late phase (Pulses II–IV). For instance, it may be the transition from cocoon surroundings by a jet to the photosphere of the matter-dominated jet. A jet break is found in the late X-ray afterglow, which is in keeping with the standard external shock afterglow model in the interstellar medium circumburst.

GRB 190530A: From Precursor, Prompt Emission to Afterglow all Originated from Synchrotron Radiation

GRB 190 530A was jointly observed by the High Energy X-ray Telescope of the Hard X-ray Modulation Telescope (Insight-HXMT/HE) and the Ground-Based Wide-Angle Camera network (GWAC-N) with the extremely large field of view. After triggered by Insight-HXMT/HE and Fermi/GBM, we observed the optical emission of GRB 190 530A, using the 30 cm telescope of GWAC (GWAC-F30) to search and locate its position. Subsequent observation of the late afterglow of GRB 190 530A was made with the 2.16 m telescope at Xinglong Observatory. In this paper, we make a detailed exploration of the origin of GRB 190 530A. In the prompt emission, a “double-tracking” pattern is presented both for the low-energy spectral index α and the peak energy E p in the Band function with Insight-HXMT/HE and Fermi/GBM data; the results of GRB 190 530A are consistent with the Amati and Yonetoku correlations; the spectral lag (τ) versus energy (E) can be estimated with . The synchrotron radiation can account for the origin of GRB 190 530A prompt emission behaviors. The α and E p of the precursor are essentially the same as that of the main prompt emission, implying that they have the same origin. For the afterglow, it can be described with the external forward shock model in ISM circumburst medium. In summary, from precursor, prompt emission to afterglow of GRB 190 530A all originated from synchrotron radiation.

On the Origin of the Multi-GeV Photons from the Closest Burst with Intermediate Luminosity: GRB 190829A

Abstract Very high energy (VHE) emission is usually interpreted in the synchrotron self-Compton scenario and expected from the low-redshift and high-luminosity gamma-ray bursts (GRBs), such as GRB 180720B and GRB 190114C. Recently, the H.E.S.S. telescopes reported VHE emission from one of the closest bursts, GRB 190829A, which was associated with the supernova 2019oyw. In this paper, we present a temporal and spectral analysis from optical bands to the Fermi-LAT energy range over multiple observational periods beginning after the trigger time and extending for almost 3 months. We show that the X-ray and optical observations are consistent with synchrotron forward-shock emission evolving between the characteristic and cooling spectral breaks during the early and late afterglow in a uniform-density medium. Modeling the light curves together with the spectral energy distribution, we show that the outflow expanded with an initial bulk Lorentz factor of Γ ∼ 30, which is high for low-luminosity GRBs and low for high-luminosity GRBs. The values of the initial bulk Lorentz factor and the isotropic-equivalent energy suggest that GRB 190829A is an intermediate-luminosity burst; consequently, it becomes the first burst of this class to be detected in the VHE gamma-ray band by an imaging atmospheric Cherenkov telescope and, in turn, the first event to not be simultaneously observed by the Fermi-LAT instrument. Analyzing the intermediate-luminosity bursts with z ≲ 0.2, such as GRB 130702A, we show that bursts with intermediate luminosities are potential candidates to be detected in VHEs.

Спектроскопия барьерного разряда низкого давления. Послесвечение с ионами Ne-=SUP=-2+-=/SUP=-, Ne-=SUP=-+-=/SUP=- и Ne-=SUP=-2+-=/SUP=-

We present the results of modeling the radiation of a decaying plasma, formed by the processes of electron-ion recombination with the participation of three neon ions: the molecular ion Ne2+ and atomic ions Ne+ and Ne2+. Such a combination of ions, simultaneously participating in the formation of the plasma spectrum, was first discovered in the afterglow of a pulsed barrier discharge of a cylindrical geometry at neon pressures less than 1 Torr and an electron density[e] ≤ 4 × 1010 cm-3. The main attention is paid to the comparative analysis of the mechanisms of impact-radiation recombination of Ne+ and Ne2+ ions based on the numerical solution of the system of differential equations for the densities of ions and long-lived excited atoms in the afterglow, taking into account the main elementary processes in decaying plasma with pulsed "heating" of electrons. The regularities of electron temperature relaxation from discharge values of several electron volts to 300 K in the late afterglow are considered in particular details. Comparison of the model solutions with the spectral intensities measured by the multichannel photon counting method shows that, given their good agreement in the case of singly charged ions, an adequate description of the evolution of ionic lines requires expanding the available information on the recombination of Ne2+ ions.

Selective nitrogen doping of graphene due to preferential healing of plasma-generated defects near grain boundaries

Hyperspectral Raman IMAging (RIMA) is used to study spatially inhomogeneous polycrystalline monolayer graphene films grown by chemical vapor deposition. Based on principal component analysis clustering, distinct regions are differentiated and probed after subsequent exposures to the late afterglow of a microwave nitrogen plasma at a reduced pressure of 6 Torr (800 Pa). The 90 × 90 µm2 RIMA mapping shows differentiation between graphene domains (GDs), grain boundaries (GBs), as well as contaminants adsorbed over and under the graphene layer. Through an analysis of a few relevant band parameters, the mapping further provides a statistical assessment of damage, strain, and doping levels in plasma-treated graphene. It is found that GBs exhibit lower levels of damage and N-incorporation than GDs. The selectivity at GBs is ascribed to (i) a low migration barrier of C adatoms compared to N-adatoms and vacancies and (ii) an anisotropic transport of C adatoms along GBs, which enhances adatom-vacancy recombination at GBs. This preferential self-healing at GBs of plasma-induced damage ensures selective incorporation of N-dopants at plasma-generated defect sites within GDs. This surprising selectivity vanishes, however, as the graphene approaches an amorphous state.

Controlling N and C-atom densities in N2/H2 and N2/CH4 microwave afterglows for selective TiO2 surface nitriding

Radiative states of N2/<5% H2 and N2/<50/00 CH4 HF(microwave) flowing afterglows of microwave plasmas at reduced gas pressures (4–20 Torr) have been analyzed by emission spectroscopy. The N2 1st pos (580 nm), 2nd pos (316 nm), N2+ 1st neg (391.4 nm), NH (336 nm) and CN (386 nm) bands in the pink (early) and the late afterglows were measured and the NO titration was used to determine the concentrations of N-atoms, O-atoms in impurity, N2(X,v > 13) and N2(A) metastable molecules, N2+ ions. Introduction of small percentages of H2 (or CH4) into N2 lead to a size reduction of the early afterglow region which was followed by a late afterglow zone where the N + N recombination is the dominant process. In addition, it also gave an effect of reducing the densities of N atoms (as well as the N2 excited states) in the afterglow regions. At the same time, it introduced other active species such as H (or C) atoms, of which the densities strongly vary depending on the percentage of H2 (or CH4) in the mixture. Such changes in the densities of active species such as N and C atoms in the afterglow region had a strong influence on the incorporation characteristics of N atoms into the surface of TiO2 films introduced into the afterglow region. These results deliver an important meaning in devising process conditions especially for selective nitrogen or carbon doping into a skin-depth layers of oxide materials, which is desired in the case of preparing a few nm-thick functional films for photovoltaic and photocatalytic applications.

Do Afterglow Synchrotron Radiations Follow the Lp,iso–Ep,z–Γ0 Relation of Gamma-Ray Bursts? The Cases of GRBs 190114C, 130427A, and 180720B

Abstract Bimodal spectral energy distributions (SEDs) of gamma-ray burst (GRB) afterglow of GRBs 190114C, 130427A, and 180720B confirm that they are originated from the synchrotron emission (Syn) and synchrotron self-Compton scattering (SSC) process of electrons accelerated in the jets. The radiation mechanism and the physics of the observed spectrum-luminosity/energy relations of GRBs remain as open questions. By extracting the Syn component through fitting their early afterglow SEDs with the Syn+SSC model, we find that their luminosity (L syn), peak energy (E p,syn,z), and the Lorentz factor of the afterglow fireball (Γ t ) follow the L p,iso–E p,z–Γ0 relation of prompt gamma rays, where L p,iso is the isotropic luminosity, E p,z is the peak energy of the spectrum in the burst frame, and Γ0 is the initial Lorentz factor of the fireball. To examine whether late afterglows are consistent with this relation, we calculate the synchrotron component at late afterglows. It is found that they also follow the same L p,iso–E p,z–Γ0 relation, albeit they are not consistent with the L p,iso–E p,z relation. Our results may imply that the L p,iso–E p,z–Γ0 relation would be a universal feature of synchrotron radiations of electrons accelerated in GRB jets throughout the prompt and afterglow phases among GRBs. Its origin is not fully understood, and possible explanations are briefly discussed.

Determination of absolute O(3P) and O2(a1 Δ g) densities and kinetics in fully modulated O2 dc glow discharges from the O2(X3 Σ g −) afterglow recovery dynamics

A method is presented for the determination of the absolute densities of O(3P) atoms and O2(a1Δg) molecules in an O2 electrical discharge, which does not depend on any calibration procedure or knowledge of optical transition strengths. It is based on observing the recovery dynamics of the O2(X3Σg −) density in the afterglow of a fully-modulated discharge, and is demonstrated in a dc glow discharge in pure О2 at pressures of 0.2–4 Torr. The time-resolved O2(X3Σg −) density was measured by VUV absorption spectroscopy using the monochromator branch of the VUV DESIRS beamline at Synchrotron SOLEIL, but this methodology could be used with another density measurement technique. During the active discharge, the O2(X3Σg −) density is depleted by a combination of О2 dissociation, excitation into metastable states (principally O2(a1Δg)) and gas heating/dilation. After discharge extinction, the O2(X3Σg −) density progressively recovers to its initial (before discharge) value, with three distinct time-constants due to: (i) gas cooling (fast), (ii) O(3P) atom recombination (intermediate), and (iii) O2(a1Δg) quenching (slow). The O(3P) and O2(a1Δg) dynamics can be separated easily, allowing the O(3P) and O2(a1Δg) afterglow loss kinetics to be determined, as well as their mole fractions in the steady-state discharge. Both the O(3P) and O2(a1Δg) mole-fractions increase with current (up to the highest current studied, 40 mA) and pass through maxima with pressure at 1 Torr, reaching 16.5% and 8%, respectively. O(3P) atoms are principally lost by recombination at the borosilicate tube surface, with a loss probability in the afterglow of ∼8 × 10−4, nearly independent of gas pressure and discharge current (in contrast to previous observations in the active discharge []). The O2(a1Δg) dynamics were also measured by IR emission spectroscopy. In the late afterglow, this agrees well with the O2(X3Σg −) recovery dynamics, corresponding to an O2(a1Δg) surface loss probability of ∼2.2 × 10−4. The initial O2(a1Δg) loss is faster than in the later afterglow, indicating that it is also quenched by O atoms.

Binary Neutron Star (BNS) Merger: What We Learned from Relativistic Ejecta of GW/GRB 170817A

Gravitational Waves (GW) from coalescence of a Binary Neutron Star (BNS) and its accompanying short Gamma-Ray Burst (GRB) GW/GRB 170817A confirmed the presumed origin of these puzzling transients and opened up the way for relating properties of short GRBs to those of their progenitor stars and their surroundings. Here we review an extensive analysis of the prompt gamma-ray and late afterglows of this event. We show that a fraction of polar ejecta from the merger had been accelerated to ultra-relativistic speeds. This structured jet had an initial Lorentz factor of about 260 in our direction, which was O ( 10 ∘ ) from the jet’s axis, and was a few orders of magnitude less dense than in typical short GRBs. At the time of arrival to circum-burst material the ultra-relativistic jet had a close to Gaussian profile and a Lorentz factor ≳ 130 in its core. It had retained in some extent its internal collimation and coherence, but had extended laterally to create mildly relativistic lobes—a cocoon. Its external shocks on the far from center inhomogeneous circum-burst material and low density of colliding shells generated slowly rising afterglows, which peaked more than 100 days after the prompt gamma-ray. The circum-burst material was somehow correlated with the merger. As non-relativistic outflows or tidally ejected material during BNS merger could not have been arrived to the location of the external shocks before the relativistic jet, circum-burst material might have contained recently ejected materials from resumption of internal activities, faulting and mass loss due to deformation and breaking of stars crusts by tidal forces during latest stages of their inspiral but well before their merger. By comparing these findings with the results of relativistic Magneto-Hydro-Dynamics (MHD) simulations and observed gravitational waves we conclude that progenitor neutron stars were most probably old, had close masses and highly reduced magnetic fields.

Nearly monoenergetic positive ion beam with self-neutralized space charge extracted from a pulsed plasma

A nearly monoenergetic Ar+ ion beam (100 eV peak energy, 3.5 eV full width at half maximum), with self-neutralized space charge, was extracted from a power modulated (pulsed) inductively coupled plasma (ICP), through a single grounded grid. The beam energy was set by applying a synchronous DC bias voltage to a boundary electrode in contact with the plasma, in the late afterglow of the power pulse. A retarding field energy analyzer and a movable Faraday cup were used to measure the ion energy distribution (IED) and the time resolved ion and electron current densities, as a function of position along the beam axis, to unravel the mechanism of self-neutralization of the space charge of the beam. When a +100 V DC bias was applied to the boundary electrode during the afterglow, a ~100 eV beam (ion density ~109 cm−3) emerged that was space-charge neutralized by periodic injection of electrons, mainly during the early afterglow, into the region downstream of the extraction grid. A background plasma (electron density ~1010 cm−3) was formed by these electrons, in conjunction with relatively low-energy ions (1–10 eV) that flew out of the ICP when the DC bias was off, as well as formed by resonant charge exchange of the 100 eV beam ions with the background Ar in the downstream region near the extraction grid. The behavior of the beam IED as a function of pressure and power of the ICP as well as the timing of application of the DC bias voltage was also studied.

Low-damage nitrogen incorporation in graphene films by nitrogen plasma treatment: Effect of airborne contaminants

Graphene films grown on copper by chemical vapor deposition were exposed to the late afterglow of a reduced-pressure N2 plasma sustained by microwave electromagnetic fields. X-ray photoelectron and Raman spectroscopies reveal extremely high incorporation of plasma-generated N atoms into the graphene film (N/C = 29%) while maintaining an unprecedentedly low-damage generation (D:G = 0.35–0.45) compared to the literature (0.5–2.5). The incorporation dynamics between graphene on copper and graphene on copper oxide are also compared and discussed. After transfer on SiO2/Si substrate, the N/C content decrease to only 6%. This reveals that a large part of the N atoms are weakly bonded to the graphene surface. Most of the nitrogen incorporation seems linked to the functionalization of weakly bonded hydrocarbons initially adsorbed from air exposure or carbon-nitrogen structures arising from plasma-surface interactions.

Treatment of graphene films in the early and late afterglows of N2 plasmas: comparison of the defect generation and N-incorporation dynamics

Graphene films grown on copper substrate by chemical vapor deposition were exposed to the flowing afterglow of a reduced-pressure N2 plasma sustained by microwave electromagnetic fields (surface-wave plasma). Two set of conditions were examined by controlling the gas flow rate: the late afterglow (LA) characterized by a high number densities of reactive N atoms and the early afterglow (EA) in which significant populations of metastable N2(A) states and positive ions (N2+ and N4+) coexist with plasma-generated N atoms. LA treatments of graphene films show monotonous and steady incorporation of nitrogen atoms along with very low damage. However, given the very mild LA treatment conditions, a large part of the N atoms remains weakly bonded to the graphene surface; a feature ascribed to the plasma-induced functionalization of airborne hydrocarbon contaminants. In such conditions, graphitic inclusion of plasma-generated N atoms is limited to native defect sites. On the other hand, the presence of highly energetic species in the EA induces significant damage combined with much higher N-incorporation. Detailed Raman analysis of EA-treated samples further reveals a transition from vacancy-type defects to much larger multi-vacancies with increasing treatment time. This complete set of data indicates that through a judicious control of the populations of reactive N atoms, metastable N2(A) states, and positive ions (N2+ and N4+), the flowing afterglow of microwave N2 plasmas represents a highly promising tool for precise, post-growth tuning of the defect generation and N-incorporation dynamics in graphene films.

Synchrotron self-absorption in GRB afterglows: the effects of a thermal electron population

In the standard synchrotron afterglow model, a power law of electrons is responsible for all aspects of photon production and absorption. Recent numerical work has shown that the vast majority of particles in the downstream medium are actually "thermal" particles, which were shock-heated but did not enter the Fermi acceleration process (the name stands in contrast to the nonthermal high-energy tail, rather than connoting a Maxwellian distribution). There are substantial differences at optical and higher energies when these thermal electrons participate in the afterglow, but early work along these lines ignored the radio end of the electromagnetic spectrum. We report here on an extension of previous Monte Carlo simulations of gamma-ray burst afterglows. The model now includes the synchrotron self-absorption (SSA) process and so can simulate afterglows across the entire EM spectrum, and several orders of magnitude in time. In keeping with earlier work, inclusion of the thermal electrons increases the SSA frequency by a factor of 30, and the radio intensity by a factor of 100. Furthermore, these changes happen with no modification to the late optical or X-ray afterglow. Our results provide very strong evidence that thermal electrons must be considered in any multiwavelength model for afterglows.

Late Time Afterglow Observations Reveal a Collimated Relativistic Jet in the Ejecta of the Binary Neutron Star Merger GW170817.

The binary neutron star (BNS) merger GW170817 was the first astrophysical source detected in gravitational waves and multiwavelength electromagnetic radiation. The almost simultaneous observation of a pulse of gamma rays proved that BNS mergers are associated with at least some short gamma-ray bursts (GRBs). However, the gamma-ray pulse was faint, casting doubt on the association of BNS mergers with the luminous, highly relativistic outflows of canonical short GRBs. Here we show that structured jets with a relativistic, energetic core surrounded by slower and less energetic wings produce afterglow emission that brightens characteristically with time, as recently seen in the afterglow of GW170817. Initially, we only see the relatively slow material moving towards us. As time passes, larger and larger sections of the outflow become visible, increasing the luminosity of the afterglow. The late appearance and increasing brightness of the multiwavelength afterglow of GW170817 allow us to constrain the geometry of its ejecta and thus reveal the presence of an off-axis jet pointing about 30° away from Earth. Our results confirm a single origin for BNS mergers and short GRBs: GW170817 produced a structured outflow with a highly relativistic core and a canonical short GRB. We did not see the bright burst because it was beamed away from Earth. However, approximately one in 20 mergers detected in gravitational waves will be accompanied by a bright, canonical short GRB.