The X-ray to radio afterglow emission of GRB 170817A / GW 170817 so far scales as F ν ∝ ν −0.6 t 0.8 with observed frequency and time, consistent with a single power-law segment of the synchrotron spectrum from the external shock going into the ambient medium. This requires the effective isotropic equivalent afterglow shock energy in the visible region to increase as ∼ t 1.7 . The two main channels for such an energy increase are (i) radial: more energy carried by slower material (in the visible region) gradually catches up with the afterglow shock and energizes it, and (ii) angular: more energy in relativistic outflow moving at different angles to our line of sight, whose radiation is initially beamed away from us but its beaming cone gradually reaches our line of sight as it decelerates. One cannot distinguish between these explanations (or combinations of them) using only the X-ray to radio F ν (t). Here we demonstrate that the most promising way to break this degeneracy is through afterglow imaging and polarization, by calculating the predicted evolution of the afterglow image (its size, shape and flux centroid) and linear polarization Π(t) for different angular and/or radial outflow structures that fit F ν (t). We consider two angular profiles -a Gaussian and a narrow core with power-law wings in energy per solid angle, as well as a (cocoon motivated) (quasi-) spherical flow with radial velocity profile. For a jet viewed off-axis (and a magnetic field produced in the afterglow shock) Π(t) peaks when the jet's core becomes visible, at ≈ 2t p where the lightcurve peaks at t p , and the image can be elongated with aspect ratios 2. A quasi-spherical flow has an almost circular image and a much lower Π(t) (peaking at ≈ t p ) and flux centroid displacement θ fc (a spherical flow has Π(t) = θ fc = 0 and a perfectly circular image).

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 t del ≈ 1.74 s. Detailed follow-up of the subsequent broadband kilonova emission revealed a two-component ejecta -a lanthanide-poor ejecta with mass M ej,blue ≈ 0.025 M that powered the early but rapidly fading blue emission and a lanthanide-rich ejecta with mass M ej,red ≈ 0.04 M that powered the longer lasting redder emission. Both the prompt gamma-ray onset delay and the existence of the blue ejecta with modest electron fraction, 0.2 Y e 0.3, can be explained if the collapse to a black hole was delayed by the formation of a hypermassive neutron star (HMNS). 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 black hole after t coll = 0.98 +0.31 −0.26 s. 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.

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 (GW 170817), and was also accompanied by an electromagnetic counterpart -the short-duration GRB 170817A. It occurred in the nearby (D ≈ 40 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 ≥ 8.9 days) and radio (at ≥ 16.4 days) emission. There was a delay of ∆t ≈ 1.74 s between the GW merger chirp signal and the prompt-GRB emission onset, and an upper limit of θ obs < 28 • 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 ∆t sets an upper limit on the prompt-GRB emission radius, R γ 2c∆t/(θ obs − θ 0 ) 2 , for a jet with sharp edges at an angle θ 0 < θ obs . GRB 170817A's relatively low isotropic equivalent γ-ray energy-output may suggest a viewing angle slightly outside the jet's sharp edge, θ obs − θ 0 ∼ (0.05 − 0.1)(Γ/100) −1 , but its peak νF ν 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 θ obs 2θ 0 . Finally, we consider the type of remnant that is produced by the NS-NS merger and find that a relatively long-lived (> 2 s) massive NS is strongly disfavored, while a hyper-massive NS of lifetime ∼ 1 s appears to be somewhat favored over the direct formation of a black hole.

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 B tor 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 θ obs . Synchrotron with B ⊥ can produce large polarizations, up to 25% Π 45%, for a top-hat jet but only for lines of sight just outside (θ obs −θ j ∼ 1/Γ) the jet's sharp edge at θ = θ j . 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 Π 15% with reasonably high fluences, but this requires steep gradients in Γ(θ). A polarization of 50% Π 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, B tor , for 1/Γ < θ obs < θ j ). 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 Π 20% is measured in most GRBs within a large enough sample, by deriving the polarization distribution for our different emission and jet models.

We analyzed broadband X-ray and radio data of the magnetar SGRJ1935+2154 taken in the aftermath of its 2014, 2015, and 2016 outbursts. The source soft X-ray spectrum <10 keV is well described with a blackbody +power-law (BB+PL) or 2BB model during all three outbursts. Nuclear Spectroscopic Telescope Array observations revealed a hard X-ray tail, with a PL photon index Γ=0.9, extending up to 50 keV, with flux comparable to the one detected <10 keV. Imaging analysis of Chandra data did not reveal small-scale extended emission around the source. Following the outbursts, the total 0.5-10 keV flux from SGRJ1935+2154 increased in concordance to its bursting activity, with the flux at activation onset increasing by a factor of ∼7 following its strongest 2016 June outburst. A Swift/X-Ray Telescope observation taken 1.5 days prior to the onset of this outburst showed a flux level consistent with quiescence. We show that the flux increase is due to the PL or hot BB component, which increased by a factor of 25 compared to quiescence, while the cold BB component kT=0.47 keV remained more or less constant. The 2014 and 2015 outbursts decayed quasiexponentially with timescales of ∼40 days, while the stronger 2016 May and June outbursts showed a quick short-term decay with timescales of about four days. Our Arecibo radio observations set the deepest limits on the radio emission from a magnetar, with a maximum flux density limit of 14 μJy for the 4.6GHz observations and 7 μJy for the 1.4GHz observations. We discuss these results in the framework of the current magnetar theoretical models.

We analyze the prompt emission of GRB 100724B and GRB 160509A, two of the brightest Gamma-Ray Bursts (GRBs) observed by Fermi at MeV energies but surprisingly faint at 100 MeV energies. Time-resolved spectroscopy reveals a sharp high-energy cutoff at energies E c ∼ 20 − 60 MeV for GRB 100724B and E c ∼ 80−150 MeV for GRB 160509A. We first characterize phenomenologically the cutoff and its time evolution. We then fit the data to two models where the high-energy cutoff arises from intrinsic opacity to pair production within the source (τ γγ ): (i) a Band spectrum with τ γγ from the internal-shocks motivated model of Granot et al. (2008), and (ii) the photospheric model of Gill & Thompson (2014). Alternative explanations for the cutoff, such as an intrinsic cutoff in the emitting electron energy distribution, appear to be less natural. Both models provide a good fit to the data with very reasonable physical parameters, providing an estimate of bulk Lorentz factors in the range Γ ∼ 100−400, on the lower end of what is generally observed in Fermi GRBs. Surprisingly, their lower cutoff energies E c compared to other Fermi /LAT GRBs arise not predominantly from the lower Lorentz factors, but also at a comparable level from differences in variability time, luminosity, and high-energy photon index. Finally, particularly low E c values may prevent detection by Fermi /LAT, thus introducing a bias in the Fermi /LAT GRB sample against GRBs with low Lorentz factors or variability times.

We present Atacama Large Millimeter/submillimeter Array 97.5 GHz total intensity and linear polarization observations of the mm-band afterglow of GRB 190114C spanning 2.2-5.2 hours after the burst. We detect linear polarization at the ≈ 5 σ level, decreasing from Π = (0.87 ± 0.13)% to (0.60 ± 0.19)%, and evolving in polarization position angle from (10 ± 5) • to (−44 ± 12) • during the course of the observations. This represents the first detection and measurement of the temporal evolution of polarized radio/millimeter emission in a γ-ray burst. We show that the optical and Xray observations between 0.03 days and ∼ 0.3 days are consistent with a fast-cooling forward shock expanding into a wind environment. However, the optical observations at 0.03 days, as well as the radio and millimeter observations, arise from a separate component, which we interpret as emission from the reverse-shocked ejecta. Using the measured linear polarization, we constrain the coherence scale of tangled magnetic fields in the ejecta to an angular size of θ B ≈ 10 −3 radian, while the rotation of the polarization angle rules out the presence of large-scale, ordered axisymmetric magnetic fields, and in particular a large scale toroidal field, in the jet.

The short-duration ( 2 s) GRB 170817A in the nearby (D = 40 Mpc) elliptical galaxy NGC 4993 is the first electromagnetic counterpart of the first gravitational wave (GW) detection of a binary neutron-star (NS-NS) merger. It was followed by optical, IR, and UV emission from half a day up to weeks after the event, as well as late time X-ray and radio emission. The early UV, optical, and IR emission showed a quasi-thermal spectrum suggestive of radioactive-decay powered kilonova-like emission. Comparison to kilonova models favors the formation of a short-lived (∼ 1 s) hypermassive NS, which is also supported by the ∆t ≈ 1.74 s delay between the GW chirp signal and the prompt GRB onset. However, the late onset of the X-ray (8.9 days) and radio (16.4 days) emission, together with the low isotropic equivalent γ-ray energy output (E γ,iso ≈ 5 × 10 46 erg), strongly suggest emission from a narrow relativistic jet viewed off-axis. Here we set up a general framework for off-axis GRB jet afterglow emission, comparing analytic and numerical approaches, and showing their general predictions for short-hard GRBs that accompany binary NS mergers. The prompt GRB emission suggests a viewing angle well outside the jet's core, and we compare the afterglow lightcurves expected in such a case to the X-ray to radio emission from GRB 170817A. We fit an afterglow off-axis jet model to the X-ray and radio data and find that the observations are explained by a viewing angle θ obs ≈ 16 • − 26 • , GRB jet energy E ∼ 10 48.5 − 10 49.5 erg, and external density n ∼ 10 −5 − 10 −1 cm −3 for a ξ e ∼ 0.1 non-thermal electron acceleration efficiency.The detection of an SGRB, GRB 170817A (von Kienlin et al. 2017), in coincidence with the first GW detection of a NS-NS merger (Abbott et al. 2017b), in the relatively nearby (D = 40 Mpc) elliptical galaxy NGC 4993, provides the long awaited "smoking gun" that binary NS mergers in-