Early and late multiwavelength observations play an important role in determining the nature of the progenitor, circumburst medium, physical processes and emitting regions associated to the spectral and temporal features of bursts. GRB 180720B is a long and powerful burst detected by a large number of observatories in multiwavelenths that range from radio bands to sub-TeV gamma-rays. The simultaneous multiwavelength observations were presented over multiple periods of time beginning just after the trigger time and extending for more than 30 days. The temporal and spectral analysis of Fermi LAT observations suggests that it presents similar characteristics to other bursts detected by this instrument. Coupled with X-ray and optical observations, the standard external-shock model in a homogeneous medium is favored by this analysis. The X-ray flare is consistent with the synchrotron self-Compton (SSC) model from the reverse-shock region evolving in a thin shell and long-lived LAT, X-ray and optical data with the standard synchrotron forward-shock model. The best-fit parameters derived with the Markov chain Monte Carlo simulations indicate that the outflow is endowed with magnetic fields and that the radio observations are in the self-absorption regime. The SSC forward-shock model with our parameters can explain the LAT photons beyond the synchrotron limit as well as the emission recently reported by the HESS Collaboration. Subject headings: Gamma-rays bursts: individual (GRB 180720B) -Physical data and processes: acceleration of particles -Physical data and processes: radiation mechanism: nonthermal -ISM: general -magnetic fields 1 https://fermi.gsfc.nasa.gov/ssc/observations/types/grbs/lat grbs/ arXiv:1905.13572v2 [astro-ph.HE]

The production of both gravitational waves and short gamma-ray bursts (sGRBs) is widely associated with the merger of compact objects. Several studies have modelled the evolution of the electromagnetic emission using the synchrotron emission produced by the deceleration of both a relativistic top-hat jet seen off-axis, and a wide-angle quasi-spherical outflow (both using numerical studies). In this study we present an analytical model of the synchrotron and synchrotron self-Compton (SSC) emission for an off-axis top-hat jet and a quasispherical outflow. We calculate the light curves obtained from an analytic model in which the synchrotron and SSC emission (in the fast-or slow-cooling regime) of an off-axis top-hat jet and a quasi-spherical outflow are decelerated in either a homogeneous or a wind-like circumburst medium. We show that the synchrotron emission of the quasi-spherical outflow is stronger than that of the off-axis jet during the first ∼ 10 -20 days, and weaker during the next 80 days. Moreover, we show that if the off-axis jet is decelerated in a windlike medium, then the SSC emission is very likely to be detected. Applying a MCMC code to our model (for synchrotron emission only), we find the best-fit values for the radio, optical and X-ray emission of GRB 170817A which are in accordance with observations. For GRB 170817A, we find using our model that the synchrotron emission generated by the quasi-spherical outflow and off-axis top-hat jet increase as F ν ∝ t α with α 0.8 and α > 3, respectively. Finally, we obtain the correspondent SSC light curves which are in accordance with the very-high-energy gamma-ray upper limits derived with the GeV -TeV observatories.

We present the afterglow light curves produced by the deceleration of a nonrelativistic ejecta mass in a stratified circumstellar medium with a density profile n(r) ∝ r −k with k = 0, 1, 1.5, 2, and 2.5. Once the ejecta mass is launched with equivalent kinetic energy parameterized by E(>β) ∝ β −α (where β is the ejecta velocity) and propagates into the surrounding circumstellar medium, it first moves with constant velocity (the free-coasting phase), and later it decelerates (the Sedov–Taylor expansion). We present the predicted synchrotron and synchrotron self-Compton light curves during the free-coasting phase and the subsequent Sedov–Taylor expansion. In particular cases, we show the corresponding light curves generated by the deceleration of several ejecta masses with different velocities launched during the coalescence of binary compact objects and the core collapse of dying massive stars, which will contribute at distinct timescales, frequencies, and intensities. Finally, using the multiwavelength observations and upper limits collected by a large campaign of orbiting satellites and ground telescopes, we constrain the parameter space of both the kilonova (KN) afterglow in GW170817 and the possibly generated KN afterglow in S190814bv. Further observations on timescales of years post-merger are needed to derive tighter constraints.

Gamma-ray bursts (GRBs) are fascinating extragalactic objects. They represent a fantastic opportunity to investigate unique properties not exhibited in other sources. Multiwavelength afterglow observations from some short- and long-duration GRBs reveal an atypical long-lasting emission that evolves differently from the canonical afterglow light curves favoring the off-axis emission. We present an analytical synchrotron afterglow scenario and the hydrodynamical evolution of an off-axis top-hat jet decelerated in a stratified surrounding environment. The analytical synchrotron afterglow model is shown during the coasting, deceleration (off- and on-axis emission), and post–jet break decay phases, and the hydrodynamical evolution is computed by numerical simulations showing the time evolution of the Doppler factor, the half-opening angle, the bulk Lorentz factor, and the deceleration radius. We show that numerical simulations are in good agreement with those derived with our analytical approach. We apply the current synchrotron model and successfully describe the delayed nonthermal emission observed in a sample of long and short GRBs with evidence of off-axis emission. Furthermore, we provide constraints on the possible afterglow emission by requiring the multiwavelength upper limits derived for the closest Swift-detected GRBs and promising gravitational-wave events.