Early and late multiwavelength observations play an important role in determining the nature of the progenitor, circumburst medium, physical processes, and emitting regions associated with the ...spectral and temporal features of bursts. GRB 180720B is a long and powerful burst detected by a large number of observatories at multiple wavelengths 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 to more than 30 days. The temporal and spectral analysis of Fermi Large Area Telescope (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 previous LAT, X-ray, and optical data with the standard synchrotron forward-shock model. The best-fit parameters derived with 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.
Binary neutron star mergers are believed to eject significant masses with a diverse range of velocities. Once these ejected materials begin to be decelerated by a homogeneous medium, relativistic ...electrons are mainly cooled down by synchrotron radiation, generating a multiwavelength long-lived afterglow. Analytic and numerical methods illustrate that the outermost matter, the merger shock-breakout material, can be parametrized by power-law velocity distributions . Considering that the shock-breakout material is moving on-axis toward the observer and the relativistic jet off-axis, we compute the light curves during the relativistic and the lateral expansion phase. As a particular case, we successfully describe the X-ray, optical, and radio light curves alongside the spectral energy distribution from the recently discovered gravitational-wave transient GW170817, when the merger shock-breakout material moves with mildly relativistic velocities and achieves the near-Newtonian phase and the jet moves with relativistic velocities. Future electromagnetic counterpart observations of this binary system could be able to evaluate different properties of these light curves.
Abstract
As we further our studies on gamma-ray bursts (GRBs), via both theoretical models and observational tools, more and more options begin to open for exploration of its physical properties. As ...GRBs are transient events primarily dominated by synchrotron radiation, it is expected that the synchrotron photons emitted by GRBs should present some degree of polarization throughout the evolution of the burst. Whereas observing this polarization can still be challenging due to the constraints on observational tools, especially for short GRBs, it is paramount that the groundwork is laid for the day we have abundant data. In this work, we present a polarization model linked with an off-axis spreading top-hat jet synchrotron scenario in a stratified environment with a density profile
n
(
r
) ∝
r
−
k
. We present this model's expected temporal polarization evolution for a realistic set of afterglow parameters constrained within the values observed in the GRB literature for four degrees of stratification
k
= 0, 1, 1.5, and 2 and two magnetic field configurations with high extreme anisotropy. We apply this model and predict polarization from a set of GRBs exhibiting off-axis afterglow emission. In particular, for GRB 170817A, we use the available polarimetric upper limits to rule out the possibility of an extremely anisotropic configuration for the magnetic field.
Very-high-energy (VHE; ≥ 10 GeV) photons are expected from the nearest and brightest gamma-ray bursts (GRBs). VHE photons, at energies higher than 300 GeV, were recently reported by the MAGIC ...Collaboration for this burst. Immediately, GRB 190114C was followed up by a massive observational campaign covering a large fraction of the electromagnetic spectrum. In this Letter, we obtain the Large Area Telescope (LAT) light curve of GRB 190114C and show that it exhibits similar features to other bright LAT-detected bursts; the first high-energy photon (≥100 MeV) is delayed with the onset of the prompt phase and the flux light curve exhibits a long-lasting emission (much longer than the prompt phase) and a short-lasting bright peak (located at the beginning of long-lasting emission). Analyzing the multi-wavelength observations, we show that the short-lasting LAT and Gamma-Ray Burst Monitor bright peaks are consistent with the synchrotron self-Compton reverse-shock model, and that the long-lasting observations are consistent with the standard synchrotron forward-shock model that evolves from a stratified stellar-wind-like medium to a uniform interstellar-medium-like medium. Given the best-fit values, a bright optical flash produced by synchrotron reverse-shock emission is expected. From our analysis we infer that the high-energy photons are produced in the deceleration phase of the outflow, and some additional processes to synchrotron in the forward shocks should be considered to properly describe the LAT photons with energies beyond the synchrotron limit. Moreover, we claim that an outflow endowed with magnetic fields could describe the polarization and properties exhibited in the light curve of GRB 190114C.
The short GRB 170817A, detected by the Fermi Gamma-ray Burst Monitor, orbiting satellites and ground-based telescopes, was the electromagnetic counterpart of a gravitational-wave transient (GW170817) ...from a binary neutron star merger. After this merger, the γ-ray light curve exhibited a faint peak at ∼1.7 s and the X-ray, optical, and radio light curves displayed extended emission that increased in brightness up to ∼160 days. In this paper, we show that the X-ray, optical and radio fluxes are consistent with the synchrotron forward-shock model viewed off-axis when the matter in the outflow is parameterized through a power-law velocity distribution. We discuss the origin of the γ-ray peak in terms of internal and external shocks. We show that the γ-ray flux might be consistent with a synchrotron self-Compton reverse-shock model observed at high latitudes. Comparing the best-fit values obtained after describing the γ-ray, X-ray, optical, and radio fluxes with our model, we find that the afterglow and γ-ray emission occurred in different regions and also find evidence to propose that the progenitor environment was entrained with magnetic fields; therefore we argue for the presence of magnetic field amplification in the binary neutron star merger.
Abstract
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.
Abstract
Sub-relativistic materials launched during the merger of binary compact objects and the core collapse of massive stars acquire velocity structures when expanding in a stratified environment. ...The remnant (either a spinning magnetized neutron star (NS) or a central black hole) from the compact object or core collapse could additionally inject energy into the afterglow via spin-down luminosity or/and by accreting fallback material, producing a refreshed shock, modifying the dynamics, and leading to rich radiation signatures at distinct timescales and energy bands with contrasting intensities. We derive the synchrotron light curves evolving in a stratified environment when a power-law velocity distribution parameterizes the energy of the shock, and the remnant continuously injects energy into the blast wave. As the most relevant case, we describe the latest multiwavelength afterglow observations (≳900 days) of the GW170817/GRB 170817A event via a synchrotron afterglow model with energy injection of a sub-relativistic material. The features of the remnant and the synchrotron emission of the sub-relativistic material are consistent with a spinning magnetized NS and the faster
blue
kilonova afterglow, respectively. Using the multiband observations of some short bursts with evidence of kilonovae, we provide constraints on the expected afterglow emission.
The detection of gravitational waves together with their electromagnetic counterpart, in the gamma-ray burst GRB 170817A, marked a new era of multi-messenger astronomy. Several theoretical models ...have been proposed to explain the atypical behavior of this event. Recently, it was shown th0at the multiwavelength afterglow of GRB 170817A was consistent with a synchrotron forward-shock model when the outflow was viewed off-axis, decelerated in a uniform medium and parameterized through a power-law velocity distribution. Motivated by the upper limits on the very high-energy emission, and the stratified medium in the close vicinity of a binary neutron star merger proposed to explain the gamma-ray flux in the short GRB 150101B, we extend the mechanism proposed to explain GRB 170817A to a more general scenario deriving the synchrotron self-Compton and synchrotron forward-shock model when the off-axis outflow is decelerated in a uniform and stratified circumburst density. As particular cases, we show that the delayed and long-lasting afterglow emission observed in GRB 080503, GRB 140903A, GRB 150101B, and GRB 160821B could be interpreted by a scenario similar to the one used to describe GRB 170817A. In addition, we show that the proposed scenario agrees with the Major Atmospheric Gamma-ray Imaging Cherenkov telescope, Fermi-Large Area Telescope, and High Energy Stereoscopic System upper limits on gamma-ray emission from GRB 160821B and GRB 170817A.
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− (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.
The production of both gravitational waves and short gamma-ray bursts (sGRBs) is widely associated with the merger of compact objects. Several studies have modeled 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 quasi-spherical 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 wind-like medium, then the SSC emission is very likely to be detected. Applying an 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 that 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 ∝ t with 0.8 and > 3, respectively. Finally, we obtain the corresponding SSC light curves that are in accordance with the very-high-energy gamma-ray upper limits derived with the GeV-TeV observatories.
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