The early optical afterglow emission of several gamma-ray bursts (GRBs) shows a high linear polarization degree (PD) of tens of percent, suggesting an ordered magnetic field in the emission region. The light curves are consistent with being of a reverse shock (RS) origin. However, the magnetization parameter, $σ$, of the outflow is unknown. If $σ$ is too small, an ordered field in the RS may be quickly randomized due to turbulence driven by various perturbations so that the PD may not be as high as observed. Here we use the "Athena++" relativistic MHD code to simulate a relativistic jet with an ordered magnetic field propagating into a clumpy ambient medium, with a focus on how density fluctuations may distort the ordered magnetic field and reduce PD in the RS emission for different $σ$ values. For a given density fluctuation, we discover a clear power-law relationship between the relative PD reduction and the $σ$ value of the outflow. Such a relation may be applied to estimate $σ$ of the GRB outflows using the polarization data of early afterglows.
Gamma-ray bursts (GRBs) are powered by relativistic jets that exhibit intermittency over a broad range of timescales - from $ \sim $ ms to seconds. Previous numerical studies have shown that hydrodynamic (i.e., unmagnetized) jets that are expelled from a variable engine are subject to strong mixing of jet and cocoon material, which strongly inhibits the GRB emission. In this paper we conduct 3D RMHD simulations of mildly magnetized jets with power modulation over durations of 0.1 s and 1 s, and a steady magnetic field at injection. We find that when the jet magnetization at the launching site …
The spin of a number of black holes (BHs) in X-ray binaries (XBs) has been predicted (and, in at least three cases, confirmed by observations) by using a binary stellar evolution model with Case-C mass transfer . The rotational energy of such BHs is sufficient to power up (long) gamma-ray bursts and hypernovae (GRBs/HNe) and still leave a Kerr BH behind. However, strong magnetic fields (B fields) and/or dynamo effects in the interior of a BH-progenitor star may be capable of rapidly depleting the angular momentum from the stellar core, hence, preventing the formation of a collapsar. Thus, even…
We use a parent sample of 118 gamma-ray burst (GRB) afterglows, with known redshift and host galaxy extinction, to separate afterglows with and without signatures of dominant reverse-shock emission and to determine which physical conditions lead to a prominent reverse-shock emission. We identify 10 GRBs with reverse shock signatures - GRBs 990123, 021004, 021211, 060908, 061126, 080319B, 081007, 090102, 090424 and 130427A. By modeling their optical afterglows with reverse and forward shock analytic light curves and using Monte Carlo simulations, we estimate the parameter space of the physical …
We investigate the expected radio emission from the reverse shock of short GRBs, using the afterglow parameters derived from the observed short GRB light curves. In light of recent results suggesting that in some cases the radio afterglow is due to emission from the reverse shock, we examine the extent to which this component is detectable for short GRBs. In some GRBs, the standard synchrotron shock model predicts detectable radio emission from the reverse shock when none was seen. Because many physical parameters play a role in these estimates, our results highlight the need to more deeply ex…
We use ideal axisymmetric relativistic magnetohydrodynamic simulations to calculate the spindown of a newly formed millisecond, B ~ 10^{15} G, magnetar and its interaction with the surrounding stellar envelope during a core-collapse supernova (SN) explosion. The mass, angular momentum, and rotational energy lost by the neutron star are determined self-consistently given the thermal properties of the cooling neutron star's atmosphere and the wind's interaction with the surrounding star. The magnetar drives a relativistic magnetized wind into a cavity created by the outgoing SN shock. For high s…
Assuming that the early optical emission is dominated by the external reverse shock (RS) in the standard model of gamma-ray bursts (GRBs), we intend to constrain RS models with the initial Lorentz factor $Γ_0$ of the outflows based on the ROTSE-III observations. We consider two cases of the RS behavior: the relativistic shock and the non-relativistic shock. For homogeneous interstellar medium (ISM) and wind circum-burst environment, the constraints can be achieved by the fact that the peak flux $F_{\rm ν}$ at the RS crossing time should be lower than the observed upper limit $F_{\rm ν,limit}$.…
Astrophysical sources of relativistic jets or outflows, such as gamma-ray bursts (GRBs), active galactic nuclei (AGN) or micro-quasars, often show strong time variability. Despite such impulsive behavior, most models of these sources assume a steady state for simplicity. Here I consider a time-dependent outflow that is initially highly magnetized and divided into many well-separated sub-shells, as it experiences impulsive magnetic acceleration and interacts with the external medium. In AGN the deceleration by the external medium is usually unimportant and most of the initial magnetic energy is…
We consider dynamical scales in magnetized GRB outflows, using the solutions to the Riemann problem of expanding arbitrarily magnetized outflows (Lyutikov 2010). For high ejecta magnetization, the behavior of the forward shock closely resembles the so-called thick shell regime of the hydrodynamical expansion. The exception is at small radii, where the motion of the forward shock is determined by the dynamics of subsonic relativistic outflows. The behaviors of the reverse shock is different in fluid and magnetized cases: in the latter case, even for medium magnetization, sigma ~ 1, the reverse …
In the dynamical models of gamma-ray burst (GRB) afterglows, the uniform assumption of the shocked region is known as provoking total energy conservation problem. In this work we consider shocks originating from magnetized ejecta, extend the energy-conserving hydrodynamical model of Yan et al. (2007) to the MHD limit by applying the magnetized jump conditions from Zhang & Kobayashi (2005). Compared with the non-conservative models, our Lorentz factor of the whole shocked region is larger by a factor $\lesssim\sqrt{2}$. The total pressure of the forward shocked region is higher than the reverse…
Using binary evolution with Case-C mass transfer, the spins of several black holes (BHs) in X$-$ray binaries (XBs) have been predicted and confirmed (three cases) by observations. The rotational energy of these BHs is sufficient to power up long gamma$-$ray bursts and hypernovae (GRBs/HNe) and still leave a Kerr BH behind. However, strong magnetic fields and/or dynamo effects in the interior of such stars deplete their cores from angular momentum preventing the formation of collapsars. Thus, even though binaries can produce Kerr BHs, most of their rotation is acquired from the stellar mantle, …
Reverse shock (RS) emission can be used to probe the properties of the relativistic ejecta, especially the degree of magnetization $σ$, in gamma-ray burst (GRB) afterglows. However, there has been confusion in the literature regarding the physical condition for the RS formation, and the role of magnetic fields in the RS dynamics in the Poynting-flux-dominated regime is not fully understood. Exploiting the shock jump conditions, we characterize the properties of a magnetized RS. We compare the RS dynamics and forming conditions from different theories and numerical simulations, and reconcile th…
We discuss the synchrotron emission of fast cooling electrons in shocks. The fast cooling electrons behind the shocks can generate a position-dependent inhomogeneous electron distribution if they do not have enough time to mix homogeneously. This would lead to a very different synchrotron spectrum in low-frequency bands to that in the homogeneous case due to the synchrotron absorption. In this paper, we calculate the synchrotron spectrum in the inhomogeneous case in a gamma-ray burst (GRB). Both the forward shock and the reverse shock are considered. We find for the reverse shock dominated cas…
We present detailed multi-frequency, multi-epoch radio observations of GRB 140304A at $z=5.283$ from 1 to 86 GHz and 0.45 d to 89 d. The radio and mm data exhibit unusual multiple spectral components, which cannot be simply explained by standard forward and reverse shock scenarios. Through detailed multi-wavelength analysis spanning radio to X-rays, we constrain the forward shock parameters to $E_{\rm K, iso}\approx4.9\times10^{54}\,$erg, $A_* \approx 2.6\times10^{-2}$, $ε_{\rm e}\approx2.5\times10^{-2}$, $ε_{\rm B}\approx5.9\times10^{-2}$, $p\approx2.6$, and $θ_{\rm jet}\approx1.1^{\circ}$, y…
Jets producing gamma-ray bursts (GRBs) are likely to carry a neutron component that drifts with respect to the proton component. The neutron-proton collisions strongly heat the jet and generate electron-positron pairs. We investigate radiation produced by this heating using a new numerical code. Our results confirm the recent claim that collisional heating generates the observed Band-type spectrum of GRBs. We extend the model to study the effects of magnetic fields on the emitted spectrum. We find that the spectrum peak remains near 1 MeV for the entire range of the magnetization parameter $0<…
The shock system that produces the afterglow to GRBs consists of a forward- and a reverse-shock. For short GRBs, observational evidence for a reverse-shock has been sparse, however, the afterglow to GRB 160821B requires a reverse-shock at early times to explain the radio observations. GRB 160821B is additionally accompanied by the best-sampled macronova without a gravitational-wave detection, and an interesting late-time X-ray afterglow behaviour indicative of a refreshed-shock. The presence of an observed reverse-shock in an on-axis short GRB means that the reverse-shock should be considered …
We consider the double-shock structure of ultra-relativistic flows produced by the interaction of magnetized wind with magnetized external medium. The contact discontinuity (CD) is a special point in the flow - density, kinetic pressure and magnetic field experience a jump or are non-analytic on the CD. To connect dynamically the outside region (the forward shock flow) with the inside region (the reverse shock flow) requires resolving flow singularities at the contact discontinuity. On the CD the pressure is communicated exclusively by the magnetic field on both sides - the CD becomes an Alfve…
We report multi-color optical imaging and polarimetry observations of the afterglow of the first TeV- detected gamma-ray burst, GRB 190114C, using RINGO3 and MASTER II polarimeters. Observations begin 31 s after the onset of the GRB and continue until $\sim 7000\,$s post-burst. The light curves reveal a chromatic break at $\sim 400- 500\,$s, with initial temporal decay $α= 1.669 \pm 0.013$ flattening to $α\sim 1$ post-break, which we model as a combination of reverse and forward-shock components, with magnetization parameter $R_{\rm B} \sim 70$. The observed polarization degree decreases from …
We consider delayed, volumetric heating in a magnetized outflow that has broken out of a confining medium and expanded to a high Lorentz factor ($Γ\sim 10^2-10^3$) and low optical depth to scattering ($τ_{\rm T} \sim 10^{-3}-10^{-2}$). The energy flux at breakout is dominated by the magnetic field, with a modest contribution from quasi-thermal gamma rays whose spectrum was calculated in Paper I. We focus on the case of extreme baryon depletion in the magnetized material, but allow for a separate baryonic component that is entrained from a confining medium. Dissipation is driven by relativistic…
We study the observed correlations between the duration and luminosity of the early afterglow plateau and the isotropic gamma-ray energy release during the prompt phase. We discuss these correlations in the context of two scenarios for the origin of the plateaus. In the first one the afterglow is made by the forward shock and the plateau results from variations of the microphysics parameters while in the second one the early afterglow is made by a long-lived reverse shock propagating in a low Lorentz factor tail of the ejecta.
The interaction of relativistic magnetized ejecta with an ambient medium is studied for a range of structures and magnetization of the unshocked ejecta. We particularly focus on the effect of the ambient medium on the dynamics of an impulsive, high-sigma shell. It is found that for sufficiently high values of the initial magnetization $σ_0$ the evolution of the system is significantly altered by the ambient medium well before the shell reaches its coasting phase. The maximum Lorentz factor of the shell is limited to values well below $σ_0$; for a shell of initial energy $E=10^{52}E_{52}$ ergs …
A generic synchrotron external shock model is the widely preferred paradigm used to interpret the broad-band afterglow data of gamma-ray bursts (GRBs), including predicted observable signatures from a reverse shock which have been confirmed by observations. Investigations of the nature of the reverse shock emission can provide valuable insights into the intrinsic properties of the GRB ejecta. Here we briefly review the standard and the extended models of the reverse shock emission, discussing the connection between the theory and observations, including the implications of the latest observati…
What is conducted in the present work is the study of self-gravity effects on vertical structure of magnetized neutrino-dominated accretion disc (termed NDAF and considered as a central engine for Gamma-ray bursts (GRBs)). Some of the disc physical time scales such as viscous, cooling and diffusion time scales, that are supposed to play a pivotal role in the late time evolutions of the disc, have been studied. It is of our interest to investigate the possibility of the X-ray flares occurrence, observed in late time GRB's extended emission through the "magnetic barrier" and "fragmentation" proc…
Neutrino-cooled accretion flow around a black hole, produced by a compact binary merger, is a promising scenario for jet formation and magnetic-driven winds to explain short duration gamma ray bursts (GRBs) central engine and kilonovae based on GW170817 gravitational wave observation. Magnetorotational instability (MRI) turbulence and Blandford-Znajek (BZ) mechanism are expected to play key roles in the thermal equilibrium of the disk (balancing neutrino cooling) and in driving accretion and creating jets. Using the open-source GRMHD HARM-COOL code, we study the magnetically-driven evolution o…
We present a leptonic model on the external shock framework to describe the long- and short- lasting GeV component of some GRBs. This model was already applied successfully to GRB 090926A, and we extend it to describe the high-energy emission of GRB 090902B and GRB 090510. We argue that the high-energy emission consists of two components, one at MeV energies with a duration of a few seconds during the prompt phase, and a second GeV component lasting hundred of seconds after the prompt phase. The short high-energy component can be described as SSC emission from a reverse shock and the longer co…
A simplified one dimensional grid is used to model the evolution of magnetized plasma flow. We implement diffusion laws similar to those so-far used to model magnetic reconnection with Cellular Automata. As a novelty, we also explicitly superimpose a background flow. The aim is to numerically investigate the possibility that Self-Organized Criticality appears in a one dimensional magnetized flow. The cellular automaton's cells store information about the parameter relevant to the evolution of the system being modelled. Under the assumption that this parameter stands for the magnetic field, the…
We argue that small pitch angle synchrotron emission provides an important dissipation mechanism which has to be taken into account in the models of formation of relativistic magnetized gamma-ray burst (GRB) outflows from the newborn black holes and/or magnetars. We show that if the GRB outflow is proton loaded, spectral energy distribution of this emission is expected to sharply peak in 0.1-1~MeV energy band. If the small pitch angle synchrotron emission efficiently cools relativistic particles of the outflow, its spectrum below the peak energy is a powerlaw with spectral index alpha ~ -1, cl…
The reverse shock (RS) model is generally introduced to interpret the optical afterglows with the rapid rising and decaying, such as the early optical afterglow of GRB 990123 (which is also called optical flash). In this paper, we collected 11 gamma-ray bursts (GRBs) early optical afterglows, which have such signatures of dominant RS emission. Since the temporal slopes of the optical flashes are determined by both the medium density distribution index $k$ and the electron spectral index $p$, we apply the RS model of the thin shell case to the optical flashes and determine the ambient medium of…
We use high-resolution relativistic MHD simulations coupled with a radiative transfer code to compute multiwavelength afterglow light curves of magnetized ejecta of gamma-ray bursts interacting with a uniform circumburst medium. The aim of our study is to determine how the magnetization of the ejecta at large distance from the central engine influences the afterglow emission, and to assess whether observations can be reliably used to infer the strength of the magnetic field. We find that, for typical parameters of the ejecta, the emission from the reverse shock peaks for magnetization $σ_0 \si…
Gamma-ray Bursts (GRBs) are bursts of $γ$-rays generated from relativistic jets launched from catastrophic events such as massive star core collapse or binary compact star coalescence. Previous studies suggested that GRB emission is erratic, with no noticeable memory in the central engine. Here we report a discovery that similar light curve patterns exist within individual bursts for at least some GRBs. Applying the Dynamic Time Warping (DTW) method, we show that similarity of light curve patterns between pulses of a single burst or between the light curves of a GRB and its X-ray flare can be …
Global linear stability analysis of a self-similar solution describing a relativistic shell decelerated by an ambient medium is performed. The system is shown to be subject to the convective Rayleigh-Taylor instability, with a rapid growth of eigenmodes having angular scale much smaller than the causality scale. The growth rate appears to be largest at the interface separating the shocked ejecta and shocked ambient gas. The disturbances produced at the contact interface propagate in the shocked media and cause nonlinear oscillations of the forward and reverse shock fronts. It is speculated tha…
Since the launch of the Fermi Gamma-ray Space Telescope on 11 June 2008, significant detections of high energy emission have been reported only in six Gamma-ray Bursts (GRBs) until now. In this work we show that the lack of detection of a GeV spectrum excess in almost all GRBs, though somewhat surprisingly, can be well understood within the standard internal shock model and several alternatives like the photosphere-internal shock (gradual magnetic dissipation) model and the magnetized internal shock model. The delay of the arrival of the >100 MeV photons from some Fermi bursts can be interpret…
The strong variability of magnetic central engines of AGN and GRBs may result in highly intermittent strongly magnetized relativistic outflows. We find a new magnetic acceleration mechanism for such impulsive flows that can be much more effective than the acceleration of steady-state flows. This impulsive acceleration results in kinetic-energy-dominated flows at astrophysically relevant distances from the central source. For a spherical flow, a discrete shell ejected from the source over a time t_0 with Lorentz factor Gamma~1 and initial magnetization sigma_0 = B_0^2/(4 pi rho_0 c^2) >> 1 quic…
Gamma-ray bursts (GRBs) are extremely energetic events at cosmological distances. They provide unique laboratory to investigate fundamental physical processes under extreme conditions. Due to extreme luminosities, GRBs are detectable at very high redshifts and potential tracers of cosmic star formation rate at early epoch. While the launch of {\it Swift} and {\it Fermi} has increased our understanding of GRBs tremendously, many new questions have opened up. Radio observations of GRBs uniquely probe the energetics and environments of the explosion. However, currently only 30\% of the bursts are…
We present extensive radio and millimeter observations of the unusually bright GRB 130427A at z=0.340, spanning 0.67 to 12 days after the burst. Taken in conjunction with detailed multi-band UV, optical, NIR, and X-ray observations we find that the broad-band afterglow emission is composed of distinct reverse shock and forward shock contributions. The reverse shock emission dominates in the radio/millimeter and at <0.1 days in the UV/optical/NIR, while the forward shock emission dominates in the X-rays and at >0.1 days in the UV/optical/NIR. We further find that the optical and X-ray data requ…
Early afterglow observations of gamma-ray bursts (GRBs) are valuable for exploring the properties of their jets and ambient medium. We report our photometric and spectroscopic observations of GRB 210104A and discuss its jet properties with multiwavelength data. Our spectroscopic observation reveals several absorption features and a tentative redshift of 0.46 is identified. A bright optical flare that has a peak brightness of $R=13$ mag at $112\pm 7$~s was observed in the $R$ band during $67\sim 165$ seconds post the GRB trigger. The flux of the $R$-band afterglow decays with a slope of $α_{\rm…
The X-ray afterglows of GRBs 060413, 060522, 060607A and 080330 are characterized by plateaus that are followed by very sharp drops. An X-ray plateau is interpretable within the framework of the external forward shock model but the sharp drop is not. In this work we interpret these peculiar X-ray afterglow data as the central engine afterglows from some magnetized central engines, plausibly magnetars. In this model, the X-ray afterglows are powered by the internal magnetic energy dissipation and the sudden drop is caused by the collapse of the magnetar. Accordingly, the X-ray plateau photons s…
Many short GRBs show prompt tails lasting up to hundreds of seconds that can be energetically dominant over the initial sub-second spike. In this paper we develop an electromagnetic model of short GRBs that explains the two stages of the energy release, the prompt spike and the prompt tail. The key ingredient of the model is the recent discovery that an isolated black hole can keep its open magnetic flux for times much longer than the collapse time and, thus, can spin-down electromagnetically, driving the relativistic wind. First, the merger is preceded by an electromagnetic precursor wind. If…
Gamma-ray bursts (GRBs) might be powered by a black hole (BH) hyperaccretion systems via the Blandford-Znajek (BZ) mechanism or neutrino annihilation from neutrino-dominated accretion flows (NDAFs). Magnetic coupling (MC) between the inner disc and BH can transfer angular momentum and energy from the fast-rotating BH to the disc. The neutrino luminosity and neutrino annihilation luminosity are both efficiently enhanced by the MC process. In this paper, we study the structure, luminosity, MeV neutrinos, and gravitational waves (GWs) of magnetized NDAFs (MNDAFs) under the assumption that both th…
We perform GRMHD and RMHD simulations of weakly and highly magnetized gamma-ray burst (GRB) jets propagating in binary neutron star (BNS) merger ejecta. Using the simulations, we first find that mixing between the jet and cocoon, which is present in all types of jets, inhibits the formation of subphotospheric collisionless shocks. However, we show that a mild magnetization may lead to the formation of collisionless subshocks which allow efficient proton acceleration. We consider shear acceleration and diffusive shock acceleration at collimation shocks, internal shocks, shock breakout, and exte…
Previous studies have considered synchrotron as the emission mechanism for prompt Gamma-Ray Bursts (GRBs). These works have shown that the electrons must cool on a timescale comparable to the dynamic time at the source in order to satisfy spectral constraints while maintaining high radiative efficiency. We focus on conditions where synchrotron cooling is balanced by a continuous source of heating, and in which these constraints are naturally satisfied. Assuming that a majority of the electrons in the emitting region are contributing to the observed peak, we find that the energy per electron ha…
We present multi-wavelength observations and modeling of the exceptionally bright long $γ$-ray burst GRB 160625B. The optical and X-ray data are well-fit by synchrotron emission from a collimated blastwave with an opening angle of $θ_j\approx 3.6^\circ$ and kinetic energy of $E_K\approx 2\times10^{51}$ erg, propagating into a low density ($n\approx 5\times10^{-5}$ cm$^{-3}$) medium with a uniform profile. The forward shock is sub-dominant in the radio band; instead, the radio emission is dominated by two additional components. The first component is consistent with emission from a reverse shoc…
Relativistic jets are universal in long-duration gamma-ray burst (GRB) models. Before breaking out, they must propagate in the progenitor envelope along with a forward shock and a reverse shock forming at the jet head. Both electrons and protons will be accelerated by the shocks. High energy neutrinos could be produced by these protons interacting with stellar materials and electron-radiating photons. The jet will probably be collimated, which may have a strong effect on the final neutrino flux. Under the assumption of a power-law stellar-envelope density profile $ρ\propto r^{-α}$ with an inde…
One of the most energetic gamma-ray bursts GRB 110731A, was observed from optical to GeV energy range by Fermi and Swift Observatories, and by the MOA and GROND optical telescopes. The multiwavelength observations over different epochs (from trigger time to more than 800 s) showed that the spectral energy distribution was better fitted by a wind afterglow model. We present a leptonic model based on an early afterglow that evolves in a stellar wind to describe the multiwavelength light curves observations. In particular, the origin of the LAT emission is explained through the superposition of s…
The X-ray afterglow plateau emission observed in many Gamma-ray Bursts (GRBs) has been interpreted as either being fueled by fallback onto a newly formed black hole, or by the spin-down luminosity of an ultra-magnetized millisecond neutron star. If the latter model is assumed, GRB X-ray afterglow light curves can be analytically reproduced. We fit a sample of GRB X-ray plateaus, interestingly yielding a distribution in the magnetic field versus spin period (B-P) diagram consistent with $B\propto P^{7/6}$. This is expected from the well-established physics of the spin-up line minimum period for…
We consider a magnetar flare model for fast radio bursts (FRBs). We show that millisecond burst of sufficient power can be generated by synchrotron maser emission ignited at the reverse shock propagating through the weakly magnetized material that forms the magnetar flare. If the maser emission is generated in an anisotropic regime (due to the geometry of the production region or presence of an intense external source of stimulating photons) the duration of the maser flashes is similar to the magnetar flare duration even if the shock front radius is large. Our scenario allows relaxing the requ…
GRB 120308A, a long duration $γ-$ray burst detected by {\it Swift}, was distinguished by a highly-polarized early optical afterglow emission that strongly suggests an ordered magnetic field component in the emitting region. In this work we model the optical and X-ray emission in the reverse and forward shock scenario and show that the strength of the magnetic field in reverse shock region is $\sim 10$ times stronger than that in the forward shock region. Consequently the outflow powering the highly-polarized reverse shock optical emission was mildly-magnetized at a degree $σ\sim$ a few percent…
A planned rapid submillimeter (submm) Gamma Ray Burst (GRBs) follow-up observations conducted using the Greenland Telescope (GLT) is presented. The GLT is a 12-m submm telescope to be located at the top of the Greenland ice sheet, where the high-altitude and dry weather porvides excellent conditions for observations at submm wavelengths. With its combination of wavelength window and rapid responding system, the GLT will explore new insights on GRBs. Summarizing the current achievements of submm GRB follow-ups, we identify the following three scientific goals regarding GRBs: (1) systematic dete…
GRB 240205B: A Reverse Shock Detected in Rapid Response Radio Observations
University of New Mexico（新墨西哥大学）；Texas Tech University（德克萨斯理工大学）；Australia Telescope National Facility, CSIRO（澳大利亚望远镜国家设施，CSIRO）；George Washington University（乔治华盛顿大学）；Trottier Space Institute at McGill（麦吉尔大学特罗蒂尔空间研究所）；McGill University（麦吉尔大学）；International Centre for Radio Astronomy Research, Curtin University（国际射电天文研究中心，科廷大学）；The University of Sydney（悉尼大学）；CSIRO Space and Astronomy（CSIRO空间与天文学）；ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)（ARC引力波发现卓越中心（OzGrav））；David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto（多伦多大学大卫·邓拉普天体物理学系）
Internal shocks provide a plausible heating mechanism in the jets of gamma-ray bursts (GRBs). Shocks occurring below the jet photosphere are mediated by radiation. It was previously found that radiation mediated shocks (RMSs) inside GRB jets are inefficient photon producers, and the photons that mediate the RMS must originate from an earlier stage of the explosion. We show that this conclusion is valid only for non-magnetized jets. RMSs that propagate in moderately magnetized plasma develop a collisionless subshock which locally heats the plasma to a relativistic temperature, and the hot elect…
Optical re-brightenings in the afterglows of some gamma-ray bursts (GRBs) are unexpected within the framework of the simple external shock model. While it has been suggested that the central engines of some GRBs are newly born magnetars, we aim to relate the behaviors of magnetars to the optical re-brightenings. A newly born magnetar will lose its rotational energy in the form of Poynting-flux, which may be converted into a wind of electron-positron pairs through some magnetic dissipation processes. As proposed by Dai (2004), this wind will catch up with the GRB outflow and a long-lasting reve…
We present the first results from a recently concluded study of GRBs at $z\gtrsim5$ with the Karl G. Jansky Very Large Array (VLA). Spanning $1$ to $85.5$ GHz and 7 epochs from 1.5 to 82.3 d, our observations of GRB 140311A are the most detailed joint radio and millimeter observations of a GRB afterglow at $z\gtrsim5$ to date. In conjunction with optical/near-IR and X-ray data, the observations can be understood in the framework of radiation from a single blast wave shock with energy $E_{\rm K,iso}\approx8.5\times10^{53}$ erg expanding into a constant density environment with density, $n_0\app…
