Structure Of Gamma-ray Bursts Jets Research Articles

Three-dimensional numerical simulations of structured gamma-ray burst jets

ABSTRACT After the detection of GRB 170817A, the first unambiguous off-axis gamma-ray burst (GRB), several studies tried to understand the structure of GRB jets. The initial jet structure (directly produced by the central engine) can be partially preserved, or can be completely modified by the interaction with the environment. In this study, we perform three-dimensional, special relativistic hydrodynamics simulations of long GRB jets evolving through a massive progenitor star. Different jet scenarios were considered: Top-hat, Gaussian jets dominated by pressure or by kinetic energy, as well as a model of a supernova (SN) plus a jet both propagating through the progenitor. We found that, while propagating inside the progenitor star, jets with different initial structures are nearly indistinguishable. Kinetic-dominated jets are faster and more collimated than pressure-dominated jets. The dynamics of jets inside the progenitor star strongly depends on the presence of an associated SN, which can substantially decelerate the jet propagation. We show that the initial structure of GRB jets is preserved, or not, mainly depending on the jet collimation. The initial structure is preserved in uncollimated jets, i.e. jets which move through low-density environments. Meanwhile, jets which move through dense environments are shaped by the interaction with the medium and remain collimated.

Observational constraints on the structure of gamma-ray burst jets

Motivated by GW170817 we examine constraints that observations put on the angular structure of long gamma-ray burst (GRB) jets. First, the relatively narrow observed distribution of $E_{X}/E_{\gamma}$ (the isotropic equivalent early X-ray afterglow to prompt $\gamma$-ray energy ratio) implies that at any angle that $\gamma$-rays are emitted the Lorentz factor must be high. Specifically, the Lorentz factor of $\gamma$-ray emitting material cannot drop rapidly with angle, and must be $\Gamma(\theta)\gtrsim 50$ even if there are angles for which the gamma-ray received energy is lower by three orders of magnitude compared to the jet core. Second, jets with an angular structure of the $\gamma$-ray emission that over-produce events with a $\gamma$-ray luminosity below the peak of the observed luminosity function are ruled-out. This eliminates models in which the $\gamma$-ray energy angular distribution isn't sufficiently steep and the Lorentz factor distribution isn't sufficiently shallow. Finally, models with a steep structure (e.g. Gaussian) which are detected away from the jet core generate afterglow light-curves that were never observed. We conclude that even if the jet kinetic energy distribution drops continuously with latitude, efficient $\gamma$-ray emission seems to be restricted to material with $\Gamma\gtrsim 50$ and is most likely confined to a narrow region around the core. While our study is confined to long GRBs, where the observed sample is larger and more complete, there are indications that similar conclusions may be applicable also to short GRBs. We discuss the possible implications to the $\gamma$-rays observed in GRB 170817A.

Constraining the Structure of Gamma‐Ray Burst Jets through the logN–logSDistribution

A general formalism is developed for calculating the luminosity function and the expected number N of observed gamma-ray bursts (GRBs) above a peak photon flux S for an arbitrary GRB jet structure. This formalism directly provides the true GRB rate for any jet model, instead of first calculating the GRB rate assuming isotropic emission and then introducing a "correction factor" to account for effects of the GRB jet structure, as was done in previous works. We apply it to the uniform jet (UJ) and universal structured jet (USJ) models for the structure of GRB jets and perform fits to the observed log N- log S distribution from the GUSBAD catalog, which contains 2204 BATSE bursts. We allow for a scatter in the peak luminosity L for a given jet half-opening angle θj (viewing angle θobs) in the UJ (USJ) model, which is implied by observations. A core angle θc and an outer edge at θmax are introduced for the structured jet, and a finite range of opening angles θmin ≤ θj ≤ θmax is assumed for the uniform jets. The efficiency for producing γ-rays, γ, and the energy per solid angle in the jet, , are allowed to vary with θj (θobs) in the UJ (USJ) model, γ ∝ θ-b and ∝ θ-a. We find that a single power-law luminosity function provides a good fit to the data. Such a luminosity function arises naturally in the USJ model, while in the UJ model it implies a power-law probability distribution for θj, P(θj) ∝ θ. The value of q cannot be directly determined from the fit to the observed log N- log S distribution, and an additional assumption on the value of a or b is required. Alternatively, an independent estimate of the true GRB rate would enable one to determine a, b, and q. The implied values of θc (or θmin) and θmax are close to the current observational limits. The true GRB rate for the USJ model is found to be RGRB(z = 0) = 0.86 Gpc-3 yr-1 (1 σ), while for the UJ model it is higher by a factor f(q), which strongly depends on the unknown value of q.

A Unified Jet Model of X‐Ray Flashes, X‐Ray–rich Gamma‐Ray Bursts, and Gamma‐Ray Bursts. I. Power‐Law–shaped Universal and Top‐Hat–shaped Variable Opening Angle Jet Models

HETE-2 has provided strong evidence that the properties of X-ray flashes (XRFs), X-ray-rich gamma-ray bursts (GRBs), and GRBs form a continuum, and therefore that these three kinds of bursts are the same phenomenon. A key feature found by HETE-2 is that the density of bursts is roughly constant per logarithmic interval in burst fluence SE and observed spectral peak energy E, and in isotropic-equivalent energy Eiso and spectral peak energy Epeak in the rest frame of the burst. In this paper, we explore a unified jet model of all three kinds of bursts, using population synthesis simulations of the bursts and detailed modeling of the instruments that detect them. We show that both a variable jet opening angle model in which the emissivity is a constant independent of the angle relative to the jet axis and a universal jet model in which the emissivity is a power-law function of the angle relative to the jet axis can explain the observed properties of GRBs reasonably well. However, if one tries to account for the properties of XRFs, X-ray-rich GRBs, and GRBs in a unified picture, the extra degree of freedom available in the variable jet opening angle model enables it to explain the observations reasonably well while the power-law universal jet model cannot. The variable jet opening angle model of XRFs, X-ray-rich GRBs, and GRBs implies that the energy Eγ radiated in gamma rays is ~100 times less than has been thought. The model also implies that most GRBs have very small jet opening angles (~half a degree). This suggests that magnetic fields may play an important role in GRB jets. It also implies that there are ~104-105 more bursts with very small jet opening angles for every burst that is observable. If this is the case, the rate of GRBs could be comparable to the rate of Type Ic core-collapse supernovae. These results show that XRFs may provide unique information about the structure of GRB jets, the rate of GRBs, and the nature of Type Ic supernovae.

Gravitational wave memory of gamma-ray burst jets

Gamma-Ray Bursts (GRBs) are now considered as relativistic jets. We analyze the gravitational waves from the acceleration stage of the GRB jets. We show that (i) the point mass approximation is not appropriate if the opening half-angle of the jet is larger than the inverse of the Lorentz factor of the jet, (ii) the gravitational waveform has many step function like jumps, and (iii) the practical DECIGO and BBO may detect such an event if the GRBs occur in Local group of galaxy. We found that the light curve of GRBs and the gravitational waveform are anti-correlated so that the detection of the gravitational wave is indispensable to determine the structure of GRB jets.