The physics of gamma-ray bursts & relativistic jets

We provide a comprehensive review of major developments in our understanding of gamma-ray bursts, with particular focus on the discoveries made within the last fifteen years when their true nature was uncovered. We describe the observational properties of photons from the radio to multi-GeV bands, both in the prompt emission and the afterglow phases. Mechanisms for the generation of these photons in GRBs are discussed and confronted with observations to shed light on the physical properties of these explosions, their progenitor stars and the surrounding medium. After presenting observational evidence that a powerful, collimated, jet moving at close to the speed of light is produced in these explosions, we describe our current understanding regarding the generation, acceleration, and dissipation of the jet and compare these properties with jets associated with AGNs and pulsars. We discuss mounting observational evidence that long duration GRBs are produced when massive stars die, and that at least some short duration bursts are associated with old, roughly solar mass, compact stars. The question of whether a black-hole or a strongly magnetized, rapidly rotating neutron star is produced in these explosions is also discussed. We provide a brief summary of what we have learned about relativistic collisionless shocks and particle acceleration from GRB afterglow studies, and discuss the current understanding of radiation mechanism during the prompt emission phase. We discuss theoretical predictions of possible high-energy neutrino emission from GRBs and the current observational constraints. Finally, we discuss how these explosions may be used to study cosmology, e.g. star formation, metal enrichment, reionization history, as well as the formation of first stars and galaxies in the universe.

/pdf/the-physics-of-gamma-ray-bursts-and-relativistic-jets-571z4a98ye.pdf

The Physics of Gamma-Ray Bursts and Relativistic Jets

Since the successful launch of NASA's dedicated gamma-ray burst (GRB) mission, Swift, the study of cosmological GRBs has entered a new era. Here I review the rapid observational and theoretical progress in this dynamical research field during the first two-year of the Swift mission, focusing on how observational breakthroughs have revolutionized our understanding of the physical origins of GRBs. Besides summarizing how Swift helps to solve some pre-Swift mysteries, I also list some outstanding problems raised by the Swift observations. An outlook of GRB science in the future, especially in the GLAST era, is briefly discussed.

http://www.physics.unlv.edu/~bzhang/zhang07-ChJAA.pdf

Gamma-Ray Bursts in the Swift Era

One of the few ways to measure the properties of dark energy is to extend the Hubble diagram (HD) to higher redshifts with gamma-ray bursts (GRBs). GRBs have at least five properties (their spectral lag, variability, spectral peak photon energy, time of the jet break, and the minimum rise time) that have correlations to the luminosity of varying quality. In this paper I construct a GRB HD with 69 GRBs over a redshift range from 0.17 to >6, with half the bursts having a redshift larger than 1.7. This paper uses over 3.6 times as many GRBs and 12.7 times as many luminosity indicators as any previous GRB HD work. For the gravitational lensing and Malmquist biases, I find that the biases are small, with an average of 0.03 mag and an rms scatter of 0.14 mag in the distance modulus. The GRB HD is well behaved and nicely delineates the shape of the HD. The reduced χ2 for the fit to the concordance model is 1.05, and the rms scatter about the concordance model is 0.65 mag. This accuracy is just a factor of 2.0 times that gotten for the same measure from all the big supernova surveys. I fit the GRB HD to a variety of models, including where the dark energy has its equation of state parameter varying as w(z) = w0 + waz/(1 + z). I find that the concordance model is consistent with the data, that is, the dark energy can be described well as a cosmological constant that does not change with time.

http://iopscience.iop.org/article/10.1086/511742/pdf

The Hubble Diagram to Redshift >6 from 69 Gamma-Ray Bursts

Without imposing any theoretical models and assumptions, we present a multivariable regression analysis to several observable quantities for a sample of 15 gamma-ray bursts (GRBs). The observables used in the analysis include the isotropic gamma-ray energy (Eγ,iso), the peak energy of the νFν spectrum in the rest frame (E), and the rest-frame break time of the optical afterglow light curves (t). A strong dependence of Eγ,iso on E and t is derived, which reads Eγ,iso/1052 ergs = (0.85 ± 0.21)(E/100 keV)1.94±0.17(t/1 day)-1.24±0.23 in a flat universe with ΩM = 0.28 and H0 = 71.3 km s-1 Mpc-1. We also extend the analysis to the isotropic afterglow energies in the X-ray and optical bands, respectively, and find that they are essentially not correlated with E and t. Regarding the Eγ,iso relationship as a luminosity indicator, we explore the possible constraints on the cosmological parameters using the GRB sample. Since there is no low-redshift GRB sample to calibrate this relationship, we weight the probability of using the relationship in each cosmology to serve as a standard candle by χ2 statistics and then use this cosmology-weighted standard candle to evaluate cosmological parameters. Our results indicate that 0.05 < ΩM < 0.50 at the 1 σ level, with the most probable value of ΩM being 0.28. The best value of ΩΛ is 0.64, but it is less constrained. Only a loose limit of ΩΛ < 1.2 is obtained at the 1 σ level. In the case of a flat universe, the 1 σ constraints are 0.13 < ΩM < 0.49 and 0.50 < ΩΛ < 0.85, respectively. The deceleration factor (q) and its cosmological evolution (dq/dz) are also investigated with an evolutionary form of q = q0 + z dq/dz. The best-fit values are (q0, dq/dz) = (-1.00, 1.12), with -2.23 < q0 < 0.26 and -0.07 < dq/dz < 3.48 at the 1 σ level. The inferred transition redshift between the deceleration and acceleration phases is 0.78 (1 σ). Through Monte Carlo simulations, we find that the GRB sample satisfying our relationship observationally tends to be a soft and bright one and that the constraints on the cosmological parameters can be much improved either by enlarging the sample size or by increasing the observational precision. Although the sample may not expand significantly in the Swift era, a significant increase of the sample is expected in the long-term future. Our simulations indicate that with a sample of 50 GRBs satisfying our multivariable standard candle, one can achieve a constraint to the cosmological parameters comparable to that derived from 157 Type Ia supernovae. Furthermore, the detections of a few high-redshift GRBs satisfying the correlation could greatly tighten the constraints. Identifying high-z GRBs and measuring their E and t are therefore essential for the GRB cosmology in the Swift era.

https://iopscience.iop.org/article/10.1086/491594/pdf

Model-independent multivariable gamma-ray burst luminosity indicator and its possible cosmological implications

We present UBVRCIC photometry of the optical afterglow of the gamma-ray burst GRB 021004 taken at the Nordic Optical Telescope between approximately 8 hours and 30 days after the burst. These data are combined with an analysis of the 87 ks Chandra X-ray observations of GRB 021004 taken at a mean epoch of 33 hr after the burst to investigate the nature of this GRB. We find an intrinsic spectral slope at optical wavelengths of ?UH = 0.39 ? 0.12 and an X-ray slope of ?X = 0.94 ? 0.03. There is no evidence for color evolution between 8.5 hr and 5.5 days after the burst. The optical decay becomes steeper approximately 5 days after the burst. This appears to be a gradual break due to the onset of sideways expansion in a collimated outflow. Our data suggest that the extragalactic extinction along the line of sight to the burst is between AV ? 0.3 and 0.5 and has an extinction law similar to that of the Small Magellanic Cloud. The optical and X-ray data are consistent with a relativistic fireball with the shocked electrons being in the slow cooling regime and having an electron index of p = 1.9 ? 0.1. The burst occurred in an ambient medium that is homogeneous on scales larger than approximately 1018 cm but inhomogeneous on smaller scales. The mean particle density is similar to what is seen for other bursts (0.1 cm-3 n 100 cm-3). Our results support the idea that the brightening seen at approximately 0.1 days was due to interaction with a clumpy ambient medium within 1017?1018 cm of the progenitor. The agreement between the predicted optical decay and that observed approximately 10 minutes after the burst suggests that the physical mechanism controlling the observed flux at t ? 10 minutes is the same as the one operating at t > 0.5 days.

http://iopscience.iop.org/article/10.1086/374235/pdf

Javier Mendez Alvarez

Papers

Optical Photometry of GRB 021004: The First Month