Conveners
Photospheric Emission in GRBs: Block 1
- Damien Begue (Bar Ilan University)
- Gregory Vereshchagin (ICRANet)
Photospheric Emission in GRBs: Block 2
- Gregory Vereshchagin (ICRANet)
- Damien Begue (Bar Ilan University)
Description
Time resolved spectra of many gamma-ray bursts present signatures of evolving thermal component in keV band, either in the prompt emission or in the early afterglow. In some bursts this component is dominant with respect to the non-thermal one, in others it is less pronounced. Such thermal component is associated with the photosphere of highly relativistic outflows launched by the central engine. In our session we will discuss basic radiation mechanisms producing observed spectra and light curves of gamma-ray bursts and their respective roles. Special attention will be given to theoretical and observational results aiming at discrimination between these mechanisms, in particular synchrotron and photospheric emission.
In recent years, there is a renewed debate about the origin of the observed prompt emission signal. Some authors found that synchrotron emission can dominate the spectra of several long bursts, and a recent analysis show that it may be possible to overcome the famous 'line of death' argument by a direct fitting procedure. On the other hand, several recent works showed that non-dissipative...
Despite years of extensive research, the launching mechanism and the nature of relativistic jets remain open questions. Using 3D RMHD simulations of GRB jets with different initial magnetizations and engine modulation timescales, we calculate the resulting prompt emission light curves by considering photospheric emission and internal shocks, and compare them with observations. Our results show...
Photospheric emission from relativistic outflows may originate in two different regimes: photon decoupling within the outflow or radiative diffusion. I will discuss observed thermal component in the early afterglows of gamma-ray bursts as emission from such diffusive photospheres. In addition, I will discuss implications of photon diffusion for dissipative models of GRBs.
The study of Gamma Ray Bursts (GRBs) has the potential to improve our understanding of high energy astrophysical phenomena. In order to reliably use GRBs to this end, we first need to have a well-developed grasp of the mechanism that produces the radiation within GRB jets and how that relates to their structure. One model for the emission mechanism of GRBs invokes radiation produced deep in...
Although the observed GRB prompt emission spectrum is well constrained, the underlying radiation mechanism is not very well understood. We have explored photospheric emission in GRB jets by modelling the Comptonization of fast cooled synchrotron photons whilst the electrons and protons are accelerated to highly relativistic energies by repeated energy dissipation events as well as Coulomb...
We explore the nature of photospheric emission in long and short GRBs by performing hydrodynamical simulations of relativistic jet and post-process radiation transfer calculations. Our simulations show that the structure of the jet developed during its propagation has strong imprints on the resulting light curves, spectra, and polarization. In particular, it is found that the viewing angle...
The photospheric emission in the prompt emission is the natural prediction
of the original fireball model for gamma-ray burst (GRB) due to the large
optical depth ($\tau >1$) at the base of the outflow, which is supported by
the quasi-thermal components found in several Fermi GRBs. However,
the main origin of the most prompt emission spectrum (photosphere or
synchrotron) is still under...
As we are now in the era of fitting physical models to the MeV spectra of GRBs, we must address the proper way to analyze data from MeV GRB as well as how we check the quality of our fits. I will discuss the mathematically proven way to assess spectral fits, compare models, and treat the data with care. We will then apply these tools to physical models such as synchrotron emission and discuss...
Gamma-ray bursts (GRBs) are highly variable and exhibit strong spectral evolution. In particular, the emission properties vary from pulse to pulse in multipulse bursts. Here we present a time-resolved Bayesian spectral analysis of a compilation of GRB pulses observed by the Fermi/Gamma-ray Burst Monitor. The pulses are selected to have at least four time bins with a high statistical...
In this talk, I will focus on the photospheric emission observed in the short gamma-ray bursts (GRBs). In our work, we find that several short GRBs are consistent with a pure, non-dissipative photospheric model, at least around the peak of the pulse. For these bursts, we find (i) a bimodal distribution in the values of the Lorentz factors and the hardness ratios and (ii) an anti-correlation...
Although the origin of gamma-ray burst (GRB) prompt emission remains an unsolved problem, progress is being made by the advent of better analysis tools and as the state of our physical models improve. It is becoming increasingly clear that the difficulties of consistently reconciling empirical models with any underlying physical processes means that we have much to gain from directly fitting...
