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Register a new userBayesian Time-Resolved Spectroscopy of GRB Pulses: $alpha$-Intensity Correlation
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H\\\"usne Dereli-B\\'egu\\'e
Publication date
2020
fields
Physics
and research's language is
English
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Gamma-ray bursts (GRBs) show different behaviours and trends in their spectral evolution. One of the methods used to understand the physical origin of these behaviours is to study correlation between the spectral fit parameters. In this work, we used a Bayesian analysis method to fit time-resolved spectra of GRB pulses that were detected by the textit{Fermi}/GBM during its first 9 years of mission. We studied single pulsed long bursts ($T_{90}geq2$ s). Among all the parameter correlations, we found that the correlation between the low-energy power-law index $alpha$ and the energy flux exhibited a systematic behaviour. We presented the properties of the observed characteristics of this behaviour and interpreted it in the context of the photospheric emission model.
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Besides light curves and spectra, polarization provides a different powerful tool of studying the $gamma-$ray burst (GRB) prompt phase. Compared with the time-integrated and energy-integrated polarization, time-resolved and energy-resolved polarization can deliver more physical information about the emitting region. Here we investigate time-resolved and energy-resolved polarization of GRB prompt emission using the synchrotron models. We find that the equal arrival time surface effect is very important in shaping the PD curves when the physical conditions of emitting region changes violently with radius. Polarization properties are neither correlated with the spectral lag nor the peak energy evolution patterns. Polarization properties with a mixed magnetic field are very similar to those for a corresponding ordered magnetic field but the former has a smaller polarization degree. The emission at the MeV peak can be highly polarized for a synchrotron model while it is unpolarized as predicted by a dissipative photosphere model. Future energy-resolved polarization observations can distinguish between these two models.
Simultaneous $gamma$-ray measurements of gamma-ray burst (GRB) spectra and polarization offer a unique way to determine the underlying emission mechanism(s) in these objects as well as probing the particle acceleration mechanism(s) that lead to the observed $gamma$-ray emission. Herein we examine the jointly-observed data from POLAR and GBM of GRB 170114A to determine its spectral and polarization properties and seek to understand the emission processes that generate these observations. We aim to develop an extensible and statistically sound framework for these types of measurements applicable to other instruments. We leverage the existing 3ML analysis framework to develop a new analysis pipeline for simultaneously modeling the spectral and polarization data. We derive the proper Poisson likelihood for $gamma$-ray polarization measurements in the presence of background. The developed framework is publicly available for similar measurements with other $gamma$-ray polarimeters. The data are analyzed within a Bayesian probabilistic context and the spectral data from both instruments are simultaneously modeled with a physical, numerical synchrotron code. The spectral modeling of the data is consistent with a synchrotron photon model as has been found in a majority of similarly analyzed single-pulse GRBs. The polarization results reveal a slight trend of growing polarization in time reaching values of ~30% at the temporal peak of the emission. Additionally, it is observed that the polarization angle evolves with time throughout the emission. These results hint at a synchrotron origin of the emission but further observations of many GRBs are required to verify these evolutionary trends. Furthermore, we encourage the development of time-resolved polarization models for the prompt emission of GRBs as the current models are not predictive enough to enable a full modeling of our current data.
Giant flares (GFs) are unusual bursts from soft gamma-ray repeaters (SGRs) that release an enormous amount of energy in a fraction of a second. The afterglow emission of these SGR-GFs or GF candidates is a highly beneficial means of discerning their composition, relativistic speed, and emission mechanisms. GRB 200415A is a recent GF candidate observed in a direction coincident with the nearby Sculptor galaxy at 3.5 Mpc. In this work, we searched for transient gamma-ray emission in past observations by Fermi-LAT in the direction of GRB 200415A. These observations confirm that GRB 200415A is observed as a transient GeV source only once. A pure pair-plasma fireball cannot provide the required energy for the interpretation of GeV afterglow emission and a baryonic poor outflow is additionally needed to explain the afterglow emission. A baryonic rich outflow is also viable, as it can explain the variability and observed quasi-thermal spectrum of the prompt emission if dissipation is happening below the photosphere via internal shocks. Using the peak energy ($E_p$) of the time-resolved prompt emission spectra and their fluxes ($F_p$), we found correlation between $E_p$ and $F_p$ or $E_p$ and isotropic luminosity $L_{rm iso}$ for GRB 200415A. This supports the intrinsic nature of $E_p$-$E_{rm iso}$ correlation found in SGRs-GFs, hence favoring a baryonic poor outflow. Our results also indicate a different mechanism at work during the initial spike, and that the evolution of the prompt emission spectral properties in this outflow would be intrinsically due to the injection process.
High resolution spectroscopy of GRB 021004 revealed a wealth of absorption lines from several intermediate ionization species. The velocity structure of the absorber is complex and material with velocity up to >3000 km/s is observed. Since only the blueshifted component is observed, the absorber is very likely to be material closely surrounding the gamma-ray burst. We use a time-dependent photoionization code to track the abundance of the ions over time. Thanks to the presence of absorption from intermediate ionization states at long times, we can estimate the location and mass of the components of the absorber. We interpret those constraints within the hypernova scenario showing that the mass loss rate of the progenitor must have been ~10^{-4} solar masses per year, suggestive of a very massive star. In addition, the wind termination shock must lie at a distance of at least 100 pc, implying a low density environment. The velocity structure of the absorber also requires clumping of the wind at those large distances.
We present experimental results for the ionization of aniline and benzene molecules subjected to intense ultrashort laser pulses. Measured parent molecular ions yields were obtained using a recently developed technique capable of three-dimensional imaging of ion distributions within the focus of a laser beam. By selecting ions originating from the central region of the focus, where the spatial intensity distribution is nearly uniform, volumetric-free intensity-dependent ionization yields were obtained. The measured data revealed a previously unseen resonant-like multiphoton ionization process. Comparison of benzene, aniline and Xe ion yields demonstrate that the observed intensity dependent structures are not due to geometric artifacts in the focus. Finally we attribute the ionization of aniline to a stepwise process going through the pi-sigma^star state which sits 3 photons above the ground state and 2 photons below the continuum.
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