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Register a new userDetection of Gamma-Ray Polarization in Prompt Emission of GRB 100826A
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Added by
Daisuke Yonetoku Dr.
Publication date
2011
fields
Physics
and research's language is
English
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We report the polarization measurement in prompt $gamma$-ray emission of GRB 100826A with the Gamma-Ray Burst Polarimeter (GAP) aboard the small solar power sail demonstrator IKAROS. We detected the firm change of polarization angle (PA) during the prompt emission with 99.9% ($3.5 sigma$) confidence level, and the average polarization degree ($Pi$) of $27 pm 11$% with 99.4% ($2.9 sigma$) confidence level. Here the quoted errors are given at 1 $sigma$ confidence level for two parameters of interest. The systematic errors have been carefully included in this analysis, unlike any previous reports. Such a high $Pi$ can be obtained in several emission models of gamma-ray bursts (GRBs), including synchrotron and photospheric models. However, it is difficult to explain the observed significant change of PA within the framework of axisymmetric jet as considered in many theoretical works. The non-axisymmetric (e.g., patchy) structures of the magnetic fields and/or brightness inside the relativistic jet are therefore required within the observable angular scale of $sim Gamma^{-1}$. Our observation strongly indicates that the polarization measurement is a powerful tool to constrain the GRB production mechanism, and more theoretical works are needed to discuss the data in more details.
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We present spectro-polarimetric analysis of thisgrb using data from asat, fermi, and swift, to provide insights into the physical mechanisms of the prompt radiation and the jet geometry. Prompt emission from thisgrb was very bright (fluence $>10^{-4}$~ergs~cm$^{-2}$) and had a complex structure composed of the superimposition of several pulses. The energy spectra deviate from the typical Band function to show a low energy peak $sim 15$~keV --- which we interpret as a power-law with two breaks, with a synchrotron origin. Alternately, the prompt spectra can also be interpreted as Comptonized emission, or a blackbody combined with a Band function. Time-resolved analysis confirms the presence of the low energy component, while the peak energy is found to be confined in the range of 100--200~keV. Afterglow emission detected by fermi-LAT is typical of an external shock model, and we constrain the initial Lorentz factor using the peak time of the emission. swift-XRT measurements of the afterglow show an indication for a jet break, allowing us to constrain the jet opening angle to $>$ 6$degr$. Detection of a large number of Compton scattered events by asat-CZTI provides an opportunity to study hard X-ray polarization of the prompt emission. We find that the burst has high, time-variable polarization, with the emission {bf have higher polarization} at energies above the peak energy. We discuss all observations in the context of GRB models and polarization arising due to {bf due to physical or geometric effects:} synchrotron emission from multiple shocks with ordered or random magnetic fields, Poynting flux dominated jet undergoing abrupt magnetic dissipation, sub-photospheric dissipation, a jet consisting of fragmented fireballs, and the Comptonization model.
Gamma-ray bursts are the strongest explosions in the Universe since the Big Bang, believed to be produced either in forming black holes at the end of massive star evolution or merging of compact objects. Spectral and timing properties of gamma-ray bursts suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe; however, the physical properties, especially the structure and magnetic topologies in the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of gamma-ray bursts should provide crucial information on the highly relativistic jets. As a result there have been many reports of gamma-ray burst polarization measurements with diverse results, see, however many such measurements suffered from substantial uncertainties, mostly systematic. After the first successful measurements by the GAP and COSI instruments, here we report a statistically meaningful sample of precise polarization measurements, obtained with the dedicated gamma-ray burst polarimeter, POLAR onboard Chinas Tiangong-2 spacelab. Our results suggest that the gamma-ray emission is at most polarized at a level lower than some popular models have predicted; although our results also show intrapulse evolution of the polarization angle. This indicates that the low polarization degrees could be due to an evolving polarization angle during a gamma-ray burst.
The radiative process responsible for gamma-Ray Burst (GRB) prompt emission has not been identified yet. If dominated by fast-cooling synchrotron radiation, the part of the spectrum immediately below the $ u F_ u$ peak energy should display a power-law behavior with slope $alpha_2=-3/2$, which breaks to a higher value $alpha_1=-2/3$ (i.e. to a harder spectral shape) at lower energies. Prompt emission spectral data (usually available down to $sim10-20,$keV) are consistent with one single power-law behavior below the peak, with typical slope $langlealpharangle=-1$, higher than (and then inconsistent with) the expected value $alpha_2=-3/2$. To better characterize the spectral shape at low energy, we analyzed 14 GRBs for which the Swift X-ray Telescope started observations during the prompt. When available, Fermi-GBM observations have been included in the analysis. For 67% of the spectra, models that usually give a satisfactory description of the prompt (e.g., the Band model) fail in reproducing the $0.5-1000,$keV spectra: low-energy data outline the presence of a spectral break around a few keV.We then introduce an empirical fitting function that includes a low-energy power law $alpha_1$, a break energy $E_{rm break}$, a second power law $alpha_2$, and a peak energy $E_{rm peak}$. We find $langlealpha_1rangle=-0.66$ ($ rm sigma=0.35$), $langle log (E_{rm break}/rm keV)rangle=0.63$ ($ rm sigma=0.20$), $langlealpha_2rangle=-1.46$ ($rm sigma=0.31$), and $langle log (E_{rm peak}/rm keV)rangle=2.1$ ($ rm sigma=0.56$).The values $langlealpha_1rangle$ and $langlealpha_2rangle$ are very close to expectations from synchrotron radiation. In this context, $E_{rm break}$ corresponds to the cooling break frequency.
Synchrotron emission polarization is very sensitive to the magnetic field configuration. Recently, polarization of synchrotron emission with a mixed (SM) magnetic field in Gamma-ray burst (GRB) afterglow phase had been developed. Here, we apply these SM models to GRB prompt phase and compare their polarization properties with that of synchrotron emission in purely ordered (SO) magnetic field. We find that the polarization properties in a SM model are very similar to these in a corresponding SO model (e.g., synchrotron emission in a mixed magnetic field with an aligned ordered part (SMA) and synchrotron emission with a purely ordered aligned magnetic field (SOA)), only with a lower polarization degree (PD). We also discuss the statistical properties of the models. We find PDs of the simulated bursts are concentrated around $25%$ for both SOA and synchrotron emission in a purely ordered toroidal magnetic field (SOT), while they can range from $0%$ to $25%$ for SMA and synchrotron emission in a mixed magnetic field with a toroidal ordered part (SMT), depending on $xi_B$ value, i.e., the ratio of magnetic reduction of the ordered magnetic field over that of random magnetic field. From statistics, if PDs of majority GRBs are non-zero, then it favours SO and SM models. Further, if there are some bright GRBs with a prominently lower PDs than that of the majority GRBs, it favours SOT (SMT) models; if all the bright GRBs have comparable PDs with the majority ones, it favours SOA (SMA) models. Finally, we apply our results to POLARs data and find that $sim10%$ time-integrated PDs of the observed bursts favor SMA and SMT models, and $xi_B$ parameter of these bursts is constrained to be around 1.135.
GRB spectra appear non-thermal, but recent observations of a few bursts with Fermi GBM have confirmed previous indications from BATSE of the presence of an underlying thermal component. Photospheric emission is indeed expected when the relativistic outflow emerging from the central engine becomes transparent to its own radiation, with a quasi-blackbody spectrum in absence of additional sub-photospheric dissipation. However, its intensity strongly depends on the acceleration mechanism - thermal or magnetic - of the flow. We aim to compute the thermal and non-thermal emissions produced by an outflow with a variable Lorentz factor, where the power injected at the origin is partially thermal (fraction epsilon_th) and partially magnetic (fraction 1-epsilon_th). The thermal emission is produced at the photosphere, and the non-thermal emission in the optically thin regime. Apart from the value of epsilon_th, we want to test how the other model parameters affect the observed ratio of the thermal to non-thermal emission. If the non-thermal emission is made by internal shocks, we self-consistently obtained the light curves and spectra of the thermal and non-thermal components for any distribution of the Lorentz factor in the flow. If the non-thermal emission results from magnetic reconnection we were unable to produce a light curve and could only compare the respective non-thermal and thermal spectra. In the different considered cases, we varied the model parameters to see when the thermal component in the light curve and/or spectrum is likely to show up or, on the contrary, to be hidden. We finally compared our results to the proposed evidence for the presence of a thermal component in GRB spectra. Focussing on GRB 090902B and GRB 10072B, we showed how these observations can be used to constrain the nature and acceleration mechanism of GRB outflows.
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