Linear polarization in gamma-ray burst prompt emission

Gill, Ramandeep; Granot, Jonathan; Kumar, Pawan

doi:10.1093/mnras/stz2976

Cited by 51 publications

(94 citation statements)

References 122 publications

(170 reference statements)

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“…These POLAR results appear to disfavor synchrotron models with globally ordered toroidal magnetic fields (Gill et al 2020). However, the time-integrated results are consistent with synchrotron emission with a radial or normal to the radial magnetic field as well as Compton drag models.…”

Section: Discussionsupporting

confidence: 62%

“…In particular, the linear polarization properties of the gamma-rays are highly dependent on the emission processes at play during the prompt emission as well as the magnetic fields and their structure within the emitting jets. For a detailed overview the reader is referred to, for example, Toma et al (2009) and Gill et al (2020). The theoretical predictions can be generalized as follows: Models predicting the majority of the emission to be the result of synchrotron radiation allow for linear PDs as high as 50% (Luytikov et al 2003).…”

Section: Introductionmentioning

confidence: 99%

“…In these models, the maximum value at the beginning of the pulse can be as high as 60%, while the minimum value decreases to a few percent as presented in Zhang et al (2011). In order to properly distinguish between such models, both a large sample of GRB measurements and measurements capable of determining the evolution of the linear polarization parameters within a single emission pulse are required (Gill et al 2020). It should be noted that circular polarization is not predicted in any of these models (Luytikov et al (2003), Lundman et al (2014), Zhang et al (2011)), and therefore for the remainder of this work the term polarization can be assumed to mean linear polarization.…”

Section: Introductionmentioning

confidence: 99%

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The POLAR gamma-ray burst polarization catalog

Kole

Angelis²,

Berlato³

et al. 2020

A&A

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Context. Despite over 50 years of research, many open questions remain about the origin and nature of gamma-ray bursts (GRBs). Linear polarization measurements of the prompt emission of these extreme phenomena have long been thought to be key to answering a range of these questions. The POLAR detector was designed to produce the first set of detailed and reliable linear polarization measurements in the 50 − 500 keV energy range. During late 2016 and early 2017, POLAR detected a total of 55 GRBs. The analysis results of 5 of these GRBs have been reported, and were found to be consistent with a low or unpolarized flux. However, previous reports by other collaborations found high levels of linear polarization, including some as high as 90%. Aims. We study the linear polarization for the 14 GRBs observed by POLAR for which statistically robust inferences are possible. Additionally, time-resolved polarization studies are performed on GRBs with sufficient apparent flux. Methods. A publicly available polarization analysis tool, developed within the Multi-Mission Maximum Likelihood framework (3ML), was used to produce statistically robust results. The method allows spectral and polarimetric data from POLAR to be combined with spectral data from the Fermi Gamma-ray Burst Monitor (Fermi-GBM) and the Neil Gehrels Swift Observatory (hereafter Swift) to jointly model the spectral and polarimetric parameters. Results. The time-integrated analysis finds all results to be compatible with low or zero polarization with the caveat that, when timeresolved analysis is possible within individual pulses, we observe moderate linear polarization with a rapidly changing polarization angle. Therefore, time-integrated polarization results, while pointing to lower polarization, are potentially an artifact of summing over the changing polarization signal and thus washing out the true moderate polarization. We therefore caution against overinterpretation of any time-integrated results inferred herein and encourage the community to wait for more detailed polarization measurements from forthcoming missions such as POLAR-2 and LEAP.

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Section: Discussionsupporting

confidence: 62%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The POLAR gamma-ray burst polarization catalog

Kole

Angelis²,

Berlato³

et al. 2020

A&A

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“…The high polarisation fraction suggests that the magnetic field is ordered on the angular scale of 1/Γ. According to the discussions presented in section 6.2, we find that the second episode of emission is likely to be synchrotron produced by the energetic electrons generated in the internal shocks in the optically thin region of the outflow, as they cool in the large scale ordered magnetic fields originating from the central engine (Toma et al 2009;Granot 2003;Mao & Wang 2018;Gill et al 2019). The cutoff observed in the spectrum can be related to the intrinsic pair opacity created via photon-photon annihilation in the optically thin region at the dissipation site.…”

Section: Second Emission Episodesupporting

confidence: 63%

Spectropolarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks

Sharma

Iyyani

Bhattacharya

et al. 2020

Monthly Notices of the Royal Astronomical Society

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GRB 160325A is the only bright burst detected by AstroSat CZT Imager in its primary field of view to date. In this work, we present the spectral and polarimetric analysis of the prompt emission of the burst using AstroSat, Fermi and Niel Gehrels Swift observations. The prompt emission consists of two distinct emission episodes separated by a few seconds of quiescent/ mild activity period. The first emission episode shows a thermal component as well as a low polarisation fraction of PF < 37 % at 1.5 σ confidence level. On the other hand, the second emission episode shows a non-thermal spectrum and is found to be highly polarised with PF > 43 % at 1.5σ confidence level. We also study the afterglow properties of the jet using Swift/XRT data. The observed jet break suggests that the jet is pointed towards the observer and has an opening angle of 1.2 • for an assumed redshift, z = 2. With composite modelling of polarisation, spectrum of the prompt emission and the afterglow, we infer that the first episode of emission originates from the photosphere with localised dissipation happening below it, and the second from the optically thin region above the photosphere. The photospheric emission is generated mainly by inverse Compton scattering, whereas the emission in the optically thin region is produced by the synchrotron process. The low radiation efficiency of the burst suggests that the outflow remains baryonic dominated throughout the burst duration with only a subdominant Poynting flux component, and the kinetic energy of the jet is likely dissipated via internal shocks which evolves from an optically thick to optically thin environment within the jet.

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“…harvard.edu/dataset.xhtml?persistentId=doi:10.7910/DVN/ MTDPTA to a subset of GRBs (Ryde et al 2010;Ahlgren et al 2015;Vianello et al 2018a). Measurements of polarization can break this degeneracy (Toma et al 2009;Gill et al 2018). Photospheric emission will typically produce unpolarized emission although a moderate polarization level is possible in special circumstances (Lundman et al 2018) and predicts very specific changes of the polarization angle (Lundman et al 2014).…”

Section: Introductionmentioning

confidence: 99%

Time-resolved GRB polarization with POLAR and GBM

Burgess

Kole

Berlato

et al. 2019

A&A

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Context. Simultaneous γ-ray measurements of γ-ray burst 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 γ-ray emission. Aims. We examine the jointly observed data from POLAR and Fermi-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. Methods. We leveraged the existing 3ML analysis framework to develop a new analysis pipeline for simultaneously modeling the spectral and polarization data. We derived the proper Poisson likelihood for γ-ray polarization measurements in the presence of background. The developed framework is publicly available for similar measurements with other γ-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. Results. 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 gamma-ray bursts. The polarization results reveal a slight trend of growing polarization in time reaching values of ∼30% at the temporal peak of the emission. We also observed that the polarization angle evolves with time throughout the emission. These results suggest 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 gamma-ray bursts as the current models are not predictive enough to enable a full modeling of our current data.

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Linear polarization in gamma-ray burst prompt emission

Cited by 51 publications

References 122 publications

The POLAR gamma-ray burst polarization catalog

The POLAR gamma-ray burst polarization catalog

Spectropolarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks

Time-resolved GRB polarization with POLAR and GBM

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