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16 pages, 572 KiB

Open AccessFeature PaperArticle

Strongly Magnetized Sources: QED and X-ray Polarization

by Jeremy Heyl and Ilaria Caiazzo

Galaxies 2018, 6(3), 76; https://doi.org/10.3390/galaxies6030076 - 21 Jul 2018

Cited by 25 | Viewed by 3369

Abstract

Radiative corrections of quantum electrodynamics cause a vacuum threaded by a magnetic field to be birefringent. This means that radiation of different polarizations travels at different speeds. Even in the strong magnetic fields of astrophysical sources, the difference in speed is small. However, [...] Read more.

Radiative corrections of quantum electrodynamics cause a vacuum threaded by a magnetic field to be birefringent. This means that radiation of different polarizations travels at different speeds. Even in the strong magnetic fields of astrophysical sources, the difference in speed is small. However, it has profound consequences for the extent of polarization expected from strongly magnetized sources. We demonstrate how the birefringence arises from first principles, show how birefringence affects the polarization state of radiation and present recent calculations for the expected polarization from magnetars and X-ray pulsars. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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12 pages, 454 KiB

Open AccessFeature PaperArticle

Photoelectric Polarimetry and the Gas Pixel Detector Yesterday, Today and Tomorrow

by Enrico Costa

Galaxies 2018, 6(3), 71; https://doi.org/10.3390/galaxies6030071 - 16 Jul 2018

Cited by 1 | Viewed by 2832

Abstract

Since the very beginning of X-ray Astronomy, polarimetry has been suggested as a tool of diagnostics, of great potentiality. While almost all measurements of X-rays were based on detectors using the photoelectric effect, the first attempt to perform polarimetry were based on Compton [...] Read more.

Since the very beginning of X-ray Astronomy, polarimetry has been suggested as a tool of diagnostics, of great potentiality. While almost all measurements of X-rays were based on detectors using the photoelectric effect, the first attempt to perform polarimetry were based on Compton scattering and Bragg diffraction. The use of photoelectric effect also for polarimetry has been hypothesized and attempted for many years but never accomplished. Only 40 years from the start of X-ray astronomy, the Gas Pixel Detector (GPD) was developed, compatible with an X-ray optics, and capable of measuring energy, time, position and polarization simultaneously. Only after 20 more years, the Imaging X-ray Polarimetry Explorer, based on the GPD detectors, will be launched. I present the story of the development of photoelectric polarimetry that arrived to the Gas Pixel Detector, and discuss the possible future evolutions. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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10 pages, 493 KiB

Open AccessFeature PaperArticle

Probing Black Hole Magnetic Fields with QED

by Ilaria Caiazzo and Jeremy Heyl

Galaxies 2018, 6(2), 57; https://doi.org/10.3390/galaxies6020057 - 24 May 2018

Cited by 1 | Viewed by 3161

Abstract

The effect of vacuum birefringence is one of the first predictions of quantum electrodynamics (QED): the presence of a charged Dirac field makes the vacuum birefringent when threaded by magnetic fields. This effect, extremely weak for terrestrial magnetic fields, becomes important for highly [...] Read more.

The effect of vacuum birefringence is one of the first predictions of quantum electrodynamics (QED): the presence of a charged Dirac field makes the vacuum birefringent when threaded by magnetic fields. This effect, extremely weak for terrestrial magnetic fields, becomes important for highly magnetized astrophysical objects, such as accreting black holes. In the X-ray regime, the polarization of photons traveling in the magnetosphere of a black hole is not frozen at emission but is changed by the local magnetic field. We show that, for photons traveling along the plane of the disk, where the field is expected to be partially organized, this results in a depolarization of the X-ray radiation. Because the amount of depolarization depends on the strength of the magnetic field, this effect can provide a way to probe the magnetic field in black-hole accretion disks and to study the role of magnetic fields in astrophysical accretion in general. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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24 pages, 1271 KiB

Open AccessArticle

Instrumentation and Future Missions in the Upcoming Era of X-ray Polarimetry

by Sergio Fabiani

Galaxies 2018, 6(2), 54; https://doi.org/10.3390/galaxies6020054 - 11 May 2018

Cited by 12 | Viewed by 4127

Abstract

The maturity of current detectors based on technologies that range from solid state to gases renewed the interest for X-ray polarimetry, raising the enthusiasm of a wide scientific community to improve the performance of polarimeters as well as to produce more detailed theoretical [...] Read more.

The maturity of current detectors based on technologies that range from solid state to gases renewed the interest for X-ray polarimetry, raising the enthusiasm of a wide scientific community to improve the performance of polarimeters as well as to produce more detailed theoretical predictions. We will introduce the basic concepts about measuring the polarization of photons, especially in the X-rays, and we will review the current state of the art of polarimeters in a wide energy range from soft to hard X-rays, from solar flares to distant astrophysical sources. We will introduce relevant examples of polarimeters developed from the recent past up to the panorama of upcoming space missions to show how the recent development of the technology is allowing reopening the observational window of X-ray polarimetry. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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9 pages, 354 KiB

Open AccessArticle

A Changing-Look AGN to Be Probed by X-ray Polarimetry

by Beatriz Agís-González, Damien Hutsemékers and Giovanni Miniutti

Galaxies 2018, 6(2), 52; https://doi.org/10.3390/galaxies6020052 - 27 Apr 2018

Cited by 1 | Viewed by 3240

Abstract

Active galactic nuclei (AGN) produce the highest intrinsic luminosities in the Universe from within a compact region. The central engine is thought to be powered by accretion onto a supermassive black hole. A fraction of this huge release of energy influences the evolution [...] Read more.

Active galactic nuclei (AGN) produce the highest intrinsic luminosities in the Universe from within a compact region. The central engine is thought to be powered by accretion onto a supermassive black hole. A fraction of this huge release of energy influences the evolution of the host galaxy, and in particular, star formation. Thus, AGN are key astronomical sources not only because they play an important role in the evolution of the Universe, but also because they constitute a laboratory for extreme physics. However, these objects are under the resolution limit of current telescopes. Polarimetry is a unique technique capable of providing us with information on physical AGN structures. The incoming new era of X-ray polarimetry will give us the opportunity to explore the geometry and physical processes taking place in the innermost regions of the accretion disc. Here we exploit this future powerful tool in the particular case of changing-look AGN, which are key for understanding the complexity of AGN physics. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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9 pages, 1361 KiB

Open AccessFeature PaperArticle

A Study of Background Conditions for Sphinx—The Satellite-Borne Gamma-Ray Burst Polarimeter

by Fei Xie and Mark Pearce

Galaxies 2018, 6(2), 50; https://doi.org/10.3390/galaxies6020050 - 24 Apr 2018

Cited by 10 | Viewed by 4103

Abstract

SPHiNX is a proposed satellite-borne gamma-ray burst polarimeter operating in the energy range 50–500 keV. The mission aims to probe the fundamental mechanism responsible for gamma-ray burst prompt emission through polarisation measurements. Optimising the signal-to-background ratio for SPHiNX is an important task during [...] Read more.

SPHiNX is a proposed satellite-borne gamma-ray burst polarimeter operating in the energy range 50–500 keV. The mission aims to probe the fundamental mechanism responsible for gamma-ray burst prompt emission through polarisation measurements. Optimising the signal-to-background ratio for SPHiNX is an important task during the design phase. The Geant4 Monte Carlo toolkit is used in this work. From the simulation, the total background outside the South Atlantic Anomaly (SAA) is about 323 counts/s, which is dominated by the cosmic X-ray background and albedo gamma rays, which contribute ∼60% and ∼35% of the total background, respectively. The background from albedo neutrons and primary and secondary cosmic rays is negligible. The delayed background induced by the SAA-trapped protons is about 190 counts/s when SPHiNX operates in orbit for one year. The resulting total background level of ∼513 counts/s allows the polarisation of ∼50 GRBs with minimum detectable polarisation less than 30% to be determined during the two-year mission lifetime. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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19 pages, 10775 KiB

Open AccessFeature PaperArticle

Practical Aspects of X-ray Imaging Polarimetry of Supernova Remnants and Other Extended Sources

by Jacco Vink and Ping Zhou

Galaxies 2018, 6(2), 46; https://doi.org/10.3390/galaxies6020046 - 10 Apr 2018

Cited by 9 | Viewed by 4779

Abstract

The new generation of X-ray polarisation detectors, the gas pixel detectors, which will be employed by the future space missions IXPE and eXTP, allows for spatially resolved X-ray polarisation studies. This will be of particular interest for X-ray synchrotron emission from extended sources [...] Read more.

The new generation of X-ray polarisation detectors, the gas pixel detectors, which will be employed by the future space missions IXPE and eXTP, allows for spatially resolved X-ray polarisation studies. This will be of particular interest for X-ray synchrotron emission from extended sources like young supernova remnants and pulsar wind nebulae. Here we report on employing a polarisation statistic that can be used to makes maps in the Stokes I, Q, and U parameters, a method that we expand by correcting for the energy-dependent instrumental modulation factor, using optimal weighting of the signal. In order to explore the types of Stokes maps that can be obtained, we present a Monte Carlo simulation program called xpolim, with which different polarisation weighting schemes are explored. We illustrate its use with simulations of polarisation maps of young supernova remnants, after having described the general science case for polarisation studies of supernova remnants, and its connection to magnetic-field turbulence. We use xpolim simulations to show that in general deep, ~2 Ms observations are needed to recover polarisation signals, in particular for Cas A, for which in the polarisation fraction may be as low as 5%. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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7 pages, 362 KiB

Open AccessFeature PaperArticle

Hot Coronae in Local AGN: Present Status and Future Perspectives

by Andrea Marinucci, Francesco Tamborra, Stefano Bianchi, Michal Dovčiak, Giorgio Matt, Riccardo Middei and Alessia Tortosa

Galaxies 2018, 6(2), 44; https://doi.org/10.3390/galaxies6020044 - 4 Apr 2018

Cited by 7 | Viewed by 4822

Abstract

The nuclear X-ray emission in radio-quiet Active Galactic Nuclei (AGN) is commonly believed to be due to inverse Compton scattering of soft UV photons in a hot corona. The radiation is expected to be polarized, the polarization degree depending mainly on the geometry [...] Read more.

The nuclear X-ray emission in radio-quiet Active Galactic Nuclei (AGN) is commonly believed to be due to inverse Compton scattering of soft UV photons in a hot corona. The radiation is expected to be polarized, the polarization degree depending mainly on the geometry and optical depth of the corona. Nuclear Spectroscopic Telescope Array (NuSTAR) observations are providing for the first time high quality measurements of the coronal physical parameters—temperature and optical depth. We hereby review the NuSTAR results on the coronal physical parameters (temperature and optical depth) and discuss their implications for future X-ray polarimetric studies. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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8 pages, 871 KiB

Open AccessFeature PaperArticle

Future X-ray Polarimetry of Relativistic Accelerators: Pulsar Wind Nebulae and Supernova Remnants

by Niccolò Bucciantini

Galaxies 2018, 6(2), 42; https://doi.org/10.3390/galaxies6020042 - 27 Mar 2018

Cited by 2 | Viewed by 4645

Abstract

Supernova remnants (SNRs) and pulsar wind nebulae (PWNs) are among the most significant sources of non-thermal X-rays in the sky, and the best means by which relativistic plasma dynamics and particle acceleration can be investigated. Being strong synchrotron emitters, they are ideal candidates [...] Read more.

Supernova remnants (SNRs) and pulsar wind nebulae (PWNs) are among the most significant sources of non-thermal X-rays in the sky, and the best means by which relativistic plasma dynamics and particle acceleration can be investigated. Being strong synchrotron emitters, they are ideal candidates for X-ray polarimetry, and indeed the Crab nebula is up to present the only object where X-ray polarization has been detected with a high level of significance. Future polarimetric measures will likely provide us with crucial information on the level of turbulence that is expected at particle acceleration sites, together with the spatial and temporal coherence of magnetic field geometry, enabling us to set stronger constraints on our acceleration models. PWNs will also allow us to estimate the level of internal dissipation. I will briefly review the current knowledge on the polarization signatures in SNRs and PWNs, and I will illustrate what we can hope to achieve with future missions such as IXPE/XIPE. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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7 pages, 550 KiB

Open AccessFeature PaperArticle

The Growth of Interest in Astronomical X-Ray Polarimetry

by Frédéric Marin

Galaxies 2018, 6(1), 38; https://doi.org/10.3390/galaxies6010038 - 19 Mar 2018

Cited by 5 | Viewed by 3464

Abstract

Astronomical X-ray polarimetry was first explored in the end of the 1960s by pioneering rocket instruments. The craze arising from the first discoveries of stellar and supernova remnant X-ray polarization led to the addition of X-ray polarimeters to early satellites. Unfortunately, the inadequacy [...] Read more.

Astronomical X-ray polarimetry was first explored in the end of the 1960s by pioneering rocket instruments. The craze arising from the first discoveries of stellar and supernova remnant X-ray polarization led to the addition of X-ray polarimeters to early satellites. Unfortunately, the inadequacy of the diffraction and scattering technologies required to measure polarization with respect to the constraints driven by X-ray mirrors and detectors, coupled with long integration times, slowed down the field for almost 40 years. Thanks to the development of new, highly sensitive, compact X-ray polarimeters in the beginning of the 2000s, observing astronomical X-ray polarization has become feasible, and scientists are now ready to explore our high-energy sky thanks to modern X-ray polarimeters. In the forthcoming years, several X-ray missions (rockets, balloons, and satellites) will create new observational opportunities. Interest in astronomical X-ray polarimetry field has thus been renewed, and this paper presents for the first time a quantitative assessment, all based on scientific literature, of the growth of this interest. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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14 pages, 1594 KiB

Open AccessFeature PaperArticle

Multi-Wavelength Polarimetry of Isolated Neutron Stars

by Roberto P. Mignani

Galaxies 2018, 6(1), 36; https://doi.org/10.3390/galaxies6010036 - 13 Mar 2018

Cited by 9 | Viewed by 3899

Abstract

Isolated neutron stars are known to be endowed with extreme magnetic fields, whose maximum intensity ranges from

10^{12}

–

10^{15}

G, which permeates their magnetospheres. Their surrounding environment is also strongly magnetized, especially in the compact nebulae powered by the relativistic [...] Read more.

Isolated neutron stars are known to be endowed with extreme magnetic fields, whose maximum intensity ranges from

10^{12}

–

10^{15}

G, which permeates their magnetospheres. Their surrounding environment is also strongly magnetized, especially in the compact nebulae powered by the relativistic wind from young neutron stars. The radiation from isolated neutron stars and their surrounding nebulae is, thus, supposed to bring a strong polarization signature. Measuring the neutron star polarization brings important information about the properties of their magnetosphere and of their highly magnetized environment. Being the most numerous class of isolated neutron stars, polarization measurements have been traditionally carried out for radio pulsars, hence in the radio band. In this review, I summarize multi-wavelength linear polarization measurements obtained at wavelengths other than radio both for pulsars and other types of isolated neutron stars and outline future perspectives with the upcoming observing facilities. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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15 pages, 12795 KiB

Open AccessFeature PaperArticle

An Overview of X-Ray Polarimetry of Astronomical Sources

by Martin C. Weisskopf

Galaxies 2018, 6(1), 33; https://doi.org/10.3390/galaxies6010033 - 6 Mar 2018

Cited by 22 | Viewed by 6416

Abstract

We review the history of astronomical X-ray polarimetry based on the author’s perspective, beginning with early sounding-rocket experiments by Robert Novick at Columbia University and his team, of which the author was a member. After describing various early techniques for measuring X-ray polarization, [...] Read more.

We review the history of astronomical X-ray polarimetry based on the author’s perspective, beginning with early sounding-rocket experiments by Robert Novick at Columbia University and his team, of which the author was a member. After describing various early techniques for measuring X-ray polarization, we discuss the polarimeter aboard the Orbiting Solar Observatory 8 (OSO-8) and its scientific results. Next, we describe the X-ray polarimeter to have flown aboard the ill-fated original Spectrum-X mission, which provided important lessons on polarimeter design, systematic effects, and the programmatics of a shared focal plane. We conclude with a description of the Imaging X-ray Polarimetry Explorer (IXPE) and its prospective scientific return. IXPE, a partnership between NASA and ASI, has been selected as a NASA Astrophysics Small Explorers Mission and is currently scheduled to launch in April of 2021. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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6 pages, 377 KiB

Open AccessFeature PaperArticle

Studying Microquasars with X-Ray Polarimetry

by Giorgio Matt and Francesco Tamborra

Galaxies 2018, 6(1), 32; https://doi.org/10.3390/galaxies6010032 - 5 Mar 2018

Cited by 2 | Viewed by 2952

Abstract

Microquasars are Galactic black hole systems in which matter is transferred from a donor star and accretes onto a black hole of, typically, 10–20 solar masses. The presence of an accretion disk and a relativistic jet made them a scaled down analogue of [...] Read more.

Microquasars are Galactic black hole systems in which matter is transferred from a donor star and accretes onto a black hole of, typically, 10–20 solar masses. The presence of an accretion disk and a relativistic jet made them a scaled down analogue of quasars—thence their name. Microquasars feature prominently in the scientific goals of X-ray polarimeters, because a number of open questions, which are discussed in this paper, can potentially be answered: the geometry of the hot corona believed to be responsible for the hard X-ray emission; the role of the jet; the spin of the black hole. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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9 pages, 3467 KiB

Open AccessFeature PaperArticle

The PoGO+ Balloon-Borne Hard X-ray Polarimetry Mission

by Mette Friis, Mózsi Kiss, Victor Mikhalev, Mark Pearce and Hiromitsu Takahashi

Galaxies 2018, 6(1), 30; https://doi.org/10.3390/galaxies6010030 - 2 Mar 2018

Cited by 13 | Viewed by 6282

Abstract

The PoGO mission, including the PoGOLite Pathfinder and PoGO+, aims to provide polarimetric measurements of the Crab system and Cygnus X-1 in the hard X-ray band. Measurements are conducted from a stabilized balloon-borne platform, launched on a 1 million cubic meter balloon from [...] Read more.

The PoGO mission, including the PoGOLite Pathfinder and PoGO+, aims to provide polarimetric measurements of the Crab system and Cygnus X-1 in the hard X-ray band. Measurements are conducted from a stabilized balloon-borne platform, launched on a 1 million cubic meter balloon from the Esrange Space Center in Sweden to an altitude of approximately 40 km. Several flights have been conducted, resulting in two independent measurements of the Crab polarization and one of Cygnus X-1. Here, a review of the PoGO mission is presented, including a description of the payload and the flight campaigns, and a discussion of some of the scientific results obtained to date. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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Review

Jump to: Research, Other

19 pages, 5052 KiB

Open AccessFeature PaperReview

Hard X-ray and Soft Gamma Ray Polarimetry with CdTe/CZT Spectro-Imager

by Ezio Caroli, Miguel Moita, Rui M. Curado Da Silva, Stefano Del Sordo, Giovanni De Cesare, Jorge M. Maia and Marcela Pàscoa

Galaxies 2018, 6(3), 69; https://doi.org/10.3390/galaxies6030069 - 8 Jul 2018

Cited by 16 | Viewed by 4710

Abstract

CdTe/CZT based spectroscopic two-dimensional (2D)/three-dimensional (3D) imagers when operated in the Compton regime can work as high performance scattering polarimeters, for high-energy astrophysics. Polarimetry in high-energy astrophysics has been little explored. To date, X- and γ-ray source emissions have been studied almost exclusively [...] Read more.

CdTe/CZT based spectroscopic two-dimensional (2D)/three-dimensional (3D) imagers when operated in the Compton regime can work as high performance scattering polarimeters, for high-energy astrophysics. Polarimetry in high-energy astrophysics has been little explored. To date, X- and γ-ray source emissions have been studied almost exclusively through spectral, imaging, and timing analysis. Polarization measurements provide two additional observational parameters: the polarization angle and the level of linear polarization. These additional parameters should allow for a better discrimination between the physical mechanisms of different emission models characterizing a celestial object. Therefore, polarimetry will play a strategic role in new instrumentations for future high-energy astronomy missions. 2D and 3D CZT/CdTe spectroscopic imagers provided with coincidence readout logic can efficiently handle scattering events to perform simultaneously polarization, spectroscopy, imaging, and timing measurements. Herein, we describe the results obtained, both experimentally and by MC simulations, with CdTe/CZT pixel detector prototypes in high-energy polarimetry. We give an overview on the achievable polarimetric performance with spectroscopic imagers and on how these performances are affected by detector configuration parameters. Finally, we address the perspective of scattering polarimetry opened by the recent implementation of new high energy focusing optics, as broadband Laue lens, in next generation of hard X- and soft γ-ray astronomy instrumentation. The unprecedented sensitivity achievable by these telescopes will definitely open the window of polarimetry in this high-energy range. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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Other

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11 pages, 369 KiB

Open AccessFeature PaperConference Report

Searching for Axion-Like Particles with X-ray Polarimeters

by Francesca Day and Sven Krippendorf

Galaxies 2018, 6(2), 45; https://doi.org/10.3390/galaxies6020045 - 10 Apr 2018

Cited by 24 | Viewed by 3649

Abstract

X-ray telescopes are an exceptional tool for searching for new fundamental physics. In particular, X-ray observations have already placed world-leading bounds on the interaction between photons and axion-like particles (ALPs). ALPs are hypothetical new ultra-light particles motivated by string theory models. They can [...] Read more.

X-ray telescopes are an exceptional tool for searching for new fundamental physics. In particular, X-ray observations have already placed world-leading bounds on the interaction between photons and axion-like particles (ALPs). ALPs are hypothetical new ultra-light particles motivated by string theory models. They can also act as dark matter and dark energy, and provide a solution to the strong CP problem. In a background magnetic field, ALPs and photons may interconvert. This leads to energy dependent modulations in both the flux and polarisation of the spectra of point sources shining through large magnetic fields. The next generation of polarising X-ray telescopes will offer new detection possibilities for ALPs. Here we present techniques and projected bounds for searching for ALPs with X-ray polarimetry. We demonstrate that upcoming X-ray polarimetry missions have the potential to place world-leading bounds on ALPs. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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8 pages, 326 KiB

Open AccessFeature PaperConference Report

On the Spectrum and Polarization of Magnetar Flare Emission

by Roberto Taverna and Roberto Turolla

Galaxies 2018, 6(1), 35; https://doi.org/10.3390/galaxies6010035 - 12 Mar 2018

Viewed by 3221

Abstract

Bursts and flares are among the distinctive observational manifestations of magnetars, isolated neutron stars endowed with an ultra-strong magnetic field (

B \approx 10^{14}

–

10^{15}

G). It is believed that these events arise in a hot electron-positron plasma, injected in [...] Read more.

Bursts and flares are among the distinctive observational manifestations of magnetars, isolated neutron stars endowed with an ultra-strong magnetic field (

B \approx 10^{14}

–

10^{15}

G). It is believed that these events arise in a hot electron-positron plasma, injected in the magnetosphere, due to a magnetic field instability, which remains trapped within the closed magnetic field lines (the “trapped-fireball” model). We have developed a simple radiative transfer model to simulate magnetar flare emission in the case of a steady trapped fireball. After dividing the fireball surface in a number of plane-parallel slabs, the local spectral and polarization properties are obtained integrating the radiative transfer equations for the two normal modes. We assume that magnetic Thomson scattering is the dominant source of opacity, and neglect contributions from second-order radiative processes, although the presence of double-Compton scattering is accounted for in establishing local thermal equilibrium in the fireball atmospheric layers. The spectra we obtained in the 1–100 keV energy range are in broad agreement with those of available observations. The large degree of polarization (≳80%) predicted by our model should be easily detectable by new-generation X-ray polarimeters, like IXPE, XIPE and eXTP, allowing one to confirm the model predictions. Full article

(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)

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