Merlin Kole

We present updated hard X-ray polarization measurements of the Crab pulsar and nebula obtained with the balloon-borne polarimeter XL-Calibur in the ∼19–64 keV energy range. During the flight, intermittent failure of the Global Positioning System (GPS) receiver resulted in poorly constrained timing for ∼38% of the Crab dataset. By implementing a new phase recovery method that reconstructs timing during extended GPS-off intervals, phase tag data are recovered for ∼95% of the GPS-off dataset, increasing the precision of the phase-resolved analysis. Phase information for the data is recovered by using the Crab pulsar, with its 33 ms period, as an external timing source. Using a Markov Chain Monte Carlo framework to jointly fit phase offsets and frequency derivatives, sufficient phase accuracy is achieved across multiple periods without GPS for a phase-resolved analysis. This enables inclusion of nearly the full dataset in the polarization study. The polarization degree of the nebular emission is found to be (27.7 ± 4.9)% at a polarization angle of 127 . ° 2 ± 5 . ° 1, confirming previous XL-Calibur results and remaining aligned with the Crab’s spin axis, consistent with synchrotron emission from the inner nebula. Phase-resolved measurements show that the off-pulse and bridge intervals exhibit a strong polarization, while the pulsar peaks, although weakly constrained, remain in agreement with the softer-energy trends of IXPE. These findings reinforce a scenario in which hard X-ray emission arises primarily in the nebular torus and wind regions. The successful recovery of precise phase tagging from GPS-off data demonstrates the capacity to use the pulsar as an external clock even in the case of sparsely populated data.

The combination of plastic scintillators with Silicon Photo-Multipliers (SiPMs) is widely used for detecting radiation in high-energy astrophysics, particle physics, neutrino physics, or medical physics. An example of application for this kind of detectors are Compton polarimeters such as POLAR-2 [1,2,3] or LEAP [4,5,6] for which a low-Z material is needed for the Compton effect to be dominant down to as low energy as possible. Such detectors aim to measure low energy Compton depositions in organic scintillators which produce small amounts of optical light because of the low energy deposited and the low scintillation efficiency, and for which optimizing the instrumental optical properties consequently imperative. The light collection efficiency of such a device was studied with a focus on the POLAR-2 Gamma-Ray Burst polarimeter. POLAR-2 consists of a segmented array of 6400 elongated plastic scintillators divided into 100 modules, all read out by SiPMs. The conversion of incoming γ-rays into readable signal goes through the production and collection of optical light, which has been optimized both through measurements and simulations. The optical elements of the POLAR-2 polarimeter prototype module were optically characterized and an optical simulation based on Geant4 was developed to fully model its optical performances. The results from simulations were used to optimize the design and finally to verify its performance. The study resulted in a detector capable of measuring energy depositions of several keV. In addition, an important finding of this work is the impact of the plastic scintillator surface roughness on the light collection. It was found that a plastic scintillator with a higher scintillation efficiency but made of a softer material, hence with a rougher surface, was not necessarily the best option to optimize the light collection. Furthermore, in order to optimize the optical crosstalk between different channels, a production technique for very thin (∼150 μm) and reusable silicone-based optical coupling pads was developed. This method can be adapted to produce either standalone pads or to directly mold a layer on any SiPM to be later coupled to the scintillators. After an introductory discussion describing the need of a large scale GRB polarimeter like POLAR-2, the optical design and characterization of the polarimeter modules that compose its sensitive part are described. The Geant4-based optical simulations of the POLAR-2 modules and the impact of the optical properties of its various elements on the light collection efficiency of the instrument are later presented. The work is finally summarized and an outlook is given on the potential applications of the POLAR-2 optical characterization and simulation work to other experiments employing similar elements.

POLAR-2 is a dedicated gamma-ray polarimeter currently foreseen to be launched towards the China Space Station around 2027. The design of the detector is based on the legacy of its predecessor mission POLAR which was launched in 2016. POLAR-2 aims to measure the polarization of the Gamma-ray Burst prompt emission within the 30–800 keV energy range. Thanks to its high sensitivity to gamma-ray polarization, as well as its large effective area, POLAR-2 will provide the most precise measurements of this type to date. Such measurements are key to improve our understanding of the astrophysical processes responsible for Gamma-Ray Bursts. The detector consists of a segmented array of plastic scintillator bars, each one of which is read out by a Silicon PhotoMultiplier channel. The flight model of POLAR-2 will contain a total of 6400 scintillators. These are divided into 100 groups of 64 bars each, in so-called polarimeter modules. In recent years, the collaboration has designed and produced the first prototypes of these polarimeter modules and subjected these to space qualification tests. In addition, in April 2023, the first of these modules were calibrated using fully polarized gamma-ray beams at the European Synchrotron Radiation Facility (ESRF) in France. In this work, we will present the results of this calibration campaign and compare these to the simulated performance of the POLAR-2 modules. Potential improvements to the design are also discussed. Finally, the measurements are used, in combination with the verified simulation framework, to estimate the scientific performance of the full POLAR-2 detector and compare it to its predecessor.

Since their first discovery in the late 1960s, gamma-ray bursts have attracted an exponentially growing interest from the international community due to their central role in the most highly debated open questions of the modern research of astronomy, astrophysics, cosmology, and fundamental physics. These range from the intimate nuclear composition of high-density material within the core of ultra-dense neuron stars, to stellar evolution via the collapse of massive stars, the production and propagation of gravitational waves, as well as the exploration of the early universe by unveiling the first stars and galaxies (assessing also their evolution and cosmic re-ionization). GRBs in the past ∼50 years have stimulated the development of cutting-edge technological instruments for observations of high-energy celestial sources from space, leading to the launch and successful operations of many different scientific missions (several of them still in data-taking mode currently). In this review, we provide a brief description of the GRB-dedicated missions from space being designed and developed for the future. The list of these projects, not meant to be exhaustive, shall serve as a reference to interested readers to understand what is likely to come next to lead the further development of GRB research and the associated phenomenology.

Since their discoveries in 1967, Gamma-Ray Bursts (GRBs) continue to be one of the most researched objects in astrophysics. Multi-messenger observations are key to gaining a deeper understanding of these events. In order to facilitate such measurements, fast and accurate localization of the gamma-ray prompt emission is required. As traditional localization techniques are often time consuming or prone to significant systematic errors, here we present a novel method which can be applied on the POLAR-2 observatory. POLAR-2 is a dedicated GRB polarimeter, which will be launched towards the China Space Station (CSS) in 2025. The CSS provides POLAR-2 with access to a GPU, which makes it possible and advantageous to run a Deep Learning model on it. In this work, we explore the possibility to identify GRBs real time and to infer their location and spectra with deep learning models. Using POLAR simulations and data a feasibility experiment was performed to implement this method on POLAR-2. Our results indicate that using this method, in combination with real time data downlinking capabilities, POLAR-2 will be able to provide accurate localization alerts within 2 minutes of the GRB onset.

Context. The last decade has seen the emergence of two new fields within astrophysics: gamma-ray polarimetry and gravitational wave (GW) astronomy. The former, which aims to measure the polarization of gamma rays in the energy range of tens to hundreds of keV, from astrophysical sources, saw the launch of the first dedicated polarimeters such as GAP and POLAR. Due to both a large scientific interest as well as their large signal-to-noise ratios, gamma-ray bursts (GRBs) are the primary source of interest of the first generation of polarimeters. Polarization measurements are theorized to provide a unique probe of the mechanisms at play in these extreme phenomena. On the other hand, GW astronomy started with the detection of the first black hole mergers by LIGO in 2015, followed by the first multi-messenger detection in 2017. Aims. While the potential of the two individual fields has been discussed in detail in the literature, the potential for joint observations has thus far been ignored. In this article, we aim to define how GW observations can best contribute to gamma-ray polarimetry and study the scientific potential of joint analyses. In addition, we aim to provide predictions on feasibility of such joint measurements in the near future. Methods. We study which GW observables can be combined with measurements from gamma-ray polarimetry to improve the discriminating power regarding GRB emission models. We then provide forecasts for the joint detection capabilities of current and future GW detectors and polarimeters. Results. Our results show that by adding GW data to polarimetry, a single precise joint detection would allow for the majority of emission models to be ruled out. We show that in the coming years, joint detections between GW and gamma-ray polarimeters might already be possible. Although these would allow one to constrain part of the model space, the probability of highly constraining joint detections will remain small in the near future. However, the scientific merit held by even a single such measurement makes it important to pursue such an endeavour. Furthermore, we show that using the next generation of GW detectors, such as the Einstein Telescope, joint detections for which GW data can better complement the polarization data become possible.