Particle Acceleration Mechanisms Research Articles

Electron stochastic acceleration in laboratory-produced kinetic turbulent plasmas

The origin of energetic charged particles in universe remains an unresolved issue. Astronomical observations combined with simulations have provided insights into particle acceleration mechanisms, including magnetic reconnection acceleration, shock acceleration, and stochastic acceleration. Recent experiments have also confirmed that electrons can be accelerated through processes such as magnetic reconnection and collisionless shock formation. However, laboratory identifying stochastic acceleration as a feasible mechanism is still a challenge, particularly in the creation of collision-free turbulent plasmas. Here, we present experimental results demonstrating kinetic turbulence with a typical spectrum k−2.9 originating from Weibel instability. Energetic electrons exhibiting a power-law distribution are clearly observed. Simulations further reveal that thermal electrons undergo stochastic acceleration through collisions with multiple magnetic islands-like structures within the turbulent region. This study sheds light on a critical transition period during supernova explosion, where kinetic turbulences originating from Weibel instability emerge prior to collisionless shock formation. Our results suggest that electrons undergo stochastic acceleration during this transition phase.

The Role of Stochastic Fermi-type Particle Acceleration in the Inner Jets of Active Galactic Nuclei

High-resolution radio observations of nearby active galactic nuclei have revealed extended, limb-brightened structures in their inner jets. This ties in with other multiwavelength observations from radio to X-ray and gamma ray, indicating that a structured jet model is required. While electrons need to be kept energized to account for the observed features, the underlying particle acceleration mechanism is still unclear. We explore the role of stochastic Fermi-type particle acceleration, i.e., classical second-order Fermi and shear acceleration, for understanding the multiwavelength observations of the inner jets of M87. An analytical Fokker–Planck description is adopted to infer characteristic spectral indices and cutoff energies for these two mechanisms. We focus on electron synchrotron radiation as the dominant emission process. We find that the multiwavelength observations of M87 can be satisfactorily accounted for in a framework, where the X-rays are produced at a larger distance from the core than the radio emission region. This provides further support to multizone, broadband emission modeling. We use our findings to also comment on the acceleration of cosmic rays entrained in the sheath.

Investigating the Properties of the Relativistic Jet and Hot Corona in AGN with X-ray Polarimetry

X-ray polarimetry has been suggested as a prominent tool for investigating the geometrical and physical properties of the emissions from active galactic nuclei (AGN). The successful launch of the Imaging X-ray Polarimetry Explorer (IXPE) on 9 December 2021 has expanded the previously restricted scope of polarimetry into the X-ray domain, enabling X-ray polarimetric studies of AGN. Over a span of two years, IXPE has observed various AGN populations, including blazars and radio-quiet AGN. In this paper, we summarize the remarkable discoveries achieved thanks to the opening of the new window of X-ray polarimetry of AGN through IXPE observations. We will delve into two primary areas of interest: first, the magnetic field geometry and particle acceleration mechanisms in the jets of radio-loud AGN, such as blazars, where the relativistic acceleration process dominates the spectral energy distribution; and second, the geometry of the hot corona in radio-quiet AGN. Thus far, the IXPE results from blazars favor the energy-stratified shock acceleration model, and they provide evidence of helical magnetic fields inside the jet. Concerning the corona geometry, the IXPE results are consistent with a disk-originated slab-like or wedge-like shape, as could result from Comptonization around the accretion disk.

Rapid variability of Markarian 421 during extreme flaring as seen through the eyes of XMM–Newton

ABSTRACT By studying the variability of blazars across the electromagnetic spectrum, it is possible to resolve the underlying processes responsible for rapid flux increases, so-called flares. We report on an extremely bright X-ray flare in the high-peaked BL Lacertae object Markarian 421 (Mrk 421) that occurred simultaneously with enhanced γ-ray activity detected at very high energies by First G-APD Cherenkov Telescope on 2019 June 9. We triggered an observation with XMM–Newton, which observed the source quasi-continuously for 25 h. We find that the source was in the brightest state ever observed using XMM–Newton, reaching a flux of 2.8 × 10−9 $\mathrm{erg\, cm^{-2}\, s^{-1}}$ over an energy range of 0.3–10 keV. We perform a spectral and timing analysis to reveal the mechanisms of particle acceleration and to search for the shortest source-intrinsic time-scales. Mrk 421 exhibits the typical harder-when-brighter behaviour throughout the observation and shows a clock-wise hysteresis pattern, which indicates that the cooling dominates over the acceleration process. While the X-ray emission in different sub-bands is highly correlated, we can exclude large time lags as the computed z-transformed discrete correlation functions are consistent with a zero lag. We find rapid variability on time-scales of 1 ks for the 0.3–10 keV band and down to 300 s in the hard X-ray band (4–10 keV). Taking these time-scales into account, we discuss different models to explain the observed X-ray flare, and find that a plasmoid-dominated magnetic reconnection process is able to describe our observation best.

X-Ray Polarization Variability of High Spectral Peak BL Lacertaes: Cases of 1ES 1959+650 and PKS 2155–304

Abstract The high-energy-peaked BL Lacertae objects (HBLs) are the main targets of the Imaging X-ray Polarimetry Explorer (IXPE) for investigating the mechanisms of radiation and particle acceleration in jets. In this Letter, we report the first IXPE observations of two HBLs, 1ES 1959+650 and PKS 2155–304. Both sources exhibit X-ray polarization with a confidence level exceeding 99%, as well as significant variability in polarization across different time intervals and energy ranges. Notably, PKS 2155–304 demonstrates the highest X-ray polarization among all blazars detected by IXPE within its entire energy band (2–8 keV), with a polarization degree of ΠX = 21.9% ± 1.9% (MDP99 ∼ 6.0%). An even higher polarization is observed in the 3–4 keV band, reaching ΠX = 28.6% ± 2.7% (MDP99 ∼ 8.1%) with a confidence level of 10.8σ. Furthermore, no polarization is detected above the 5 keV energy band. For 1ES 1959+650, the highest detected polarization degree in the 2–8 keV band is ΠX = 12.4% ± 0.7% (MDP99 ∼ 2.2%), with an electric vector position angle (EVPA) of ψ X = 19.°7 ± 1.°6. The X-ray polarization of 1ES 1959+650 exhibits evident variability, accompanied by the variations of ψ X, flux, spectrum, and energy bin. We discuss possible implications of these observational findings, including the variability in polarization, rotation of EVPA, and transition between synchrotron and synchrotron–self-Compton. We speculate that the X-rays observed during different IXPE observations originate from distinct regions in the jet and may involve diverse mechanisms for particle acceleration.

ZWCL 1856.8: A Rare Double Radio Relic System Captured within NuSTAR and Chandra Field of View

Observations of galaxy cluster mergers provide insights on the particle acceleration and heating mechanisms taking place within the intracluster medium. Mergers form shocks that propagate through the plasma, which result in shock/cold fronts in the X-ray, and radio halos and/or relics in the radio regime. The connection between these tracers and the mechanisms driving non-thermal processes, such as inverse Compton, are not well understood. ZWCL 1856.8 is one of the few known double radio relic systems that originate from nearly head-on collisions observed close to the plane of the sky. For the first time, we study NuSTAR and Chandra observations of this system, which contains both relics within their field of view. The spectro-imaging analyses results of the system suggest weak shock fronts with M numbers within 2σ of the radio-derived values, and provide evidence of inverse Compton emission at both relic sites. Our findings have great uncertainties due to the shallow exposure times available. Deeper NuSTAR and Chandra data are crucial for studying the connection of the radio and X-ray emission features and for constraining the thermal versus non-thermal emission contributions in this system. We also present methods and approaches on how to investigate X-ray properties of double relic systems by taking full advantage of the complementary properties of NuSTAR and Chandra missions.

Magnetic field properties inside the jet of Mrk 421

Aims. We aim to probe the magnetic field geometry and particle acceleration mechanism in the relativistic jets of supermassive black holes. Methods. We conducted a polarimetry campaign from radio to X-ray wavelengths of the high-synchrotron-peak (HSP) blazar Mrk 421, including Imaging X-ray Polarimetry Explorer (IXPE) measurements from 2022 December 6–8. During the IXPE observation, we also monitored Mrk 421 using Swift-XRT and obtained a single observation with XMM-Newton to improve the X-ray spectral analysis. The time-averaged X-ray polarization was determined consistently using the event-by-event Stokes parameter analysis, spectropolarimetric fit, and maximum likelihood methods. We examined the polarization variability over both time and energy, the former via analysis of IXPE data obtained over a time span of 7 months. Results. We detected X-ray polarization of Mrk 421 with a degree of ΠX = 14 ± 1% and an electric-vector position angle ψX = 107 ± 3° in the 2–8 keV band. From the time variability analysis, we find a significant episodic variation in ψX. During the 7 months from the first IXPE pointing of Mrk 421 in 2022 May, ψX varied in the range 0° to 180°, while ΠX remained relatively constant within ∼10–15%. Furthermore, a swing in ψX in 2022 June was accompanied by simultaneous spectral variations. The results of the multiwavelength polarimetry show that ΠX was generally ∼2–3 times greater than Π at longer wavelengths, while ψ fluctuated. Additionally, based on radio, infrared, and optical polarimetry, we find that the rotation of ψ occurred in the opposite direction with respect to the rotation of ψX and over longer timescales at similar epochs. Conclusions. The polarization behavior observed across multiple wavelengths is consistent with previous IXPE findings for HSP blazars. This result favors the energy-stratified shock model developed to explain variable emission in relativistic jets. We considered two versions of the model, one with linear and the other with radial stratification geometry, to explain the rotation of ψX. The accompanying spectral variation during the ψX rotation can be explained by a fluctuation in the physical conditions, for example in the energy distribution of relativistic electrons. The opposite rotation direction of ψ between the X-ray and longer wavelength polarization accentuates the conclusion that the X-ray emitting region is spatially separated from that at longer wavelengths. Moreover, we identify a highly polarized knot of radio emission moving down the parsec-scale jet during the episode of ψX rotation, although it is unclear whether there is any connection between the two events.

Optothermal Needle-Free Injection of Vaterite Nanocapsules.

The propulsion and acceleration of nanoparticles with light have both fundamental and applied significance across many disciplines. Needle-free injection of biomedical nano cargoes into living tissues is among the examples. Here a new physical mechanism of laser-induced particle acceleration is explored, based on abnormal optothermal expansion of mesoporous vaterite cargoes. Vaterite nanoparticles, a metastable form of calcium carbonate, are placed on a substrate, underneath a target phantom, and accelerated toward it with the aid of a short femtosecond laser pulse. Light absorption followed by picosecond-scale thermal expansion is shown to elevate the particle's center of mass thus causing acceleration. It is shown that a 2µm size vaterite particle, being illuminated with 0.5W average power 100fsec IR laser, is capable to overcome van der Waals attraction and acquire 15msec-1 velocity. The demonstrated optothermal laser-driven needle-free injection into a phantom layer and Xenopus oocyte in vitro promotes the further development of light-responsive nanocapsules, which can be equipped with additional optical and biomedical functions for delivery, monitoring, and controllable biomedical dosage to name a few.

The Planck clusters in the LOFAR sky

Context. It is well established that shock waves in the intracluster medium launched by galaxy cluster mergers can produce synchrotron emission, which is visible to us at radio frequencies as radio relics. However, the particle acceleration mechanism producing these relics is still not fully understood. It is also unclear how relics relate to radio halos, which trace merger-induced turbulence in the intracluster medium. Aims. We aim to perform the first statistical analysis of radio relics in a mass-selected sample of galaxy clusters, using homogeneous observations. Methods. We analysed all relics observed by the Low Frequency Array Two Metre Sky Survey Data Release 2 (LoTSS DR2) at 144 MHz, hosted by galaxy clusters in the second Planck catalogue of SZ sources (PSZ2). We measured and compared the relic properties in a uniform, unbiased way. In particular, we developed a method to describe the characteristic downstream width in a statistical manner. Additionally, we searched for differences between radio relic-hosting clusters with and without radio halos. Results. We find that, in our sample, ∼10% of galaxy clusters host at least one radio relic. We confirm previous findings, at higher frequencies, of a correlation between the relic-cluster centre distance and the longest linear size, as well as the radio relic power and cluster mass. However, our findings suggest that we are still missing a population of low-power relics. We also find that relics are wider than theoretically expected, even with optimistic downstream conditions. Finally, we do not find evidence of a single property that separates relic-hosting clusters with and without radio halos.

Theory and prospects of the pulsar study

After the pulsar was discovered by Hewish, the relevant research has become one of the hottest research fields. Astronomers have noticed that pulsars are of great academic significance in the field of basic scientific research. Due to the large mass and small radius of pulsars, their surface gravitational field is very strong. The existence of general relativity effects cannot be ignored, making pulsars a natural laboratory for the study of strong gravitational fields. the strong magnetic field provides an identical place for the study of radio radiation process, magnetospheric particle acceleration mechanism and high energy radiation. Pulsar as the product of supernova burst after the collapse of massive stars, it is very important for the study of supernova burst theory and understand the formation mechanism of pulsars. In terms of application research, due to the high stability of its rotation period, it has significant applicating prospect in measuring time standard and X-ray pulsar navigation system. The Pulsar has made great remarkable achievements in the past 50 years since its discovery. In short, the discovery and research significance of pulsars are highly anticipated.

An X-rays-to-radio investigation of the nuclear polarization from the radio-galaxy Centaurus A

ABSTRACT Centaurus A is one of the closest radio galaxies to Earth. Its proximity allowed us to extensively study its active galactic nucleus but the core emission mechanism remains elusive because of local strong dust and gas obscuration. The capability of polarimetry to shave-off contaminating emission has been exploited without success in the near-infrared by previous studies but the very recent measurement of the 2–8 keV polarization by the Imaging X-ray Polarimetry Explorer (IXPE) brought the question back to the fore. To determine what is the prevalent photon generation mechanism to the multiwavelength emission from the core of Centaurus A, we retrieved from the archives the panchromatic polarization measurements of the central compact component. We built the total and polarized flux spectral energy distributions of the core and demonstrated that synchrotron self-Compton models nicely fit the polarized flux from the radio to the X-ray band. The linear polarization of the synchrotron continuum is perpendicular to the jet radio axis from the optical to the radio band, and parallel to it at higher energies. The observed smooth rotation of the polarization angle in the ultraviolet band is attributed to synchrotron emission from regions that are getting closer to the particle acceleration site, where the orientation of the jet’s magnetic fields become perpendicular to the jet axis. This phenomenon support the shock acceleration mechanism for particle acceleration in Centaurus A, in line with IXPE observations of several high-synchrotron peak blazars.

Interplay of Three‐Dimensional Instabilities and Magnetic Reconnection in the Explosive Onset of Magnetospheric Substorms

AbstractMagnetospheric substorms are preceded by a slow growth phase of magnetic flux loading and current sheet thinning in the tail. Extensive data sets have provided evidence of the triggering of instabilities at substorm onset, including magnetic reconnection and ballooning instabilities. Using an exact kinetic magnetotail equilibrium we present particle‐in‐cell simulations which capture the explosive nature of substorms through a disruption of the dipolarization front by the ballooning instability. We use self‐consistent particle tracking to determine the nonthermal particle acceleration mechanisms.

Striped Jets in Post–Neutron Star Merger Systems

Models invoking magnetic reconnection as the particle acceleration mechanism within relativistic jets often adopt a gradual energy dissipation profile within the jet. However, such a profile has yet to be reproduced in first-principles simulations. Here we perform a suite of 3D general relativistic magnetohydrodynamic simulations of post–neutron star merger disks with an initially purely toroidal magnetic field. We explore the variations in both the microphysics (e.g., nuclear recombination, neutrino emission) and system parameters (e.g, disk mass). In all of our simulations, we find the formation of magnetically striped jets. The stripes result from the reversals in the poloidal magnetic flux polarity generated in the accretion disk. The simulations display large variations in the distributions of stripe duration, τ, and power, 〈P Φ〉. We find that more massive disks produce more powerful stripes, the most powerful of which reaches 〈P Φ〉 ∼ 1049 erg s−1 at τ ∼ 20 ms. The power and variability that result from the magnetic reconnection of the stripes agree with those inferred in short-duration gamma-ray bursts. We find that the dissipation profile of the cumulative energy is roughly a power law in both radial distance, z, and τ, with a slope in the range of ∼1.7–3; more massive disks display larger slopes.

Particle acceleration in self-driven turbulent reconnection

The theoretical prediction that magnetic reconnection spontaneously drives turbulence has been supported by magnetohydrodynamic (MHD) and kinetic simulations. While reconnection with externally driven turbulence is accepted as an effective mechanism for particle acceleration, the acceleration during the reconnection with self-driven turbulence is studied for the first time in this work. By using high-resolution 3D MHD simulations of reconnection with self-generated turbulence, we inject test particles into the reconnection layer to study their acceleration process. We find that the energy gain of the particles takes place when they bounce back and forth between converging turbulent magnetic fields. The particles can be efficiently accelerated in self-driven turbulent reconnection with the energy increase by about 3 orders of magnitude in the range of the box size. The acceleration proceeds when the particle gyroradii become larger than the thickness of the reconnection layer. We find that the acceleration in the direction perpendicular to the local magnetic field dominates over that in the parallel direction. The energy spectrum of accelerated particles is time-dependent with a slope that evolves toward -2.5. Our findings can have important implications for particle acceleration in high-energy astrophysical environments.

Observation of an Electron Microburst With an Inverse Time‐Of‐Flight Energy Dispersion

AbstractInteractions between whistler mode chorus waves and electrons are a dominant mechanism for particle acceleration and loss in the outer radiation belt. One form of this loss is electron microburst precipitation: a sub‐second intense burst of electrons. Despite previous investigations, details regarding the microburst‐chorus scattering mechanism—such as dominant resonance harmonic—are largely unconstrained. One way to observationally probe this is via the time‐of‐flight energy dispersion. If a single cyclotron resonance is dominant, then higher energy electrons will resonate at higher magnetic latitudes: sometimes resulting in an inverse time‐of‐flight dispersion with lower‐energy electrons leading. Here we present a clear example of this phenomena, observed by a FIREBIRD‐II CubeSat on 27 August 2015, that shows good agreement with the Miyoshi‐Saito time‐of‐flight model. When constrained by this observation, the Miyoshi‐Saito model predicts that a relatively narrowband chorus wave with a ∼0.2 of the equatorial electron gyrofrequency scattered the microburst.

IXPE and Multiwavelength Observations of Blazar PG 1553+113 Reveal an Orphan Optical Polarization Swing

The lower-energy peak of the spectral energy distribution of blazars has commonly been ascribed to synchrotron radiation from relativistic particles in the jets. Despite the consensus regarding jet emission processes, the particle acceleration mechanism is still debated. Here, we present the first X-ray polarization observations of PG 1553+113, a high-synchrotron-peak blazar observed by the Imaging X-ray Polarimetry Explorer (IXPE). We detect an X-ray polarization degree of (10 ± 2)% along an electric-vector position angle of ψ X = 86° ± 8°. At the same time, the radio and optical polarization degrees are lower by a factor of ∼3. During our IXPE pointing, we observed the first orphan optical polarization swing of the IXPE era, as the optical angle of PG 1553+113 underwent a smooth monotonic rotation by about 125°, with a rate of ∼17° day–1. We do not find evidence of a similar rotation in either radio or X-rays, which suggests that the X-ray and optically emitting regions are separate or, at most, partially cospatial. Our spectropolarimetric results provide further evidence that the steady-state X-ray emission in blazars originates in a shock-accelerated and energy-stratified electron population.

A MeerKAT-meets-LOFAR study of Abell 1413: a moderately disturbed non-cool-core cluster hosting a ~500 kpc ‘mini’-halo

ABSTRACT Many relaxed cool-core clusters host diffuse radio emission on scales of hundreds of kiloparsecs: mini-haloes. However, the mechanism responsible for generating them, as well as their connection with central active galactic nuclei, is elusive and many questions related to their physical properties and origins remain unanswered. This paper presents new radio observations of the galaxy cluster Abell 1413 performed with MeerKAT (L-band; 872 to 1712 MHz) and LOFAR HBA (120 to 168 MHz) as part of a statistical and homogeneous census of mini-haloes. Abell 1413 is unique among mini-halo clusters as it is a moderately disturbed non-cool-core cluster. Our study reveals an asymmetric mini-halo up to 584 kpc in size at 1283 MHz, twice as large as first reported at similar frequencies. The spectral index is flatter than previously reported, with an integrated value of α = −1.01 ± 0.06, shows significant spatial variation, and a tentative radial steepening. We studied the point-to-point X-ray/radio surface brightness correlation to investigate the thermal/non-thermal connection: our results show a strong connection between these components, with a superlinear slope of $b = 1.63^{+0.10}_{-0.10}$ at 1283 MHz and $b = 1.20^{+0.13}_{-0.11}$ at 145 MHz. We also explore the X-ray surface brightness/radio spectral index correlation, finding a slope of $b = 0.59^{+0.11}_{-0.11}$. Both investigations support the evidence of spectral steepening. Finally, in the context of understanding the particle acceleration mechanism, we present a simple theoretical model which demonstrates that hybrid scenarios – secondary electrons (re-)accelerated by turbulence – reproduce a superlinear correlation slope.

Extreme ion acceleration at extragalactic jet termination shocks

Context. Extragalactic plasma jets are some of the few astrophysical environments able to confine ultra-high-energy cosmic rays, but whether they are capable of accelerating these particles is unknown. Aims. In this work, we revisit particle acceleration at relativistic magnetized shocks beyond the local uniform field approximation by considering the global transverse structure of the jet. Methods. Using large two-dimensional particle-in-cell simulations of a relativistic electron-ion plasma jet, we show that the termination shock forming at the interface with the ambient medium accelerates particles up to the confinement limit. Results. The radial structure of the jet magnetic field leads to a relativistic velocity shear that excites a von Kármán vortex street in the downstream medium trailing behind an over-pressured bubble filled with cosmic rays. Particles are efficiently accelerated at each crossing of the shear flow boundary layers. Conclusions. These findings support the idea that extragalactic plasma jets may be capable of producing ultra-high-energy cosmic rays. This extreme particle acceleration mechanism may also apply to microquasar jets.

Study of particle acceleration by Laguerre–Gaussian ultra intense laser plasma interactions

In this work, simulations of multi-petawatt lasers in the range of ∼0.2 PW–∼100 PW with varying Laguerre–Gaussian (LG) azimuthal modes as well as linearly polarized (LP) and circularly polarized (CP) laser beams striking near critical density targets were studied by using three-dimensional particle in cell (PIC) codes. Particle acceleration mechanisms have a dependence on laser polarization and it affects the energy gained by the particles. It is known that laser pulses can be polarized helically by applying the LG distribution function to the fundamental Gaussian laser profile. In this study, differently polarized laser beams with varying powers were employed to study laser driven particle acceleration and compares accelerated charged particles’ energy spectra and angular distribution. It is seen that LG laser beams can accelerate higher energetic particles due to higher conversion efficiency compared to LP and CP laser beams. It is also seen that LG laser beams can collimate ions with a narrower spread compared to LP and CP beams. Furthermore, ions can have a smaller divergence angle with increasing azimuthal mode index when the laser is LG polarized. We also studied the energy deposition of these particles in a water cell obtained by the PIC codes for different laser parameters by using Geant4 Monte Carlo simulations which suggests that LG laser beam can be useful for the future hadron therapy applications.

The diffuse radio emission in the high-redshift cluster PSZ2 G091.83+26.11: Total intensity and polarisation analysis with Very Large Array 1–4 GHz observations

Context. Diffuse radio emission in galaxy clusters, namely radio halos and radio relics, is usually associated with merger events. Despite the tremendous advances in observations in the last decades, the particle (re-)acceleration and magnetic field amplification mechanisms and the connection with the stage and geometry of the cluster merger are still uncertain. Aims. In this paper, we present the peculiar case of PSZ2 G091.83+26.11 at z = 0.822. This cluster hosts a megaparsec-scale radio halo and an elongated radio source whose morphology resembles that of a radio relic. However, the location of this diffuse radio source with respect to the intracluster medium (ICM) distribution and to the cluster centre is not consistent with a simple merger scenario. Methods. We use Karl Jansky Very Large Array data at 1–4 GHz to investigate the spectral and polarisation properties of the diffuse radio emission. We combine these data with previously published data from the Low Frequency Array (LOFAR) in the 120–168 MHz band and from the upgraded Giant Metrewave Radio Telescope (uGMRT) at 250–500 and 550–900 MHz. Finally, we complement the radio data with Chandra X-ray observations in order to compare the thermal and non-thermal emission of the cluster. Results. The elongated radio emission east of the cluster is visible up to 3.0 GHz and has an integrated spectral index of α144 MHz3.0 GHz = −1.24 ± 0.03, with a steepening from −0.89 ± 0.03 to −1.39 ± 0.03. These values correspond to Mach numbers ℳradio, int = 3.0 ± 0.19 and ℳradio, inj = 2.48 ± 0.15. Chandra data revealed a surface brightness discontinuity at the location of the radio source with a compression factor of C = 2.22 −0.30+0.39 (i.e. MXray = 1.93−0.32+0.42). We also found that the source is polarised at GHz frequencies. Using QU-fitting, we estimate an intrinsic polarisation fraction of p0 ∼ 0.2, a Rotation Measure of ∼50 rad m−2 (including the Galactic contribution), and an external depolarisation of σRM ∼ 60 rad m−2. The polarisation B-vectors are aligned with the major axis of the source, suggesting magnetic field compression. Hence, we classify this source as a radio relic. Finally, we found a trend consistent with a linear or super-linear correlation between the non-thermal and thermal emission. Conclusions. We propose an off-axis merger and/or multiple merger events to explain the position and orientation of the relic with the respect to the ICM emission. Given the properties of the radio relic, we speculate that PSZ2 G091.83+26.11 is in a fairly young merger state.