Two-dimensional relativistic particle-in-cell simulations of radiative magnetic reconnection in pair plasmas with multiple interacting current sheets are carried out to mimic the dynamics in high-energy astrophysical environments, such as particle acceleration regions in pulsar wind nebulae and relativistic outflows, where the magnetic field is expected to reverse polarity multiple times. Initially, due to reconnection within each isolated sheet, particles are accelerated and synchrotron emission beyond the burn-off limit is confirmed, even if the particle distribution function shows steep slopes. After this phase, plasmoids lead to cross-sheet interactions and merging, with new current sheets formed. In this regime, a Kolmogorov-like spectrum for the magnetic energy develops over a couple of decades, followed by a dissipation range starting around 5 de (electron inertial lengths), showing that multi-sheet reconnection evolves nonlinearly into well-developed turbulence. This phase provides secondary acceleration and further cooling by synchrotron emission, with intermittent radiative bursts. We show that high energy accelerated particles by the primary current sheets are further energized during the turbulent phase, while the distribution of the most energetic particles remains steep.

The Enhanced X-ray Polarimetry Observatory (EXPO) is a mission concept proposed to ESA as an M8 candidate, with a prospective launch in 2041. Building on the scientific success of IXPE, EXPO is designed to overcome its two main limitations, the narrow 2-8 keV energy band and the very slow repointing time, and to enable new scientific capabilities. A wide energy band and fast repointing are essential for investigating the hard X-ray emission of magnetars and black-hole binaries, particle acceleration in supernova remnants and pulsar-wind nebulae, radiative transfer in highly magnetized plasmas, X-ray reflection in accretion flows and active galactic nuclei, and the prompt and afterglow emission of gamma-ray bursts and magnetar flares. EXPO comprises five focusing X-ray telescopes and gas photoelectric polarimeters based on the Timepix ASIC family with InGrid amplification, enabling three-dimensional track imaging and operation in the 2-35 keV band through optimized low- and medium-energy detector configurations. The mirror modules use proven electroformed nickel technology with Au-C coatings and an XMM-like focal length of 7.5 m. The polarimeters are complemented by a coded-mask Wide Field Instrument (WFI), derived from SVOM/ECLAIRs for continuous monitoring of a 2 sr field of view; a Spectral Imaging Camera (SIC), based on stacked CMOS and CdTe detectors for broadband imaging spectroscopy and accurate spectro-polarimetric decomposition; and an Instrument Control Unit (ICU) for payload management, onboard WFI image reconstruction, transient identification, and autonomous spacecraft repointing requests. These capabilities extend X-ray polarimetry into the hard X-ray domain and open a new observational window on fast transients, time-domain astrophysics, and multi-messenger astronomy.

RCW 86 is a supernova remnant whose origin has recently been linked to an off-center explosion within a cavity created by its progenitor star. In the southwestern region, the forward shock is thought to have reached the cavity wall, encountering diverse environmental conditions. We report on the spatially resolved X-ray polarimetric observation of RCW 86 with the Imaging X-ray Polarimetry Explorer (IXPE). In the 2-4.5 keV energy band we find no significant detection of polarization. Employing a dedicated background subtraction procedure and Bayesian spectropolarimetric fitting, we derive 99% upper limits on the polarization degree of the synchrotron component: 15% in higher-statistics regions and 30%-40% in lower-statistics regions. These upper limits on the polarization degree in several regions exclude the possibility of a strongly coherent magnetic field down to the subparsec scale, and that of a moderately coherent one on the scale of the synchrotron features as resolved by IXPE. The results indicate that the shocks in the southwestern rim of RCW 86 propagate more slowly than the unshocked ejecta at their locations, yet exceed the measured proper motion speeds. This behavior is consistent with reflected shocks occurring in tenuous regions of the shocked ejecta, distinct from regions that are radio-bright.

We report on the average and orbital phase-resolved polarization of Cyg X-3 in the hard state (HS) during the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observational campaign. We find the polarization degree of 21.4% +/- 0.4% and polarization angle of 92.degrees 2 +/- 0.degrees 5 , compatible within 3 sigma with the first HS IXPE observation in 2022. As the observed polarization depends on both the accretion geometry and the X-ray emission mechanism, and assuming it arises from reflection from the optically thick envelope surrounding the central source, our result indicates that both are similar in the two HS epochs on year-long timescales despite the source transitioning through an intermediate state in between. We discuss time- and energy-dependent polarization properties and their implications for the geometry and stability of the accretion funnel, finding significant orbital modulation of the polarization properties.

The extended X-ray emission observed in the direction of several molecular clouds in the central molecular zone (CMZ) of our Galaxy exhibits spectral and temporal properties consistent with the X-ray echo scenario. This concept postulates that the observed signal is a light-travel-time delayed reflection of a short (delta t <1.5 yr) and bright (L-X > 10(39) erg s(-1)) flare that was most probably produced a few hundred years ago by Sgr A*. This scenario also predicts a distinct polarization signature for the reflected X-ray continuum, with the polarization vector being perpendicular to the direction toward the primary source and the polarization degree being determined by the scattering angle. We report the results of two deep observations of the currently brightest (in reflected emission) molecular complex Sgr A taken with the Imaging X-ray Polarimetry Explorer (IXPE) in 2022 and 2023. We confirm the previous polarization measurement for a large region encompassing the Sgr A complex with high significance. We also reveal an inconsistent polarization pattern for the brightest reflection region in its center. Specifically, the X-ray polarization from this region is almost perpendicular to the expected direction in the case of Sgr A* illumination, and it shows a smaller degree of polarization compared to the large region. Taken at face value, this could indicate the simultaneous propagation of several illumination fronts throughout the CMZ, with the origin of one of them not being Sgr A*. The primary source could be associated with the Arches stellar cluster or a currently unknown source located closer to the illuminated cloud, potentially lowering the required luminosity of the primary source. Although significantly deeper observations with IXPE would be required to unequivocally distinguish between the scenarios, a combination of high-resolution imaging and micro-calorimetric spectroscopy offers an additional promising path forward.

Hard X-ray emission from Cas A has been attributed to jitter radiation, implying magnetic turbulence on scales near or below the ion Larmor radius. We propose these fields arise from ion-scale micro-instabilities (e.g., mirror and firehose) in the shock downstream, and use high-resolution hybrid simulations to demonstrate the generation of such small-scale turbulence. Its spectral index, once numerical dissipation is accounted for, is consistent with that inferred for Cas A, suggesting that jitter radiation is a direct probe of ion-scale turbulence in supernova remnants.

The Lighthouse pulsar (PSR J1101-6101) sports a bright X-ray trail and filament. The synchrotron emission from both structures is expected to be polarized, with electric vector position angle (EVPA) perpendicular to the magnetic field direction and polarization degree (PD) indicating the local degree of magnetic turbulence. We present a 1 Ms Imaging X-ray Polarimetry Explorer observation of the Lighthouse complex. At the 99% confidence level, we detect the filament polarization with PD 55% +/- 18% and EVPA indicating a magnetic field parallel to the filament axis. The large PD implies a turbulent magnetic field weaker than the background field, in conflict with some existing models. We also detect polarization from the pulsar and trail. The trail's X-ray polarization is nearly orthogonal to the radio polarization, suggesting spatial separation between the X-ray- and radio-emitting leptons. The pulsar polarization is well fit by the rotating vector model.

Pulsar Wind Nebulae (PWNe) are among the most important sources emitting in the very-high-energy gamma-ray band. Predicting their long-term evolution is crucial for forthcoming high-energy observatories like ASTRI and CTA. In this work, We investigate the dynamical evolution of middle-aged PWNe - probably the major contributors to the Galactic TeV emission - and test the robustness of current approaches. To understand the diversity of these systems, we derive the Pulsar and Supernova Remnant (SNR) parameters governing PWN evolution. SNR evolution is set by supernova kinetic energy, ejecta mass, and ambient density, while pulsar energy injection powers the PWN expansion. Adopting standard distributions, we generate a synthetic PWN-SNR population and define a region of interest encompassing the majority of these objects. We use a semi-analytical framework for the early evolution and a 1D Lagrangian code to track their interaction with parent SNRs (the reverberation phase). Within our region of interest, we find large diversity in the late-stage evolution. Despite this, all systems converge toward a relaxed state consistent with the Sedov solution. To address 1D limitations, we perform 2D simulations - optimized to reduce computational cost while preserving physical accuracy - to study instability growth and long-term mixing. We find that instability growth depends on initial perturbations but does not significantly alter global dynamics. While effective volume evolution agrees with 1D predictions, multidimensional effects can increase the apparent size by up to 50

The Crab pulsar experienced two relatively small glitches separated by only 20 days in 2025 September and October. Imaging X-ray Polarimetry Explorer (IXPE) observed the source twice, with delay times since the glitch epoch ranging between 35 and 75 days, depending on the observation. We carried out a multimethod analysis to investigate whether there is evidence for significant changes in the polarization properties of the pulsar, underlying possible variations in the pulsar magnetosphere itself following the glitches. Specifically, we performed (1) phase-averaged polarimetry of the Crab pulsar before and after the glitches, following an approach similar to that adopted in 2019 by PolarLight, a nonimaging CubeSat-class photoelectric polarimeter which observed a change in the X-ray polarization within 100 days after a stronger glitch in 2019 July, and (2) a comparison, before and after the glitch, of phase-resolved X-ray polarimetry with IXPE, not possible with PolarLight. Furthermore, we investigated, by means of phase-resolved optical (OPTIMA) polarimetry, whether a significant change in the X-to-optical lag was present in the data before and after the glitch. We find no evidence of a change in the polarization for the pulsar emission before and after the glitch. We use the upper limits obtained to estimate the maximum change in magnetic obliquity allowed by the data, using the standard rotating vector model and assuming that the glitch is due to a neutron-star quake. We constrain this maximum change to be no greater than +/- 4 degrees at the 95% confidence level.

Bow-shock pulsar wind nebulae are valuable sources to investigate the dynamics of relativistic pulsar winds and the mechanisms by which they are converted into cosmic-ray leptons at the highest energies. The Lighthouse Nebula is one such object, famous for the high velocity of its pulsar and a long misaligned X-ray jet that is understood as a specific escape channel for the most energetic particles. We aim to get a better understanding of how the bulk of non-thermal particles are released into the interstellar medium. We focus on GHz radio observations, which probe lower-energy particles that are dominant in number and long-lived, thus offering a picture of how escape proceeds in the long run. We analyze 10.5h of MeerKAT observations in the 0.9-1.7GHz band. MeerKAT observations reveal a highly structured synchrotron nebula downstream of pulsar PSR J1101-6101. A cometary tail is detected up to beyond 5pc from the pulsar, while a system of multiple transverse two-sided emission streaks is observed for the first time. No radio counterpart of the misaligned X-ray jet is seen. The radio streaks are interpreted as the occasional charge-independent release of energetic leptons from the tail into the surrounding medium, as a result of dynamical instabilities and reconfiguration in the downstream flow. The intensity layout suggests that most of the particle content of the nebula is discharged into the ambient medium within several parsec. Once escaped, particles light up the ambient magnetic field, which appears to have a coherence length of at least a few parsec. The length and persistence of the streaks indicate a low level of magnetic turbulence, possibly slightly enhanced with respect to average cosmic-ray transport conditions in the Galaxy. Such a confinement may result from self-generated turbulence by resonant streaming instability, or be due to past activity of the progenitor star.

Relativistically hot plasmas are well known astrophysical sources of synchrotron emission, and the degree of linear polarization is affected by the level of turbulence in the source. Here we show, by means of a series of 3D numerical simulations, how the properties of decaying turbulence in hot plasmas depend on the magnetization of both the initial guide field and fluctuations, and how the turbulent Kolmogorov-type cascade proceeds in time. Dissipation occurs in thin, intermittent current sheets, variance anysotropy and non-Gaussian deviations appear at small scales. The computed synthetic polarization maps and degree depend on the plasma dynamics and on the angle of the line-of-sight direction with respect to the guide field. We describe how observations of these quantities may be used to infer the turbulence properties in the source.

We present the first X-ray polarization measurements of the PSR/PWN complex within SNR Kes 75. Two $\rm {\sim}\,500\,ks$ IXPE observations were conducted in October/November 2024 and April 2025. The second observation yields a significant phase-average 2-8 keV polarization degree $\rm PD = 9.9\% \pm 2.5\%$ at $\rm PA=36.8^\circ \pm 7.3^\circ$, implying a toroidal field aligned with the PWN symmetry axis. The first epoch, however, has only a polarization upper limit. During this epoch, an additional pulsed component is visible at $Δϕ\approx 0.5$, detected at ${\sim}\,3.7σ$. An unbinned phase-resolved analysis reveals a high-PD rotating vector model PA sweep at the ${\sim}\,99.5\%$ confidence level, with angles fixed at those inferred from the PWN morphology; this can explain the loss of phase-average polarization. Additional observations are needed to pin down the nature of the anomalous pulse.

Young massive stellar clusters (YMSCs) have emerged as potential γ-ray sources after the recent association of a dozen YMSCs with extended γ-ray emission. The large size of the detected halos, comparable to that of the wind-blown bubble expected around YMSCs, makes the γ-ray detection of individual YMSCs rather challenging. As a result, the emission from most of the Galactic YMSCs could be unresolved, thus contributing to the diffuse γ-ray radiation observed along the Galactic Plane. In this study, we estimate the possible contribution to the Galactic diffuse γ-ray emission from a synthetic population of YMSCs, and we compare it with observations obtained with different experiments, from 1 GeV to hundreds of teraelectronvolt in three regions of the Galactic Plane. As the population of galactic YMSCs is only known locally, we evaluated the contribution of γ-ray emission relying on the simulation of synthetic populations of YMSCs based on the observed properties of local clusters. We computed the γ-ray emission from each cluster assuming that the radiation is purely hadronic in nature and produced by cosmic rays that are accelerated at the cluster's collective wind termination shock. We find that the γ-ray emission from unresolved YMSCs can significantly contribute to the observed Galactic diffuse flux, especially in the inner part of the Galaxy, and that an important role is played by kinetic power injected by the Wolf-Rayet stellar winds. The predicted γ-ray flux should be considered as a lower limit, given that our calculation does not include the contribution of supernovae exploding in YMSCs.

We report the Imaging X-ray Polarimetry Explorer (IXPE) polarimetric and simultaneous multiwavelength observations of the high-energy-peaked BL Lacertae object (HBL) 1ES 1959+650, performed in 2022 October and 2023 August. In 2022 October, IXPE measured an average polarization degree Π _X = 9.4% ± 1.6% and an electric-vector position angle ψ _X = 53° ± 5°. The polarized X-ray emission can be decomposed into a constant component, plus a rotating component, with the rotation velocity ω _EVPA = (−117 ± 12) deg day ^−1 . In 2023 August, during a period of pronounced activity of the source, IXPE measured an average Π _X = 12.4% ± 0.7% and ψ _X = 20° ± 2°, with evidence (∼0.4% chance probability) for a rapidly rotating component with ω _EVPA = 1864 ± 34 deg day ^−1 . These findings suggest the presence of a helical magnetic field in the jet of 1ES 1959+650 or stochastic processes governing the field in turbulent plasma. Our multiwavelength campaigns from radio to X-ray reveal variability in both polarization and flux from optical to X-rays. We interpret the results in terms of a relatively slowly varying component dominating the radio and optical emission, while rapidly variable polarized components dominate the X-ray and provide minor contribution at optical wavelengths. The radio and optical data indicate that on parsec scales the magnetic field is primarily orthogonal to the jet direction. On the contrary, X-ray measurements show a magnetic field almost aligned with the parsec jet direction. Confronting with other IXPE observations, we guess that the magnetic field of HBLs on subparsec scale should be rather unstable, often changing its direction with respect to the Very Long Baseline Array jet.

We present a phase-dependent analysis of the polarized emission from the Crab pulsar based on three sets of observations by the Imaging X-ray Polarimetry Explorer (IXPE). We found that a phenomenological model involving a simple linear transformation of the Stokes parameters adequately describes the IXPE data. This model enabled us to establish a connection between the polarization properties of the Crab pulsar in the optical and soft X-ray bands for the first time, which suggests a common underlying emission mechanism in these bands that likely is synchrotron radiation. In particular, the phase-dependent polarization degree in X-rays for the pure pulsar emission shows similar features, but is reduced by a factor approximate to(0.46 - 0.56) compared to the optical band (when we accounted for the contribution of the knot in the optical), which implies an energy-dependent polarized emission. Using this model, we also studied the polarization angle swing in the X-rays and identified a potentially variable phase shift at the interpulse relative to the optical band, alongside a phase shift that is marginally consistent with zero and persists at the main pulse. While the origin of this variability is unknown and presents a new challenge for the theoretical interpretation, our findings suggest that the emission mechanism for the main pulse is likely located far from the neutron star surface, perhaps near to or beyond the light cylinder, and that it does not operate in the inner magnetosphere, where vacuum birefringence is expected to be at work. Ignoring the phase shifts would result in identical phase-dependent polarization angles between the optical and X-ray bands for the pure pulsar emission.

Aims. The emission from the relativistically hot plasmas of high-energy astrophysical synchrotron sources, pulsar wind nebulae (PWNe) in particular, depends on the level of magnetic fluctuations. Recent observations by the X-ray polarimeter IXPE support the presence of turbulence, with varying conditions even in different regions of a same source. We model such emission, and in particular the degree of linear polarization, by using 3D relativistic magnetohydrodynamic (MHD) turbulence simulations for the first time.Methods. Thanks to a novel accelerated version of the ECHO code, a series of 3D relativistic MHD simulations were performed assuming a relativistically hot plasma and various degrees of magnetization, mimicking different conditions encountered in synchrotron sources. Magnetic fluctuations at random directions with respect to a background field were initialized at large scales. After the full development of the turbulent cascade, the statistical properties of the plasma and of the synchrotron emission maps were analyzed.Results. Turbulence rapidly relaxes to a sort of Alfv & eacute;nic equilibrium and a Kolmogorov cascade with a slope of -5/3 soon develops, with differences depending on the initial ratio, eta, of magnetic fluctuations over the background field. Dissipation mostly occurs in thin current sheets, where (numerical) reconnection takes place and intermittency and deviation from isotropic Gaussian distributions are observed. Synthetic synchrotron maps and their statistical properties depend on eta too, approaching analytical estimates for large eta. The integrated degree of linear polarization is found to cover the whole range of observed values in PWNe, and its dependence on the relative amplitude of turbulent fluctuations shows a good agreement with analytical estimates, even in the presence of anisotropy.

The X-ray polarization observations, made possible with the Imaging X-ray Polarimetry Explorer (IXPE), offer new ways of probing high-energy emission processes in astrophysical jets from blazars. Here, we report the first X-ray polarization observation of the blazar S4 0954 +65 in a high optical and X-ray state. During our multi-wavelength (MWL) campaign of the source, we detected an optical flare whose peak coincided with the peak of an X-ray flare. This optical-X-ray flare most likely took place in a feature moving along the parsec-scale jet, imaged at 43 GHz by the Very Long Baseline Array (VLBA). The 43 GHz polarization angle of the moving component underwent a rotation near the time of the flare. In the optical band, prior to the IXPE observation, we measured the polarization angle to be aligned with the jet axis. In contrast, during the optical flare, the optical polarization angle was perpendicular to the jet axis; after the flare, it reverted to being parallel to the jet axis. Due to the smooth behavior of the optical polarization angle during the flare, we favor shocks as the main acceleration mechanism. We also infer that the ambient magnetic field lines in the jet were parallel to the jet position angle. The average degree of optical polarization during the IXPE observation was (14.3 +/- 4.1)%. Despite the flare, we only detected an upper limit of 14% (at 3 sigma level) on the X-ray polarization degree; however, a reasonable assumption on the X-ray polarization angle results in an upper limit of 8.8% (3 sigma). We modeled the spectral energy distribution (SED) and spectral polarization distribution (SPD) of S4 0954 +65 with leptonic (synchrotron self-Compton) and hadronic (proton and pair synchrotron) models. Our combined MWL polarization observations and SED modeling tentatively disfavor the use of hadronic models for the X-ray emission in S4 0954 +65.

We report the detection of a spatial variation of X-ray polarization in the southwestern (SW) shell of supernova remnant SN 1006 (SN 1006 SW) using the Imaging X-ray Polarimetry Explorer (IXPE). The shell has an average X-ray polarization degree (PD) of 21.6% ± 4.5% and polarization angle of −48 ^∘ ± 5 ^∘ in the 2–4 keV energy band, similar to those in the northeastern shell. The PD varies along SN 1006 SW, with a peak PD = 40% ± 8% in the south and a significantly lower PD ≲ 27% (99% upper limit) in the west, where the shell has been proposed to be interacting with an interstellar cloud. The correlation between the PD, which reflects the magnetic orderliness, and the preshock density provides observational evidence that magnetic turbulence and amplification are environment-dependent. The high PD detected in the southern region of the shell constrains the magnetic turbulence scale to ≲0.1 pc. Moreover, by comparing the IXPE X-ray and MeerKAT radio polarization measurements for SN 1006 SW, we found that magnetic fields traced by X-ray polarization are nearly radially distributed, whereas those traced by radio polarization tend to follow a direction parallel to the Galactic plane. This suggests that the X-ray polarization probes freshly amplified magnetic fields from small-scale structures in the immediate postshock region, while the radio traces more extended regions influenced by the preexisting ambient magnetic fields.

X-ray polarization is a unique new probe of the particle acceleration in astrophysical jets made possible through the Imaging X-ray Polarimetry Explorer. Here we report on the first dense X-ray polarization monitoring campaign on the blazar Mrk 421. Our observations were accompanied by an even denser radio and optical polarization campaign. We find significant short-timescale variability in both X-ray polarization degree and angle, including an ∼90° angle rotation about the jet axis. We attribute this to random variations of the magnetic field, consistent with the presence of turbulence but also unlikely to be explained by turbulence alone. At the same time, the degree of lower-energy polarization is significantly lower and shows no more than mild variability. Our campaign provides further evidence for a scenario in which energy-stratified shock-acceleration of relativistic electrons, combined with a turbulent magnetic field, is responsible for optical to X-ray synchrotron emission in blazar jets.

We present the X-ray polarization observation of G21.5−0.9, a young Galactic supernova remnant (SNR), conducted with the Imaging X-ray Polarimetry Explorer (IXPE) in 2023 October, with a total livetime of approximately 837 ks. Using different analysis methods, such as a space-integrated study of the entire region of the pulsar wind nebula (PWN) and a space-resolved polarization map, we detect significant polarization from the PWN at the center of the SNR, with an average polarization degree of ∼10% oriented at ∼33° (north through east). No significant energy-dependent variation in polarization is observed across the IXPE band (2–8 keV). The polarization map, corrected for the effect of polarization leakage, reveals a consistent pattern in both degree and angle, with little change across the nebula. Our findings indicate the presence of a highly polarized central torus, suggesting low levels of turbulence at particle acceleration sites. Unlike Vela, but similar to the Crab Nebula, we observe substantial differences between radio and X-ray polarization maps. This suggests a clear separation in energy of the emitting particle populations and hints at an important, yet poorly understood, role of instabilities in the turbulence dynamics of PWNe.