Giorgio Matt

We report on the observation of the Compton-thin Seyfert galaxy NGC 5506 performed with the Imaging X-ray Polarimetry Explorer (IXPE). The observation, with a net observing time of 975 ks, was partly simultaneous with NuSTAR (52 ks) and XMM-Newton (37 ks). Using a simple baseline model with IXPE data only, we find an upper limit on the 2–8 keV polarization fraction at the 99% confidence level: Π < 3.1%. We then performed a spectropolarimetric analysis combining data from the three X-ray observatories. While pure spectral modeling confirms the presence of both relativistic and cold distant reflection, in addition to the primary continuum power law, their contributions lead to somewhat different constraints on the coronal Π, depending on the specific assumptions. We also performed Monte Carlo simulations with the radiative transfer code MONK, exploring various coronal geometries. Comparing these results with the IXPE data, and assuming a radially extended corona, as suggested by previous observations of similar sources, the corresponding polarization upper limits require inclination angles below 40°.

Context. Swift J1727.8-1613 is an X-ray binary that was observed multiple times by IXPE , NICER , NuSTAR , and INTEGRAL during its 2023–2024 outburst. It was observed in a hard state at the beginning and the end of the outburst and in a soft state in between. Aims. We aim to reproduce the spectro-polarimetric signatures observed during two hard states: one bright at the beginning of the outburst and the other dim at the end. The two show similar X-ray polarimetric signatures, but their X-ray fluxes differ by two orders of magnitude. Methods. We modelled these hard state spectra in the JED–SAD framework, which combines a jet-emitting disk (JED) in the inner region of the accretion flow and a standard accretion disk (SAD) in the outer region. The X-ray polarisation properties of the best-fit JED–SAD configurations were then computed with the MONK general relativistic radiative transfer code. Results. The spectral characteristics of both the bright and dim hard states are well reproduced within the JED–SAD framework. Within our modelling, however, the dominant radiative process differs between the two states: self-Comptonised bremsstrahlung in the bright hard state and self-Comptonised synchrotron in the dim hard state. In the bright hard state, the resulting polarisation fraction remains below the observed value of ∼4%. Moreover, due to the JED’s large vertical optical depth (∼10), the polarisation angle is aligned with the accretion disk surface, i.e. there is a 90° difference with the observations. In contrast, the dim hard state’s simulated polarisation fraction is about 3% and the polarisation angle is perpendicular to the disk, both of which are broadly consistent with the IXPE measurements. Our simulations of the dim hard state are largely insensitive to the magnetic field configuration due to the strong depolarisation of the synchrotron seed photons by Faraday rotation. Conclusions. While the spectro-polarimetric properties of the dim hard state are well reproduced by our model, the bright hard state polarisation properties do not agree with the observations. We discuss different processes that could be responsible for the origin of the observed polarisation in the bright hard state: the presence of a non-thermal particle distribution, which would be consistent with the presence of a hard tail observed above 50 keV; and the electron scattering in an accretion-disk wind, which could modify both the polarisation fraction and angle. We discuss the limitations of the current JED–SAD framework and outline possible improvements to better reproduce the IXPE data.

This paper, originally published on 17 August 2026, was replaced with a revised version on 6 October 2026. Figure 11 has been added to provide a comparison between the sensitivity of the mission EXPO and other proposed or flying missions. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance. Download the erratum for additional details.

Context. 4U 1822–37 is a high-inclination eclipsing neutron-star low-mass X-ray binary and a prototype accretion-disc corona (ADC) source, in which the direct inner emission is largely obscured and the observed X-ray flux is dominated by scattered radiation. X-ray polarimetry provides a direct probe of this scattering geometry. Aims. We investigated the geometry of 4U 1822–37 through its first X-ray spectro-polarimetric observation, testing the high-inclination ADC scenario with simultaneous broadband spectroscopy and polarimetry. Methods. We analysed coordinated IXPE , XMM-Newton , NuSTAR , and Swift observations. We performed broadband spectral modelling and model-independent and spectro-polarimetric analyses of the IXPE data, including energy and orbital phase-resolved polarimetry. Results. The broadband spectrum is described by a soft thermal component, a Comptonised continuum, a hard power-law tail, and relativistically blurred reflection. The observed luminosity, L _obs ≃ 6.1 × 10 ³⁶ ergs ⁻¹ , is well below the intrinsic luminosity expected from the orbital and spin evolution, consistent with the presence of an extended, optically thin corona that scatters only a fraction of the intrinsic radiation into the line of sight. In the 2–8 keV band, IXPE measures a polarisation degree (PD) of 7.9 ± 0.6% and a polarisation angle (PA) of −24° ±2°. The PD increases with energy, while the PA remains approximately constant. Spectro-polarimetric modelling confirms the energy trend, although the relative contributions of the Comptonised continuum and reflection remain degenerate. The orbital-phase-resolved analysis suggests a lower PD during an eclipse, PD = 5.5 ± 1.7%, with no significant PA change or major broadband-continuum variation above 2 keV. Conclusions. The high polarisation degree, the stable polarisation angle, the increase in PD with energy, and the decrease in PD during an eclipse all support the view that 4U 1822–37 is observed in an extreme high-inclination, scattering-dominated regime. In this picture, the extended corona plays a central role in shaping the observed X-ray emission and its polarisation properties.

The Enhanced X-ray Polarimetry Observatory (EXPO) is a mission concept proposed to ESA as an M8 candidate, with a prospective launch around 2041. Building on the scientific success of the Imaging X-ray Polarimetry Explorer (IXPE), EXPO is designed not only to overcome its two main limitations, the narrow 2–8 keV energy band and the very slow repointing time, but also to unlock new scientific capabilities. A wide energy band and fast repointing are essential for addressing key questions in high-energy astrophysics, including the origin of the hard X-ray emission in 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 of magnetar flares. The payload combines broadband imaging X-ray polarimetry with rapid time-domain capability. Five focusing X-ray telescopes employ gas photoelectric polarimeters based on the Timepix ASIC family with InGrid amplification, enabling true three-dimensional track reconstruction and operation over the 2–35 keV band through optimized low- and medium-energy detector configurations. The mirror modules are based on proven electroformed nickel technology with Au/C coatings and an XMM-like focal length of 7.5 m. The polarimeters are complemented by three dedicated detection systems: (i) a coded-mask Wide Field Instrument (WFI), derived from SVOM /ECLAIRs, providing continuous monitoring of a ∼2 sr field of view and autonomous transient localization; (ii) a Spectral Imaging Camera (SIC), based on stacked CMOS and CdTe detectors, providing simultaneous broadband imaging spectroscopy for accurate spectro-polarimetric decomposition; and (iii) an Instrument Control Unit (ICU), responsible for payload management, onboard WFI image reconstruction, transient identification, and autonomous spacecraft repointing. Together, these capabilities will 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. This paper presents the concept of the mission EXPO, its payload architecture, and the scientific opportunities provided by its unique combination of broadband polarimetry and rapid response.

X-ray thermal emission is inherent in neutron-star and black-hole X-ray binary systems. Within these systems, it may reflect from optically thick matter, which will create characteristic observable X-ray spectro-polarimetric features. We computed rest-frame reflection spectra and the corresponding energy-dependent linear polarization degree and angle for (un)polarized single-temperature blackbody spectra impinging on a partially ionized, constant-density, optically thick slab. We used a combination of a Monte Carlo simulation that takes into account scattering, absorption, and spectral lines, and a radiative transfer pre-computation of the ionization structure of the slab in photo-ionization equilibrium without assuming a local thermodynamic equilibrium. We discuss the impact of the reflector’s ionization and the incident spectral shape on the obtained energy dependence of polarization. For isotropic incident intensity and for parallel rather than perpendicular unidirectional illumination with respect to the slab normal direction, despite the presence of highly polarized absorption features and low-polarized spectral lines, an underlying scattering-induced increase of polarization degree with energy in mid to hard X-rays naturally arises due to multiple Compton-scattering energy shifts. Such a re-processing effect is particularly apparent in 2–8 keV for steep incident X-ray spectra reflecting from highly ionized and optically thick media. Integration of the resulting local reflection tables in specific large-scale reflection geometries occurring in X-ray binary systems, including relativistic effects, will be presented in a follow-up paper. Nonetheless, we anticipate that the obtained rest-frame energy-dependent features will imprint to the observed X-ray polarization; and they could contribute to the increase of the total polarization degree with energy in the 2–8 keV range observed in some accreting systems by the IXPE mission.

We present the first X-ray polarimetric measurement of the neutron star low-mass X-ray binary system AX J1745.6−2901 conducted by the Imaging X-ray Polarimetry Explorer (IXPE) satellite. This transient source, located within ∼1 . ′ 5 of the Galactic center, was observed serendipitously during a MAXI J1744−294 observation with a duration of 150 ks. The complex nature of the region in which AX J1745.6−2901 is located poses a challenge for studying its polarization. By performing a detailed analysis of the contamination from MAXI J1744−294 and the Galactic center diffuse emission, we find the source polarization degree PD = 14.7% ± 4.0% and polarization angle PA = 122° ± 8°. The phase-resolved analysis shows increase in polarization during the eclipse phase, with PD = 34.2% ± 8.7%, suggesting that the polarization-inducing mechanisms are of scattering nature, probably originating from disk winds.

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 . ° 2 ± 0 . ° 5 , compatible within 3 σ 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.

Context. Low-luminosity active galactic nuclei (LLAGNs) often host weak radio jets whose propagation and impact on their host galaxies are strongly influenced by the surrounding interstellar medium. Constraining the connection between nuclear activity, radio variability, and large-scale radio structures in these systems remains a key observational challenge. Aims. We investigate the radio properties of the nearby LLAGN NGC 7213 to assess its ability to launch collimated outflows beyond the nuclear region and to characterise the nature, origin, and variability of newly identified radio components on parsec to kiloparsec scales. Methods. We present new radio observations obtained with MeerKAT, uGMRT, ATCA, and the Australian Long Baseline Array (LBA), covering frequencies from 300 MHz to 9 GHz. We analysed the morphology and spectra of the radio emission on kiloparsec scales and performed a dedicated LBA monitoring campaign to probe the parsec-scale core and its variability. Results. We discovered a pair of compact radio knots located symmetrically at a projected distance of ∼5 kpc north and south of the nucleus of NGC 7213. The two knots exhibit nearly identical flux densities and flat radio spectra from 300 MHz up to at least 5.5 GHz, with no significant spectral or geometric asymmetry. ATCA observations confirm the flat spectra at higher frequencies. The LBA monitoring shows that the nuclear radio source remains unresolved at all epochs, constraining the 8 GHz emission to sub-parsec scales. Significant variability is detected on both decade-long timescales, relative to archival LBA data, and on month-long timescales during our monitoring, with a flux-density increase of ∼40 mJy over six months. Conclusions. The symmetry, spectra, and physical properties of the kiloparsec-scale knots strongly support their interpretation as compact termination shocks of a weak or intermittent jet launched by the LLAGN in NGC 7213 and confined by the dense, disturbed interstellar medium of the host galaxy. The unresolved yet variable parsec-scale core traced by the LBA indicates that the observed high-frequency radio variability originates in the innermost jet region, which is likely linked to the recent increase in nuclear activity. NGC 7213 thus provides a nearby example of how weak jets in low-accretion AGN can produce both compact nuclear variability and symmetric kiloparsec-scale structures when interacting with complex environments.

Context. NGC 1068 has long served as the reference Compton-thick Seyfert 2 galaxy. Decades of X-ray observations have established the presence of both neutral reflection and ionized circumnuclear emission. However, the limited spectral capabilities of previous CCD and grating data in the Fe–K band have prevented a clean separation of these components, thus leaving large uncertainties on key properties, such as the optical depth, structure, and kinematics of the reprocessing and emitting regions, and on their connection to nuclear outflows and feedback. Aims. We exploited high-resolution X-ray spectroscopy to disentangle the neutral and ionized iron emission in NGC 1068; constrained the physical properties of the cold reflector through line energies, widths, fluorescence ratios, and Compton-shoulder diagnostics; and investigated the origin, kinematics, and potential feedback role of the highly ionized Fe XXV and Fe XXVI emission. Methods. We analyzed an XRISM/Resolve observation of NGC 1068, focusing on the Fe K α and Fe K β fluorescent lines and on the Fe XXV and Fe XXVI emission complexes. Line centroid energies, intrinsic widths, flux ratios, and constraints on the Compton shoulder were derived through local spectral fitting, and compared with atomic calculations and theoretical predictions. Results. The centroid energies of the Fe K α and Fe K β lines tightly constrain the emitting material to be neutral or near neutral. The observed Fe K β /K α ratio, together with the stringent upper limit on the Compton shoulder (≲8−11% of the core flux), disfavor reflection dominated by a homogeneous classical Compton-thick medium, indicating that most of the neutral Fe K α emission arises in optically thin or moderately Compton-thick gas. The Fe XXV and Fe XXVI emission lines exhibit remarkably large velocity widths, of several thousand km s ⁻¹ . These broad profiles closely resemble the integrated optical and infrared [O III ] and [O IV ] lines associated with the large-scale biconical outflow, and are naturally interpreted as the X-ray signature of a more highly ionized, faster, and more spatially confined phase of the same outflow. Conclusions. The iron-K emission of NGC 1068 reveals a stratified circumnuclear environment in which neutral and highly ionized components arise in physically distinct regions. The neutral Fe K fluorescence originates predominantly in optically thin or mildly Compton-thick material, despite the persistently Compton-thick line-of-sight obscuration, indicating a geometrically complex cold reprocessor. The highly ionized iron emission lines trace a fast component consistent with a bipolar outflow on parsec scales, whose large velocities and inferred energetics suggest that it may represent an efficient channel for feedback. If similar ionized outflows and reflection geometries are common in heavily obscured Seyferts, they may constitute an underappreciated aspect of feedback and circumnuclear structure in this class of sources.