Herman L. Marshall

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°.

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.

The Globe Orbiting Soft X-ray Polarimeter (GOSoX) is a spectropolarimeter for the soft X-ray band. GOSoX is based on the Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX∗), a NASA-funded sounding rocket payload. Like REDSoX, GOSoX consists of Wolter I X-ray optics from NASA/MSFC, critical-angle transmission (CAT) gratings made at MIT, and multilayer (ML) coated mirrors from LBNL that polarize the X-rays. Colleagues at U. Tübingen will adapt commercially available sCMOS sensors for the focal plane. The grating dispersion is matched to the lateral grading of ML mirrors set at 45°. GOSoX can measure polarization across the entire 0.2-0.4 keV band with a spectral resolution E/dE ∼100. Minimum detectable polarizations (MDPs) of 3-10% are expected for over a dozen targets in a one-year mission. The mission was selected by NASA for launch in 2030.

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.

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.

We present the first X-ray polarization measurement of the neutron star low-mass X-ray binary and Z-source, GX 340+0, in the normal branch (NB) using a 200 ks observation with the Imaging X-ray Polarimetric Explorer (IXPE). This observation was performed in 2024 August. Along with IXPE, we also conducted simultaneous observations with NICER, AstroSat, Insight-HXMT, the Australia Telescope Compact Array (ATCA), and the Giant Metrewave Radio Telescope (GMRT) to investigate the broadband spectral and timing properties in the X-ray and radio wavelengths. During the campaign, the source traced a complete Z-track during the IXPE observation but spent most of the time in the NB. We measure X-ray polarization degree (PD) of 1.3% ± 0.3% in the 2–8 keV energy band with a polarization angle (PA) of 38° ± 6°. The PD in the NB is observed to be weaker than in the horizontal branch (HB) but aligned in the same direction. The PD of the source exhibits a marginal increase with energy, while the PA shows no energy dependence. The joint spectro-polarimetric modeling is consistent with the observed X-ray polarization originating from a single spectral component from the blackbody, the Comptonized emission, or reflection feature, while the disk emission does not contribute toward the X-ray polarization. GMRT observations at 1.26 GHz during HB had a tentative detection at 4.5 ± 0.7 mJy, while ATCA observations a day later during the NB detected the source at 0.70 ± 0.05 mJy and 0.59 ± 0.05 mJy in the 5.5 and 9 GHz bands, respectively, suggesting an evolving jet structure depending on the Z-track position.

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 ( δt < 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.

We report the third observation of the Seyfert-1.9 active galactic nucleus (AGN) MCG-05-23-16 with the Imaging X-ray Polarimetry Explorer ( IXPE ), together with optical spectropolarimetry obtained at the Very Large Telescope (VLT), and combined with archival near-ultraviolet, optical, and near-infrared polarimetric data. We detect no X-ray polarization in the 2–8 keV band, with a 99% confidence upper limit of ≤2.9%, which is further reduced to ≤2.5% when combined with the two previous IXPE observations of the same target. Monte Carlo simulations suggest that equatorial coronal models are disfavored if the AGN is indeed a type 1.9/2 AGN, while coronae coplanar to the accretion disk remain consistent if the source is less inclined than previously assumed. Data from VLT /FORS2 reveal a typical type 2 spectrum in total flux, a broad H α line in polarized flux, and a polarization degree and angle that depend strongly on wavelength. The polarization angle rotates by nearly 70° across the optical band. Comparison with historical measurements confirms the long-term stability of the polarization spectrum and a ∼90° rotation in the near-ultraviolet. Interpreting the multiwavelength polarization relative to the AGN ionization axis indicates that the main obscurer is not a compact circumnuclear torus, but rather a distant kiloparsec-scale dust lane crossing the galaxy. This result implies that MCG-05-23-16 is, in fact, a type 1 AGN seen through foreground dust. The low X-ray column density becomes consistent with the absence of polarization, provided that the nuclear inclination is low.

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.

Observations from the Imaging X-ray Polarimetry Explorer (IXPE) have revealed electric vector position angle (EVPA) rotation in several neutron star low-mass X-ray binaries, including the Galactic X-ray burster GX 13+1. We developed a novel Bayesian nested sampling framework—“Q-U Event-by-Event Nested sampling for Bayesian EVPA Evolution” (QUEEN-BEE)—to model the unbinned Stokes parameters and infer optimal EVPA rotation rates in IXPE data. We then applied this framework to three previous IXPE observations of GX 13+1. In the first observation, QUEEN-BEE recovers a rotation rate of 42° ± 4° day ⁻¹ , consistent with prior binned analysis. Energy-binned QUEEN-BEE analysis of this first observation suggests a slablike coronal geometry, providing the first constraints between slab and shell coronae for this source. We also explore alternative EVPA rotation scenarios in GX 13+1, including variable disk wind behavior. The second observation of this source shows no evidence of rotation, and the third observation shows transient rotating behavior with an EVPA rotation rate when exiting a light curve dip of 17 0 − 40 + 20 ° day ⁻¹ . The results show marginal but consistent increases in the overall measured polarization degree for epochs where the EVPA rotation is identified. These results demonstrate that QUEEN-BEE can identify evolving polarization signatures in both time- and energy-resolved regimes, even where binned methods fall below detection thresholds. Our findings highlight the diagnostic potential of QUEEN-BEE as a tool for discriminating between competing physical models of coronal geometry and probing disk-wind-related polarization behavior, highlighting the promising potential for application of this framework in a variety of other IXPE observations.