Roberto Taverna

We report on the detection of polarized X-ray emission from the new Galactic magnetar EP J223759.5+531421, discovered by the Einstein Probe Wide-field X-ray Telescope on 2026 June 28. The Imaging X-ray Polarimetry Explorer (IXPE) follow-up observation started on 2026 July 6 and detected the source at a (absorbed) flux of ≈4.5 × 10 ⁻¹¹ erg cm ⁻² s ⁻¹ (2–7.5 keV range), confirming the presence of two thermally emitting regions on the star surface, with temperatures ≈0.5 and ≈1 keV. A significant (at >4 σ confidence level) phase- and energy-integrated polarization degree of ≈7% was detected with the polarization angle ≈−2 ^∘ East of the celestial North. The two thermal components exhibit quite different polarization properties. The difference is larger in the phase-folded data, with the polarization degree reaching ∼63% in the range 4–7.5 keV, in correspondence with the secondary peak of the pulse profile, and never exceeding ∼30% at lower energies. The polarization angle continuously oscillates from −90 ^∘ to +90 ^∘ over one rotational cycle and is well fit by the rotating vector model, with an inclination of the line of sight and of the magnetic dipole axis relative to the star spin axis of ≈28 ^∘ and ≈109 ^∘ , respectively. The data point to an emission geometry in which the warm thermal component could originate from two antipodal caps, possibly in a magnetically condensed state, while the hot one could come from a smaller region covered by an atmosphere.

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.

Magnetars are now known to be among the most strongly polarized celestial sources in X-rays. Here, we report on the 500 ks observation of the magnetar 1E 1547.0−5408 performed by the Imaging X-ray Polarimetry Explorer (IXPE) in 2025 March. The IXPE spectrum is well reproduced by a single thermal component with blackbody temperature kT _BB ∼ 0.67 keV and emission radius R _BB ∼ 1.2 km. The source exhibits a high linear polarization degree in the 2–6 keV band (PD = 47.7% ± 2.9%) with polarization angle PA = 75 . ° 8 ± 1 . ° 8 , measured west of celestial north. While PA does not appear to vary with energy, there is some evidence (at the 1 σ confidence level) of a minimum in PD between 3 and 4 keV, compatible with what is expected by partial mode conversion at the vacuum resonance in a magnetized atmosphere. Phase-resolved spectral and polarimetric analyses reveal that X-ray thermal radiation likely originates from a single, fairly small hot spot with a nonuniform temperature distribution. Fitting the phase-dependent PA measured by IXPE with a rotating vector model (RVM) constrains the source geometry and indicates that both the dipole axis and line-of-sight are misaligned with respect to the spin axis. Under these conditions, the high polarization of the source cannot be regarded as compelling evidence for the presence of vacuum birefringence in the star’s magnetosphere. Nevertheless, the fact that the RVM successfully reproduces the modulation of the X-ray polarization angle and the behavior of PD with the energy hint once more at the presence of QED effects in magnetars.

We present the first IXPE spectro-polarimetric observation of the black hole candidate MAXI J1744−294, a transient X-ray source observed during a bright 2025 outburst in the Galactic center region. The source has recently been identified as most likely a repeat outburst of the 2016 transient Swift J174540.2−290037. During the ∼150 ks observation, the source was detected in the soft state, and its spectrum was well described by an absorbed multicolor disk with a minor high-energy tail. We did not detect any significant polarization from the source, and hence we derived a 3 σ upper limit on the polarization degree of 1.3% in the 2–8 keV energy band. This result is consistent with previous findings for soft-state black hole binaries observed at low to intermediate inclination angles. By comparing the polarization degree upper limit with theoretical predictions for standard accretion disk emission, we constrain the disk inclination to i ≲ 38°–72°, depending on the black hole spin and the disk atmosphere albedo, consistent with inclination estimates obtained during the 2016 outburst of Swift J174540.2−290037.

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.

The X-ray polarimetric observing window reopening is shedding new light on our current understanding of compact accreting sources. This is particularly true for stellar-mass black hole sources observed in the thermally dominated state, for which the polarization signal is expected to depend on the accretion disk inclination and the black hole spin. Two main effects determine the polarization properties of the accretion disk emission: the absorption and scattering processes occurring before the radiation leaves the disk atmosphere, and the relativistic effects influencing its propagation toward the observer at infinity. In this work, we investigate these effects together, considering only the contribution of direct radiation. We analyze how the ionization state of the disk atmosphere, approximated with a constant-density surface layer assumed to be in either collisional ionization equilibrium or photoionization equilibrium, can influence the spectro-polarimetric properties of the radiation at the emitting disk surface. Subsequently, we study how these are modified by the propagation in a strong gravitational field.

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