Ruth M. E. Kelly

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.

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.

X-ray polarimetry offers a unique window into neutron star physics and can provide answers to questions that cannot otherwise be probed. The upcoming Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX) sounding rocket mission will be the first experiment equipped with a detector able to explore polarized X-rays below 1 keV, observing in the 0.2–0.4 keV range. Although REDSoX will only be capable of short, one-off observations, it will crucially test the instrument performance. In this paper we investigate how a fully fledged orbital mission with a longer lifetime, based on an instrument design similar to REDSoX, will allow us to study thermal emission from X-ray dim isolated neutron stars (XDINSs) and magnetars, probing their magnetic fields and the physics of their outer surface layers, including vacuum effects and quantum electrodynamics (QED) mode conversion at the vacuum resonance. We discuss the potentially observable features for promising values of the star’s surface temperature, magnetic field, and viewing geometry. Assuming emission from the whole surface, we find that, for a source with a magnetic field B = 5 × 10¹³ G and surface temperature T ≈ 10⁷ K, the instrument can resolve a proton cyclotron absorption feature in the spectrum with high significance when collecting ≈25,000 counts across a single observation. Similarly, for a source with B = 10¹⁴ G and T ≈ 10⁷ K, a switch in the dominant polarization mode, caused by mode conversion at the vacuum resonance, can be detected by collecting ≈25,000 counts, allowing for a long-sought observational test of the presence of QED effects. We then present two case studies for XDINS targets: RX J1856.5–3754 and RX J0720.4–3125.

The Imaging X-ray Polarimetry Explorer (IXPE) observed for the first time highly polarized X-ray emission from the magnetar 1E 1841−045, targeted after a burst-active phase in 2024 August. To date, IXPE has observed four other magnetars during quiescent periods, highlighting substantially different polarization properties. 1E 1841−045 exhibits a high, energy-dependent polarization degree, which increases monotonically from ≈15% at 2–3 keV up to ≈55% at 5.5–8 keV, while the polarization angle, aligned with the celestial north, remains fairly constant. The broadband spectrum (2–79 keV) obtained by combining simultaneous IXPE and NuSTAR data is well modeled by a blackbody and two power-law components. The unabsorbed 2–8 keV flux (≈2 × 10⁻¹¹ erg cm⁻² s⁻¹) is about 10% higher than that obtained from archival XMM-Newton and NuSTAR observations. The polarization of the soft, thermal component does not exceed ≈25%, and may be produced by a condensed surface or a bombarded atmosphere. The intermediate power law is polarized at around 30%, consistent with predictions for resonant Compton scattering in the star magnetosphere; meanwhile, the hard power law exhibits a polarization degree exceeding 65%, pointing to a synchrotron/curvature origin.

The Imaging X-ray Polarimetry Explorer ( observations of accreting X-ray pulsars (XRPs) continue to provide novel insights into the physics and geometry of these sources. We present the first X-ray polarimetric study of the persistent wind-fed XRP based on five observations totaling 360 ks, conducted in March and October 2024. We detect marginally significant polarization in the combined data set in the full 2--8 keV energy band, with a polarization degree (PD) of $3.0±1.1$% and polarization angle (PA) of -18 The energy-resolved analysis shows a clear energy dependence of the polarization properties, with a remarkable ∼70 switch in PA between low and high energies. Similarly, the pulse phase-resolved spectro-polarimetric analysis reveals different signatures at low and high energies. At low energies (2--3 keV), the PD spans from ∼2% up to ∼18%, accompanied by large-amplitude swings in the PA. At higher energies (4--8 keV), the PD varies between ∼3% and ∼12%, and the PA not only is similarly highly variable but also exhibits a markedly different phase dependence. Fitting the rotating vector model to the pulse phase dependence of the PA at the lower energies, we constrain the geometric configuration of the pulsar. The analysis favors a high spin-axis inclination of $>50 which agrees with both previous pulse-phase-dependent spectral fitting of the cyclotron line region and the known high orbital inclination of the binary system. The magnetic obliquity is estimated to be $30 and the spin position angle to be $19 A sharp switch in PA around 3 keV presents a particular theoretical challenge, as it is not consistent with the right-angle switch that was only seen in one other pulsar Vela X-1

The Imaging X-ray Polarimetry Explorer (IXPE) observations of accreting X-ray pulsars (XRPs) continue to provide novel insights into the physics and geometry of these sources. We present the first X-ray polarimetric study of the persistent wind-fed XRP 4U 1538-52, based on five IXPE observations totaling 360 ks, conducted in March and October 2024. We detect marginally significant polarization in the combined data set in the full 2--8 keV energy band, with a polarization degree (PD) of 3.0+-1.1% and polarization angle (PA) of -18 degrees. The energy-resolved analysis shows a clear energy dependence of the polarization properties, with a remarkable ~70 degrees switch in PA between low and high energies. Similarly, the pulse phase-resolved spectro-polarimetric analysis reveals different signatures at low and high energies. At low (2--3 keV) energies, the PD ranges between ~2% and ~18%, with the PA varying between -16 and 70 degrees. At higher (4--8 keV) energies, the PD varies between ~3% and ~12%, with a drastically different PA behavior. Fitting the rotating vector model to the pulse phase dependence of the PA at the lower energies, we constrain the geometric configuration of the pulsar. The analysis favors a high spin-axis inclination of >50 which agrees with both previous pulse-phase-dependent spectral fitting of the cyclotron line region and the known high orbital inclination of the binary system. The magnetic obliquity is estimated to be 30 degrees and the spin position angle to be 19 degrees. A sharp switch in PA around 3 keV presents a particular theoretical challenge, as it is not consistent with the right-angle switch that was only seen in one other pulsar Vela X-1.

Magnetars are neutron stars that host huge, complex magnetic fields which require supporting currents to flow along the closed field lines. This makes magnetar atmospheres different from those of passively cooling neutron stars because of the heat deposited by backflowing charges impinging on the star surface layers. This particle bombardment is expected to imprint the spectral and, even more, the polarisation properties of the emitted thermal radiation. We present solutions for the radiative transfer problem for bombarded plane-parallel atmospheres in the high magnetic field regime. The temperature profile is assumed a priori, and selected in such a way to reflect the varying rate of energy deposition in the slab (from the impinging currents and/or from the cooling crust). We find that thermal X–ray emission powered entirely by the energy released in the atmosphere by the magnetospheric back–bombardment is linearly polarised and X-mode dominated, but its polarisation degree is significantly reduced (down to 10%–50%) when compared with that expected from a standard atmosphere heated only from the cooling crust below. By increasing the fraction of heat flowing in from the crust the polarisation degree of the emergent radiation increases, first at higher energies (∼10 keV) and then in the entire soft X-ray band. We use our models inside a ray-tracing code to derive the expected emission properties as measured by a distant observer and compare our results with recent IXPE observations of magnetar sources.

Magnetars, the most strongly magnetised neutron stars, are among the most promising targets for X-ray polarimetry. The Imaging X-ray Polarimetry Explorer (IXPE), the first satellite devoted to exploring the sky in polarised X-rays, has observed four magnetars to date. A proper interpretation of IXPE results requires the development of new atmospheric models that can take into proper account the effects of the magnetised vacuum on par with those of the plasma. Here we investigate the effects of mode conversion at the vacuum resonance on the polarisation properties of magnetar emission by computing plane-parallel atmospheric models under varying conditions of magnetic field strength/orientation, effective temperature and allowing for either complete or partial adiabatic mode conversion. Complete mode conversion results in a switch of the dominant polarisation mode, from the extraordinary (X) to the ordinary (O) one, below an energy that decreases with increasing magnetic field strength, occurring at ≈0.5 keV for a magnetic field strength of B = 10¹⁴ G. Partial adiabatic mode conversion results in a reduced polarisation degree when compared with a standard plasma atmosphere. No dominant mode switch occurs for B = 10¹⁴ G while there are two switches for lower fields of B = 3 × 10¹³ G. Finally, by incorporating our models in a ray-tracing code, we computed the expected polarisation signal at infinity for different emitting regions on the star surface and for different viewing geometries. The observability of QED signatures with IXPE and with future soft X-ray polarimeters as REDSoX is discussed.