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.

We report results from a multi-mission observational campaign of the transient X-ray pulsar 2S 1417-624 during its 2025 outburst, using data from NICER, IXPE, and NuSTAR. Phase-averaged and phase-resolved spectroscopy with NICER and NuSTAR reveal that a typical broken power-law model with a high-energy cut-off well describes the broadband spectra. Several spectral parameters, however, show clear and systematic modulations with pulse phase, indicating variations in the physical conditions of the emitting plasma over the neutron star's rotation. IXPE provides the first polarimetric measurements of this source, yielding a phase-averaged polarization degree (PD) of 4.8 ± 1.2

We regret that this statement “This work was supported by China’s Space Origins Exploration Program.” was omitted in the beginning of the Acknowledgements.

We present the first broadband spectral and timing study of the Be/X-ray pulsar XTE J0111.2−7317 (SXP31.0) during the first major outburst since its discovery in 1998. This giant type II outburst, observed between April and September 2025, marks the source’s return to activity after nearly three decades of quiescence. Using NuSTAR observations together with data from Swift/XRT and SRG/ART-XC, we followed the outburst’s evolution, with the source reaching a bolometric luminosity of Lbol = 3.6 × 1038 erg s−1. The broadband spectra are well described by an absorbed cutoff power law, two blackbody components (hot and soft), and a narrow Fe Kα line. No cyclotron absorption features were detected in either the phase-averaged or phase-resolved spectra in the 5–50 keV band. Most notably, we report the discovery of a previously undetected quasiperiodic oscillation (QPO) at 0.8 ± 0.1 mHz, characterized by a fractional root-mean-square (rms) amplitude of 14% at a super-Eddington bolometric luminosity of Lbol = 2.5 × 1038 erg s−1. In contrast, the previously reported 1.27 Hz QPO was not detected. While the 0.8 mHz QPO is present, the pulsed fraction (PF) is low in soft X-rays, which is consistent with other super-Eddington pulsars exhibiting mHz QPOs; however, it rises above 20 keV to reach 35%. The QPO vanishes in subsequent observations coinciding with a sharp increase in the PF and a distinct change in pulse profile morphology. It was not observed in any follow-up observations at luminosities above or below its initial detection, suggesting it is a transient phenomenon.

We report the detection of cyclotron resonant scattering features (CRSFs) in the spectrum of the unique bursting pulsar GRO J1744–28, observed during its recent outburst in 2021 with the Neutron Star Interior Composition Explorer (NICER). Clear pulsations at a frequency of 2.141128 Hz as well as Type II X-ray bursts were observed. The pulse profile exhibits a single-peaked shape in all energy bands, with the pulse fraction showing a positive correlation with energy. We find that the persistent X-ray continuum of the accreting pulsar is well described by typical phenomenological models, and we confirm the presence of the cyclotron line at ∼5 keV as reported in previous studies. In addition, we detect a candidate absorption feature with a centroid energy of 2 keV. If confirmed, this feature could be interpreted as a CRSF, which would correspond to a magnetic field of ∼1.8 × 1011 G. Pulse-phase-resolved analysis also reveals this absorption line around the peak pulse phases. These NICER observations provide tentative evidence for the cyclotron line candidate, establishing GRO J1744–28 as a key laboratory for studying accretion physics in an intermediate-strength magnetic field.

We report the detection of mHz quasi-periodic oscillations (QPOs) in four NuSTAR observations of 4U 1626-67 during its recent spin-down episode. By using a novel method based on the Hilbert-Huang Transform (HHT), we present the first QPO-phase-resolved timing and spectral analysis of accreting X-ray pulsars in low mass X-ray binaries. Broadband QPO waveforms have been reconstructed and exhibit approximately sinusoidal shapes, with fractional amplitudes that vary with energy. In addition, we find that spin pulse profiles exhibit stable shapes between different QPO phases with different instantaneous fluxes, while the fractional root-mean-square (rms) is distinct for different observations. In this source, both QPO-phase-resolved and averaged spectra can be modeled with a negative and positive powerlaws exponential (NPEX) model, and their spectral evolutions show a similar trend, suggesting that the QPO modulation is caused by accretion rate variability instead of a geometric obscuration. These results provide new constraints on accretion physics in strongly magnetized neutron stars and the underlying mechanisms of QPOs.

We performed a systematic investigation of millihertz quasiperiodic oscillations (mHz QPOs) in the low-mass X-ray binary GS 1826−238 observed with NICER and Insight-HXMT. We discovered 37 time intervals exhibiting mHz QPOs out of 106 Good Time Interval (GTI) samples in the frequency range of 3–17 mHz at a significance level of >99.99%. The source remains in a soft state in our study. No significant differences are found between the samples with and without mHz QPOs according to positions in the color–color and hardness–intensity diagrams. These QPOs were discovered at an accretion rate of ∼ 0.1 M ̇ Edd , similar to other sources. The broadband spectrum of GS 1826–238 can be modeled as a combination of a multicolor blackbody from the accretion disk and a Comptonization with seed photons emitted from the neutron star (NS) surface. The flux modulations of mHz QPOs are related to variations of the temperature of Comptonization seed photons, consistent with the marginally stable burning theory.

X-ray polarimetry of the accreting compact object has revealed fast time variations in the polarization angle (PA), suggesting that the geometry and/or optical depth of the Comptonization region is changing rapidly. This prompts investigations into how fast such variability can be. Conventionally, the data are often binned to examine the time variability such that the measurement in each bin is above the minimum detectable polarization (MDP). Here we demonstrate that this is unnecessary, and even below the MDP, one can infer the posterior distribution of PA reliably using the Bayesian approach and still be able to place useful constraints on the physics in many cases due to small relative uncertainties on the PA (e.g., ΔPA ≈ 10°−30° compared with a dynamical range of 180°). With this approach, we discovered that the PA variation in one of the Imaging X-ray Polarimetry Explorer (IXPE) observations of GX 13+1 is not following a linear rotation mode as suggested previously. Instead, the PA swings between two discrete angles, suggesting that there are two emitting components, e.g., the boundary layer and the spreading layer, competing with each other. In XTE J1701−462, we confirmed previous results for a variable PA in the normal branch and, furthermore, revealed that the variation timescale could be as short as 1.5 hr. During the IXPE observation of Sco X-1, a hint is found for the PA in the highest flux level to be different from the average but consistent with previous measurement results with PolarLight and OSO-8.

In this paper we present the current status of the enhanced X-ray Timing and Polarimetry mission, which has been fully approved for launch in 2030. eXTP is a space science mission designed to study fundamental physics under extreme conditions of matter density, gravity, and magnetism. The mission aims at determining the equation of state of matter at supra-nuclear density, measuring the effects of quantum electro-dynamics, and understanding the dynamics of matter in strong-field gravity. In addition to investigating fundamental physics, the eXTP mission is poised to become a leading observatory for time-domain and multi-messenger astronomy in the 2030's, as well as providing observations of unprecedented quality on a variety of galactic and extragalactic objects. After briefly introducing the history and a summary of the scientific objectives of the eXTP mission, this paper presents a comprehensive overview of: 1) the cutting-edge technology, technical specifications, and anticipated performance of the mission's scientific instruments; 2) the full mission profile, encompassing spacecraft design, operational capabilities, and ground segment infrastructure.

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.

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.

Accretion on to neutron stars (NSs) in X-ray pulsars (XRPs) results in intense X-ray emission, and under specific conditions, high-energy nuclear interactions that produce gamma-ray photons at discrete energies. These interactions are enabled by the high free-fall velocities of accreting nuclei near the NS surface and give rise to characteristic gamma-ray lines, notably at 2.2, 5.5, and 67.5 MeV. We investigate the production mechanisms of these lines and estimate the resulting gamma-ray luminosities, accounting for the suppression effects of radiative deceleration in bright XRPs and the creation of electron-positron pairs in strong magnetic fields. The resulting annihilation of these pairs leads to a secondary emission line at similar to 511 keV. We also discuss the possibility that non-stationary pair creation in the polar cap region could drive coherent radio emission, though its detectability in accreting systems remains uncertain. Using a numerical framework incorporating general relativistic light bending and magnetic absorption, we compute the escape fraction of photons and distinguish between actual and apparent gamma-ray luminosities. Our results identify the parameter space - defined by magnetic field strength, accretion luminosity, and NS compactness - where these gamma-ray signatures may be observable by upcoming MeV gamma-ray missions. In particular, we highlight the diagnostic potential of detecting gravitationally redshifted gamma-ray lines and annihilation features for probing the mass-radius relation and magnetospheric structure of NSs.

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.

By combining data from several X-ray telescopes (Swift/XRT, NICER, Chandra) we reconstructed a profile of the 2019 outburst of accreting X-ray pulsar 4U 1901+03 from its peak down to the return to the ‘‘low’’ state. Softening of the X-ray spectrum and disappearance of the pulsations at a later stages of the outburst tentatively indicates that the source transitioned to the ‘‘propeller’’ state at luminosity about 10^36 erg s ^-1 , which corresponds to a magnetic field strength of B≲ 10^12 G at the neutron star surface. We also investigated the fast X-ray variability that was observed during the peak of 2003 outburst. The shape of the power spectrum could be described with a broken power law, with a break frequency of 7.5 Hz. Assuming that this frequency corresponds to a viscous frequency at a magnetospheric boundary we estimated magnetic field strength as B≈ 7× 10^11 G, which is similar to the estimate obtained from observed transition to the ‘‘propeller’’ regime.

Polarimetric observations of X-ray pulsars (XRPs) have provided us with the key to unlocking their geometrical properties. Thanks to the Imaging X-ray Polarimetry Explorer (IXPE), the geometries of several XRPs have been determined, providing new insights into their emission mechanisms and magnetic field structures. The polarimetric properties of Vela X-1 have a clear dependence on energy, showing a 90 degrees swing in the polarization angle (PA) between low and high energies. Due to the complex energy-dependent nature of the polarization properties, until now it was not possible to determine the pulsar geometry. In this work we present the results of a detailed analysis of the pulse-phase-resolved polarization properties of Vela X-1 at different energies. By separating the polarimetric analysis into low and high energy ranges, we are able to disentangle the contributions of the soft and hard spectral components to the polarization, revealing the pulse phase dependence of the polarization degree and PA in each energy band. The PA pulse-phase dependence at high energies (5-8 keV) allows us, for the first time, to determine the pulsar geometry in Vela X-1. Based on the fit with the rotating vector model, we estimate the pulsar spin position angle to be around 127 degrees and the magnetic obliquity to be 13 degrees. We discuss two possible scenarios that could explain the 90 degrees swing in the PA between high and low energies: a two-component spectral model and vacuum resonance.

We present a comprehensive analysis of 2RXP J130159.6-635806, a persistent low-luminosity Be/X-ray pulsar, focusing on itstransition to a spin equilibrium state and the discovery of a bimodal luminosity distribution that possibly reveals a new accretionregime. Using data from theNuSTAR,Swift,XMM-Newton, andChandraobservatories, we investigated changes in the pulsar'stiming and spectral properties. After more than 20 years of continuous spin-up, the pulsar's spin period has stabilized, markingthe onset of spin equilibrium. This transition was accompanied by the emergence of a previously unobserved accretion regime atLbol=(2.0(-1.0)(+2.3)) x 10(34 )erg s(-1), an order of magnitude lower than its earlier quiescent state. After that, the source occasionally switchedbetween these regimes, remaining in each state for extended periods, with the transition from a luminosity of 10(35 )erg s(-1 )to 10(34 )erg s(-1 )taking less than 2.3 days. The analysis of the spectral data collected during this new low-luminosity state revealed a two-hump shapethat is different from the cutoffpower-law spectra observed at higher luminosities. The discovery of pulsations in this state, togetherwith the hard spectral shape, indicates ongoing accretion. We estimate the magnetic field strength to be similar to 10(13 )G based on indirectmethods. Additionally, we report a hint of a previously undetected similar to 90-day orbital period in the system.

We present observations of the accretion-powered X-ray pulsar 4U 1907+09 conducted with the Imaging X-ray Polarimetry Explorer, which has delivered the first high-quality polarization measurements of this source. 4U 1907+09 was observed twice during its brightest periods, close to the periastron. We observe a stronger polarization in the first observation, with a phase-averaged polarization degree (PD) of 6.0 +/- 1.6% and a polarization angle (PA) of 69 degrees +/- 8 degrees. The second observation provides weaker constraints on the polarimetric properties, PD = 2.2 +/- 1.6% and PA = 46 degrees +/- 23 degrees, as determined from the spectro-polarimetric analysis. Combining the data from the two observations results in PD = 3.7 +/- 1.1% and PA = 63 degrees +/- 9 degrees. We detect an energy-dependent PA in the phase-averaged analyses with a significance of 1.7 sigma. In the phase-resolved analyses, we observe a potential PA rotation of approximately 90 degrees between adjacent energy bands (4-5 and 5-6 keV) within the single phase bin of 0.25-0.375. We also investigate the influence of short flares on the polarization properties of this source. The results suggest that flares do not significantly affect the energy-phase-dependent PA, implying that the pulsar's geometry remains stable during flare events.

In this paper we present the science potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission, in its new configuration, for studies of strongly magnetized compact objects. We discuss the scientific potential of eXTP for quantum electrodynamic (QED) studies, especially leveraging the recent observations made with the NASA IXPE mission. Given eXTP’s unique combination of timing, spectroscopy, and polarimetry, we focus on the perspectives for physics and astrophysics studies of strongly magnetized compact objects, such as magnetars and accreting X-ray pulsars. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is expected to launch in early 2030.

We present simultaneous high-precision optical polarimetric and near-infrared (NIR) to ultraviolet (UV) photometric observations of the low-mass black hole X-ray binary A0620-00 in a quiescent state. Subtracting interstellar polarization, estimated from a sample of field stars, we derived the intrinsic polarization of A0620-00. We show that the intrinsic polarization degree (PD) varies with the orbital period with an amplitude of similar to 0.3%, at least in the R band, where the signal-to-noise ratio of our observations is the best. This implies that some fraction of the optical polarization is produced by a scattering of stellar radiation off the matter that follows the black hole in its orbital motion. In addition, we see a rotation of the orbit-average intrinsic polarization angle (PA) with the wavelength from 163 degrees in the R to 177 degrees in the B band. All of the above, combined with the historical NIR-to-optical polarimetric observations, demonstrates the complex behavior of the average intrinsic polarization of A0620-00: the PA continuously rotates from the infrared to the blue band by similar to 53 degrees in total, while the PD of similar to 1% remains nearly constant over the entire spectral range. The spectral dependence of the PA can be described by Faraday rotation with a rotation measure of -0.2 rad mu m(-2), implying a magnetic field of a few gauss in the plasma surrounding the black hole accretion disk. However, our preferred interpretation of the peculiar wavelength dependence is an interplay between two polarized components with different PAs. Polarimetric measurements in the UV range can help in distinguishing between these scenarios.

The Be/X-ray pulsar 4U 0115+63 underwent a type II outburst in 2023. After the outburst, similar to the outbursts in 2015 and 2017, the source decayed into a quiescent state. Two out of three XMM-Newton observations conducted after the 2023 outburst confirmed the source to be in a low-luminosity state at a level of L-X similar to 10(33) erg s(-1). X-ray pulsations were detected at approximate to 0.277 Hz in both observations with a pulsed fraction exceeding 50%. The power density spectra show no significant low-frequency red noise in either observation, suggesting that the radiation is not driven by accretion. The energy spectra in this state can be described by a single blackbody component, with an emitting area smaller than the typical size of the polar caps during the accretion phase. Based on the timing and spectral properties, we suggest that the propeller effect is active during the quiescent state, resulting in a total quenching of accretion. We discuss possible mechanisms for the generation of pulsations in this regime and consider the scenario of neutron star crust cooling.