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.

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.

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.degrees 2 +/- 0.degrees 5 , compatible within 3 sigma 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 report on the performance of the hard X-ray telescope (HXT) onboard the XL-Calibur balloon-borne mission, before and after its 2022 long duration balloon (LDB) flight. XL-Calibur aims to achieve high-sensitivity hard X-ray polarization observations, with the HXT providing the large effective area essential for this mission. The HXT was developed in Japan; its initial performance measurement was conducted at SPring-8 BL20B2 in June 2021. Subsequently, XL-Calibur completed its first flight in July 2022. In preparation for the second XL-Calibur flight in 2024, we conducted a similar characterization at BL20B2 to assess whether the HXT maintained its performance after recovery. We observe that the on-axis effective area of the HXT increases from 276.4(-0.4)(+0.5) to 288.2 +/- 0.6 cm(2 )(20 keV) and that the half-power diameter degrades from 1.8 +/- 0.1 arcmin to 2.0 +/- 0.1 arcmin (20 keV). Detailed analysis shows that in the part of the HXT that experienced the maximum change, reflectors tilt up to 0.4 arcmin, with a focal distance shift of similar to 22 cm (a similar to 2% change). Despite these changes, the performance of the HXT remains well within the tolerances required to meet the scientific requirements.

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 (delta t <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.

Transition-edge sensor (TES)-based microcalorimeter detectors are an attractive option for future hard X-ray and gamma-ray telescopes due to their excellent energy resolution. As high performance single photon detectors, TESs offer unprecedented energy resolution compared to scintillator detectors, cryogenic high-purity Germanium detectors and room temperature semiconductor detectors that are currently used as telescope detectors in the hard X-ray and gamma ray regime. TES detectors typically utilize a thin film of superconducting material and an absorbing structure made with a high-Z material as the photon absorber. The photon collection efficiency is directly influenced by the material and the design of the absorbing structure. For astrophysical missions, where one requires the sensitivity, and therefore the collection efficiency to be maximal for a minimal detector mass, optimization of these absorbers is crucial. Sn (Z = 50) absorbers have been identified as a good candidate to be used in TESs in hard X-ray and gamma-ray detectors. However, the high precision in surface quality and their small (mm) size make uniform production challenging. Current fabrication methods involve mechanical processing, which often results in uneven edges and surfaces. In addition, manually attaching these irregular absorbers compromises their precise placement and increases the dead space between neighboring pixels in the detector array, thereby reducing the overall detector efficiency. In this study, we report the development process of using electroforming techniques to fabricate Sn absorbers with high dimensional accuracy. The primary goal is to minimize the dead space between neighboring absorbers. In this method, a 3-D printed mold with predefined structural features, attached to a hard polymer substrate, was used to deposit Sn via the electroforming technique. In this article, we will discuss the preliminary results of the electroformed Sn absorbers.

The 511 keV electron-positron annihilation feature near the galactic center has been detected for more than half a century, yet its origin remains a mystery. We describe a concept for a balloon-borne 511 keV gamma-ray mission called the 511-Spectrometer Mission. The mission will use transition-edge sensor (TES) arrays with thick metal absorbers that are thermally coupled to the TES. The strength of the approach is a projected energy resolution of 200-eV full-width half maximum (FWHM) at 511 keV, enabling detailed studies of the shape and substructure of the 511 keV emission from the galactic center region. A first mission equipped with 8192 gamma-ray detectors and a fully active shield and collimator could detect the galactic center with 35 sigma statistical significance. We present the mission concept as well as the first results obtained with a prototype detector equipped with 1.35 & times;1.35 & times;2 mm(3) Bi absorbers. The detector has a quantum efficiency of 15% for 511 keV photons in photoelectric effect interactions. In tests with a Cs-137 source, these prototype detectors show an energy resolution of 525 eV FWHM at 662 keV. We end with a discussion of follow-up missions that use coded mask imaging or use concentrating or focusing optics to scrutinize the sources of 511 keV gamma-rays on smaller angular scales.

Charge exchange is a process observed where highly charged ions interact with neutral atoms or molecules, capturing electrons and emitting X-rays as the excited states decay toward the ground state. Charge exchange-induced X-rays provide information on the ion composition, temperature, and density of high-temperature plasmas. Accurate charge exchange cross section data from ground-based laboratory experiments are required to interpret the charge exchange-induced X-ray emissions observed in astrophysical and laboratory plasma environments. In this work, we describe the development of a laboratory setup, a 0.7 T miniature-Electron Beam Ion Trap (mini-EBIT), for studying charge exchange interactions between highly charged neon ions and neutral gases (H or He), using a transition-edge sensor (TES) microcalorimeter array. We have designed and built a cost-effective pulsed gas injection system to introduce short, well-defined pulses of neutral gas into the EBIT to better model charge exchange events and to provide precise timing control. Detailed information about the characteristic emission lines of the neon ions would allow accurate determination of the cross sections involved in charge exchange processes. We present the initial results obtained from the mini-EBIT using this advanced TES detector array. We will also present time-of-flight spectra from a microchannel plate time-of-flight detector, as well as X-ray emission lines from a silicon drift detector, to characterize the highly charged ion production in the mini-EBIT.

The location in which γ-ray are created and emitted within extra-galactic jets is a matter of active debate. One particularly well-suited source for determining the location is the nearby bright radio galaxy 3C 84, harbouring a powerful jet. We investigated the origin of γ-rays that were measured during a recent γ-ray flare by analysing the linear polarisation signal of close-in-time very long baseline interferometry (VLBI) observations at centimetre and millimetre wavelengths. While 3C 84 is almost unpolarised overall, we find that close in time to the γ-ray flare peak regions at parsec-scale distances from the central engine, the linear polarisation increases fractionally. Under the physically well-motivated assumption of a causal relation between this polarisation enhancement and the γ-ray flare, and combined with insights from concurrent X-ray polarisation measurements, a physically motivated scenario is that the γ-rays are created in this region, in a process consistent with the synchrotron self-Compton mechanism.

We present updated hard X-ray polarization measurements of the Crab pulsar and nebula obtained with the balloon-borne polarimeter XL-Calibur in the similar to 19-64 keV energy range. During the flight, intermittent failure of the Global Positioning System (GPS) receiver resulted in poorly constrained timing for similar to 38% of the Crab dataset. By implementing a new phase recovery method that reconstructs timing during extended GPS-off intervals, phase tag data are recovered for similar to 95% of the GPS-off dataset, increasing the precision of the phase-resolved analysis. Phase information for the data is recovered by using the Crab pulsar, with its 33 ms period, as an external timing source. Using a Markov Chain Monte Carlo framework to jointly fit phase offsets and frequency derivatives, sufficient phase accuracy is achieved across multiple periods without GPS for a phase-resolved analysis. This enables inclusion of nearly the full dataset in the polarization study. The polarization degree of the nebular emission is found to be (27.7 +/- 4.9)% at a polarization angle of 127 .degrees 2 +/- 5 .degrees 1, confirming previous XL-Calibur results and remaining aligned with the Crab's spin axis, consistent with synchrotron emission from the inner nebula. Phase-resolved measurements show that the off-pulse and bridge intervals exhibit a strong polarization, while the pulsar peaks, although weakly constrained, remain in agreement with the softer-energy trends of IXPE. These findings reinforce a scenario in which hard X-ray emission arises primarily in the nebular torus and wind regions. The successful recovery of precise phase tagging from GPS-off data demonstrates the capacity to use the pulsar as an external clock even in the case of sparsely populated data.

Lorentz symmetry is the fundamental symmetry of Einstein's theory of Special Relativity and has been tested to great precision. Nevertheless, the possibility remains that it is violated at the Planck scale, as predicted by some theories of quantum gravity. While the Planck scale is not directly accessible to experiments, minute residual deviations from Lorentz symmetry at attainable energies may be observable. The polarization of light from astrophysical sources is a particularly powerful probe because tiny differences accumulate as light travels over astrophysical distances, and polarization is sensitive to light travel time differences between polarization modes on the order of the oscillation period of the electromagnetic wave. Here, we report on new constraints on Lorentz invariance violation derived from X-ray polarization measurements of active galactic nuclei. The new constraints, presented in the framework of the Standard-Model Extension, improve on our previous work, which used optical polarization measurements, by four orders of magnitude.

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 balloon-borne hard X-ray polarimetry mission XL-Calibur observed the black hole X-ray binary (BHXRB) Cygnus X-1 (Cyg X-1) during its nearly 6 day long-duration balloon flight from Sweden to Canada in 2024 July. The XL-Calibur observations allowed us to derive the most precise constraints to date of the polarization degree (PD) and polarization angle (PA) of the hard X-ray emission from a BHXRB. XL-Calibur observed Cyg X-1 in the hard state and measured a ∼19–64 keV PD of ( 5 . 0 − 3.0 + 2.7 )% (equivalent to an upper limit, at the 99% level, of 11.1%) at a PA of −28° ± 17°, with an 8.7% chance probability of detecting larger PDs than the one observed, given an unpolarized signal. The XL-Calibur results are thus comparable to the 2–8 keV PD and PA found by Imaging X-ray Polarimetry Explorer (IXPE), with a similar agreement between the hard X-ray PA and the radio jet direction. We also discuss the implications of our polarization measurements in the context of models describing the origin of the broadband X-ray and γ -ray emission, to which XL-Calibur provides independent constraints on any proposed emission modeling.

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.

A Super-Conducting ENergetic x-ray Telescope (ASCENT) is a concept for a future balloon-borne high-energy X-ray telescope in the energy range 60–85 keV to study gamma-ray emissions of 67.87 keV and 78.32 keV from the radioactive isotope 44Ti. For the focal plane instrumentation, ASCENT will use Mo-Cu/Mo-Au bilayer transition edge sensor (TES) microcalorimeter gamma-ray detectors with tin (Sn) absorbers. ‘Spectrometer to Leverage Extensive Development of Gamma-ray TESs for Huge Arrays using Microwave Multiplexed Enabled Readout’ (SLEDGEHAMMER), a detector development project at the National Institute of Standards and Technology, acts as the basis for the detector arrays for ASCENT. SLEDGEHAMMER has tin (Sn) absorbers attached to the SU-8 epoxy posts, lithographically placed on the detectors, but we are also considering other geometries for the chips where the absorbers are attached to the chips separated from the TESs, which could help to avoid parallel path for a current flow around the detectors with these BiSn sphere attachments. In this work, we are reporting on developing a method to attach Tin (Sn) absorbers to the transition edge sensors (TESs) with 0.2 mm diameter BiSn solder spheres replacing epoxy. The goal is to improve the thermal conductivity between the absorbers and the TESs compared to what was achieved using epoxy, potentially reducing the presence of an athermal component in the tails of signal pulses. We describe our efforts toward finding optimal temperature and pressure conditions for making this contact and the progress toward contact resistance measurements of these joints.

The last years have seen the first cryogenic detectors to be proposed for usage on balloon-borne missions. In such missions, the instrument will be exposed to the high radiation environment of the upper atmosphere. This radiation can induce a significant background to the measurements, something which can be mitigated through the use of an anti-coincidence shield. For hard X-ray and gamma-ray detectors such a shield typically consists of photomultiplier tubes or, more recently, silicon photomultipliers coupled to scintillators placed around the detector. When using cryogenic detectors, the shield can be placed around the entire cryostat which will make it large, heavy and expensive. For the ASCENT (A SuperConducting ENergetic X-ray Telescope) mission, which uses Transition Edge Sensor microcalorimeter detectors, it was therefore considered to instead place the shield inside. This comes with the challenge of operating it at cryogenic temperatures. For this purpose, we tested the performance of 2 different types of GAGG:Ce scintillators down to 15 mK for the first time. Although significant variations of both the decay time and the light yield were found when varying the temperature, at 4 K its performance was found to be similar to that at room temperature. Furthermore, unexpected behavior around 2 K was found for both types of GAGG:Ce, leading to more in-depth studies around these temperatures. Overall, the studies show that the combination of materials will allow to produce a functional anti-coincidence shield at several Kelvin.