The correlation between optical starlight polarization and polarized thermal dust emission can be used to infer intrinsic dust properties. This correlation is quantified by the ratio RP/p, which has been measured by the Planck Collaboration to be 5.42 ± 0.05 MJy sr−1 at 353 GHz when averaged over large areas of the sky. We investigated this correlation using newly published stellar polarimetric data densely sampling a continuous sky region of about four square degrees at intermediate Galactic latitude. We combined RoboPol optical polarization measurements for 1430 stars with submillimeter data from the Planck satellite at 353 GHz. We performed linear fits between the Planck (Qs, Us) and optical (qv, uv) Stokes parameters, taking into account the differences in resolution between the two datasets as well as the distribution of clouds along the line of sight. We find in this region of the sky that the RP/p value is 3.67 ± 0.05 MJy sr−1, indicating a significantly shallower slope than that found previously using different stellar samples. We also find significant differences in the fitted slopes when fitting the Qs–qv and Us–uv data separately. We explore two explanations using mock data: the miscalibration of the polarization angle and the variations in RP/p along the line of sight due to multiple clouds. We show that the former can produce differences in the correlations of Qs–qv and Us–uv, but large miscalibration angles would be needed to reproduce the magnitude of the observed differences. Our simulations favor the interpretation that RP/p differs between the two dominant clouds that overlap on the sky in this region. The difference in RP/p suggests that the two clouds may have distinct dust polarimetric properties. With knowledge from the tomographic decomposition of the stellar polarization, we find that one cloud appears to dominate the correlation of Us–uv, while both clouds contribute to the correlation of the Qs–qv data.

The magnetohydrodynamics (MHD) equations plus 'non-ideal' (Ohmic, Hall, ambipolar) resistivities are widely used to model weakly-ionized astrophysical systems. We show that if gradients in the magnetic field become too steep, the implied charge drift speeds become much faster than microphysical signal speeds, invalidating the assumptions used to derive both the resistivities and MHD equations themselves. Generically this situation will excite microscale instabilities that suppress the drift and current. We show this could be relevant at low ionization fractions especially if Hall terms appear significant, external forces induce supersonic motions, or dust grains become a dominant charge carrier. Considering well-established treatments of super-thermal drifts in laboratory, terrestrial, and Solar plasmas as well as conduction and viscosity models, we generalize a simple prescription to rectify these issues, where the resistivities are multiplied by a correction factor that depends only on already-known macroscopic quantities. This is generalized for multi-species and weakly-ionized systems, and leaves the equations unchanged in the drift limits for which they are derived, but restores physical behavior (driving the system back towards slow drift by diffusing away small-scale gradients in the magnetic field) if the limits are violated. This has important consequences: restoring intuitive behaviors such as the system becoming hydrodynamic in the limit of zero ionization; suppressing magnetic structure on scales below a critical length which can comparable to circumstellar disk sizes; limiting the maximum magnetic amplification; and suppressing the effects of the Hall term in particular. This likely implies that the Hall term does not become dynamically important under most conditions of interest in these systems.

Striations are diffuse, linear, quasi-periodic, and magnetized structures located in the outskirts of molecular clouds. These structures seem to play an important role during the earliest stages of star formation. Theoretical models suggest that magnetic fields play an important role in the formation of striations. With its unprecedented resolution and sensitivity, the polarization module of the PRIMAger instrument onboard the PRIMA space observatory will enable studies related to the magnetic properties of striations in nearby molecular clouds. We plan to target three nearby (≲ 350 pc) molecular clouds (the Polaris Flare, Taurus, and Musca) with prominent striations that are strongly coupled to the large-scale magnetic field properties, as traced by low-resolution sub-millimeter polarization data. We will search for the unique imprint of the passage of magnetohydrodynamic waves in the polarization angle maps, which traces the magnetic field morphology, in bands 3 (172μm) and 4 (235μm) of PRIMAger. Each of the target regions is approximately 1 square degree in size. All three regions combined, can be mapped to more than five-sigma detection in averaged polarized intensity in ∼ 59 hours. The proposed survey promises to provide important information on the early phases of star formation.

The Serkowski relation is the cornerstone of studies of starlight polarization as a function of wavelength. Although empirical, its extensive use since its inception to describe polarization induced by interstellar dust has elevated the relation to the status of an indisputable "law", serving as the benchmark for validating interstellar dust grain models. We revisit the effects of the 3D structure of the interstellar medium (ISM) on the wavelength dependence of interstellar polarization. We use analytical models to show how the wavelength dependence of both the polarization fraction and direction is affected by the presence of multiple clouds along the line of sight (LOS), accounting for recent developments in dust distribution modelling and utilizing an expanded archive of stellar polarization measurements. We highlight concrete examples of stars whose polarization profiles are severely affected by LOS variations of the dust grain and magnetic field properties, and we provide a recipe to accurately fit multiple cloud Serkowski models to such cases. We present, for the first time, compelling observational evidence that the 3D structure of the magnetized ISM often results to the violation of the Serkowski relation. We show that 3D effects impact interstellar cloud parameters derived from Serkowski fits. In particular, the dust size distribution in single - cloud sightlines may differ from analyses that ignore 3D effects, with important implications for dust modelling in the Galaxy. Our results suggest that multiband stellar polarization measurements offer an independent probe of the LOS variations of the magnetic field, constituting a valuable new tool for the 3D cartography of the ISM. We caution that, unless 3D effects are explicitly accounted for, a poor fit to the Serkowski relation does not, by itself, constitute conclusive evidence that a star is intrinsically polarized.

Context. The magnetic field has been shown to play a crucial role in star formation. Dust polarization is one of the most effective tools for probing the properties of the magnetic field, yet it does not directly trace its strength. To bridge this gap, several methods have been developed, combining polarization and spectroscopic data, to estimate the strength of the magnetic field. The most widely applied method was developed by Davis (1951, Phys. Rev., 81, 890) and Chandrasekhar & Fermi (1953, ApJ, 118, 113), hereafter DCF, and relates the polarization angle dispersion to magnetic field strength under the assumption of Alfv & eacute;nic turbulence. Skalidis & Tassis (2021, A&A, 647, A186), hereafter ST, relaxed this assumption to account for the compressible modes, and derived more accurate estimates of the magnetic field strength than the DCF in clouds with no self-gravity. The accuracy of these methods in self-gravitating regions is poorly explored. Aims. We aim to evaluate the accuracy of these magnetic-field estimation methods in star-forming regions and propose a systematic approach for calculating the key observational parameters they involve: the velocity dispersion (delta v), the polarization angle dispersion (delta theta), and the cloud density (rho). Methods. We used a three-dimensional magnetohydrodynamic chemo-dynamical simulation of a turbulent collapsing molecular cloud. We generated synthetic observations for seven different inclination angles with respect to the mean component of the magnetic field, which encompass a comprehensive set of observables, including emission line spectra, Stokes parameters, and column density maps. We employed various approaches for estimating the parameters delta v, delta theta, and rho, and identified the best approach that most effectively probes the plane-of-sky (POS) component of the magnetic field. Results. We find that the approach used to calculate the parameters delta v, delta theta, and rho plays a crucial role in estimating the magnetic field strength, regardless of the specific method used (i.e., the DCF or the ST methods). We show that the value probed by both methods corresponds to the median of the molecular-species-weighted POS component of the magnetic field. We also find that ST outperforms DCF. The magnetic field strength values derived with the ST method accurately follow the expected cosine trend with respect to the inclination angle of the magnetic field and consistently remain within 1 sigma of the median component of the magnetic field strength. In self-gravitating clouds, we propose the following approach to accurately constrain the intrinsic parameters involved in the magnetic field estimation methods: rho using radiative transfer analysis, delta v using the second moment maps, and delta theta by fitting Gaussians to the polarization angle distributions to remove the contribution of the hourglass morphology.

Polarimetry of stars at optical and near-infrared wavelengths is an invaluable tool for tracing interstellar dust and magnetic fields. Recent studies have demonstrated the power of combining stellar polarimetry with distances from the Gaia mission, in order to gain accurate, 3D information on the properties of the interstellar magnetic field and the dust distribution. However, access to optical polarization data is limited, as observations are conducted by different investigators, with different instruments, and are made available in many separate publications. To enable a more widespread accessibility of optical polarimetry for studies of the interstellar medium, we compile a new catalog of stellar polarization measurements. The data are gathered from 81 separate publications spanning two decades since the previous, widely used agglomeration of catalogs by C. Heiles. The compilation contains a total of 55,742 measurements of stellar polarization. We combine this database with stellar distances based on the Gaia Early Data Release 3, thereby providing polarization and distance data for 42,482 unique stars. We provide two separate data products: an extended catalog (containing all polarization measurements) and a unique source catalog (containing a subset of sources excluding duplicate measurements). We propose the use of a common tabular format for the publication of stellar polarization catalogs to facilitate accessibility and increase discoverability in the future.

The WALOPControl software is designed to facilitate comprehensive control and operation of the Wide Area Linear Optical Polarimeter (WALOP) polarimeters, ensuring safe and concurrent management of various instrument components and functionalities. This software encompasses several critical requirements, including control of the filter wheel, calibration half-wave plate, calibration polarizer, guider positioning, focusers, and four concurrent charge-coupled device cameras. It also manages the host telescope and dome operations while logging operational parameters, user commands, and environmental conditions for troubleshooting and stability. It provides a user-friendly graphical user interface and secure access control, a notification system for errors, and a modular configuration for troubleshooting that are integral to the software's architecture. It is accessible over the internet with the backend developed using NodeJS and ExpressJS, featuring a RESTful API that interacts with a MongoDB database, facilitating real-time status updates and data logging. The frontend utilizes the React.JS framework, with Redux for state management and Material UI for the graphical components. The system also allows for automatic observations based on user-defined schedules. A continuous integration and continuous deployment pipeline ensures the software's reliability through automated testing and streamlined deployment. The WALOPControl software is a key component of the Polar-Areas Stellar Imaging in Polarimetry High Accuracy Experiment project, which aims to study the dust and magnetic field of the Milky Way by observing the polarization of starlight. (C) 2025 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI:10.1117/1.JATIS.11.3.037001]

Observations of linear polarization in the 2–8 keV energy range with the Imaging X-ray Polarimetry Explorer (IXPE) explore the magnetic field geometry and dynamics of the regions generating nonthermal radiation in relativistic jets of blazars. These jets, particularly in blazars whose spectral energy distribution peaks at X-ray energies, emit X-rays via synchrotron radiation from high-energy particles within the jet. IXPE observations of the X-ray-selected BL Lac–type blazar 1ES 1959+650 on 2022 May 3–4 showed a significant linear polarization degree of Π x = 8.0% ± 2.3% at an electric-vector position angle ψ x = 123° ± 8°. However, on 2022 June 9–12, only an upper limit of Π x ≤ 5.1% could be derived (at the 99% confidence level). The degree of optical polarization at that time, Π O ∼ 5%, is comparable to the X-ray measurement. We investigate possible scenarios for these findings, including temporal and geometrical depolarization effects. Unlike some other X-ray-selected BL Lac objects, there is no significant chromatic dependence of the measured polarization in 1ES 1959+650, and its low X-ray polarization may be attributed to turbulence in the jet flow with dynamical timescales shorter than 1 day.

Cosmic rays (CRs) may drive outflows and alter the phase structure of the circumgalactic medium, with potentially important implications on galaxy formation. However, these effects ultimately depend on the dominant mode of transport of CRs within and around galaxies, which remains highly uncertain. To explore potential observable constraints on CR transport, we investigate a set of cosmological FIRE-2 CR-MHD simulations of L$_{\ast}$ galaxies which evolve CRs with transport models motivated by self-confinement (SC) and extrinsic turbulence (ET) paradigms. To first order, the synchrotron properties diverge between SC and ET models due to a CR physics driven hysteresis. SC models show a higher tendency to undergo `ejective' feedback events due to a runaway buildup of CR pressure in dense gas due to the behavior of SC transport scalings at extremal CR energy densities. The corresponding CR wind-driven hysteresis results in brighter, smoother, and more extended synchrotron emission in SC runs relative to ET and constant diffusion runs. The differences in synchrotron arise from different morphology, ISM gas and \textbf{B} properties, potentially ruling out SC as the dominant mode of CR transport in typical star-forming L$_{\ast}$ galaxies, and indicating the potential for non-thermal radio continuum observations to constrain CR transport physics.

We present X-ray polarimetry observations from the Imaging X-ray Polarimetry Explorer (IXPE) of three low spectral peak and one intermediate spectral peak blazars, namely 3C 273, 3C 279, 3C 454.3, and S5 0716+714. For none of these objects was IXPE able to detect X-ray polarization at the 3$\sigma$ level. However, we placed upper limits on the polarization degree at $\sim$10-30\%. The undetected polarizations favor models where the X-ray band is dominated by unpolarized photons upscattered by relativistic electrons in the jets of blazars, although hadronic models are not completely eliminated. We discuss the X-ray polarization upper limits in the context of our contemporaneous multiwavelength polarization campaigns.

Context . Carbon monoxide (CO) is a poor tracer of H 2 in the diffuse interstellar medium (ISM), where most of the carbon is not incorporated into CO molecules, unlike the situation at higher extinctions. Aims . We present a novel, indirect method for constraining H 2 column densities ( N H 2 ) without employing CO observations. We show that previously recognized nonlinearities in the relation between the extinction, A V (H 2 ), derived from dust emission and the H I column density ( N H I ) are due to the presence of molecular gas. Methods . We employed archival ( N H 2 ) data, obtained from the UV spectra of stars, and calculated A V (H 2 ) toward these sight lines using 3D extinction maps. The following relation fits the data: log N H 2 = 1.38742 (log A V (H 2 ) )3 − 0.05359 (log A V (H 2 ) )2 + 0.25722 log A V (H 2 ) + 20.67191. This relation is useful for constraining N H 2 in the diffuse ISM as it requires only N H I and dust extinction data, which are both easily accessible. In 95% of the cases, the estimates produced by the fitted equation have deviations of less than a factor of 3.5. We constructed a N H 2 map of our Galaxy and compared it to the CO integrated intensity ( W CO ) distribution. Results . We find that the average ratio ( X CO ) between N H 2 and W CO is approximately equal to 2 × 10 20 cm −2 (K km s −1 ) −1 , consistent with previous estimates. However, we find that the X CO factor varies by orders of magnitude on arcminute scales between the outer and the central portions of molecular clouds. For regions with N H 2 ≳ 10 20 cm −2 , we estimate that the average H 2 fractional abundance, f H 2 = 2 N H 2 /(2 N H 2 + N H I ), is 0.25. Multiple (distinct) largely atomic clouds are likely found along high-extinction sightlines ( A V ≥ 1 mag), hence limiting f H 2 in these directions. Conclusions . More than 50% of the lines of sight with N H 2 ≥ 10 20 cm −2 are untraceable by CO with a J = 1−0 sensitivity limit W CO = 1 K km s −1 .

We perform the first broadband study of Mrk421 from radio to TeV gamma rays with simultaneous measurements of the X-ray polarization from IXPE. The data were collected within an extensive multiwavelength campaign organized between May and June 2022 using MAGIC, Fermi-LAT, NuSTAR, XMM-Newton, Swift, and several optical and radio telescopes to complement IXPE. During the IXPE exposures, the measured 0.2-1 TeV flux is close to the quiescent state and ranges from 25% to 50% of the Crab Nebula without intra-night variability. Throughout the campaign, the VHE and X-ray emission are positively correlated at a $4\sigma$ significance level. The IXPE measurements unveil a X-ray polarization degree that is a factor of 2-5 higher than in the optical/radio bands; that implies an energy-stratified jet in which the VHE photons are emitted co-spatially with the X-rays, in the vicinity of a shock front. The June 2022 observations exhibit a rotation of the X-ray polarization angle. Despite no simultaneous VHE coverage being available during a large fraction of the swing, the Swift-XRT monitoring unveils an X-ray flux increase with a clear spectral hardening. It suggests that flares in high synchrotron peaked blazars can be accompanied by a polarization angle rotation, as observed in some flat spectrum radio quasars. Finally, during the polarization angle rotation, NuSTAR data reveal two contiguous spectral hysteresis loops in opposite directions (clockwise and counter-clockwise), implying important changes in the particle acceleration efficiency on $\sim$hour timescales.

The Wide Area Linear Optical Polarimeter North is an optical polarimeter designed for the needs of the Polar-Areas Stellar Imaging in Polarimetry High-Accuracy Experiment survey. It will be installed on the 1.3-m telescope at the Skinakas Observatory in Crete, Greece. After commissioning, it will measure the 30x30 arcmin2 polarization of millions of stars at high galactic latitude, aiming to measure hundreds of stars per square degree. The astronomical filter used in the instrument is a modified, polarimetrically neutral broadband Sloan Digital Sky Survey-r. This instrument will be a pioneering one due to its large field of view (FoV) of and high-accuracy polarimetry measurements. The accuracy and sensitivity of the instrument in polarization fraction will be at the 0.1% and 0.05% levels, respectively. Four separate 4kx4k charge-coupled devices will be used as the instrument detectors, each imaging one of the 0-, 45-, 90-, and 135-deg polarized FoV separately, therefore making the instrument a four-channel, one-shot polarimeter. Here, we present the overall optical design of the instrument, emphasizing the aspects of the instrument that are different from Wide Area Linear Optical Polarimeter South. We also present a customized design of filters appropriate for polarimetry along with details on the management of the instrument size and its polarimetric calibration.

Wide-Area Linear Optical Polarimeter (WALOP)-South is the first wide-field and survey-capacity polarimeter in the optical wavelengths. On schedule for commissioning in 2024, it will be mounted on the 1 m SAAO telescope in Sutherland Observatory, South Africa to undertake the PASIPHAE sky survey. PASIPHAE program will create the first polarimetric sky map in the optical wavelengths, spanning more than 2000 square degrees of the southern Galactic region. In a single exposure, WALOP-South's innovative design will enable it to measure the linear polarization (Stokes parameters q and u) of all sources in a field of view (FoV) of 35 x 35 arc-minutes-squared in the SDSS-r broadband and narrowband filters between 500-750 nm with 0.1 % polarization accuracy. The unique goals of the instrument place very stringent systems engineering goals, including on the performance of the optical, polarimetric, optomechanical, and electronic subsystems. In particular, the major technical hurdles for the project included the development of: (a) an optical design to achieve imaging quality PSFs across the FoV, (b) an optomechanical design to obtain high accuracy optical alignment in conjugation with minimal instrument flexure and stress birefringence on optics (which can lead to variable instrumental polarization), and (c) an on-sky calibration routine to remove the strong polarimetric cross-talk induced instrumental polarization to obtain 0.1% across the FoV. All the subsystems have been designed carefully to meet the overall instrument performance goals. As of May 2024, all the instrument optical and mechanical subsystems have been assembled and are currently getting tested and integrated. The complete testing and characterization of the instrument in the lab is expected to be completed by August 2024. While the instrument was initially scheduled for commissioning in 2022, it got delayed due to various technical challenges; WALOP-South is now on schedule for commissioning in second half of 2024. In this paper, we will present (a) the design and development of the entire instrument and its major subsystems, focusing the instrument's opto-mechanical design which has not been reported before, and (b) assembly and integration of the instrument in the lab and early results from lab characterization of the instrument's optical performance.

We present the first degree-scale tomography map of the dusty magnetized interstellar medium (ISM) from stellar polarimetry and distance measurements. We used the RoboPol polarimeter at Skinakas Observatory to conduct a survey of the polarization of starlight in a region of the sky of about four square degrees. We propose a Bayesian method to decompose the stellar-polarization source field along the distance to invert the three-dimensional (3D) volume occupied by the observed stars. We used this method to obtain the first 3D map of the dusty magnetized ISM. Specifically, we produced a tomography map of the orientation of the plane-of-sky component of the magnetic field threading the diffuse, dusty regions responsible for the stellar polarization. For the targeted region centered on Galactic coordinates ( l , b ) ≈ (103.3°, 22.3°), we identified several ISM clouds. Most of the lines of sight intersect more than one cloud. A very nearby component was detected in the foreground of a dominant component from which most of the polarization signal comes and which we identified as being an intersection of the wall of the Local Bubble and the Cepheus Flare. Farther clouds, with a distance of up to 2 kpc, were similarly detected. Some of them likely correspond to intermediate-velocity clouds seen in H I spectra in this region of the sky. We found that the orientation of the plane-of-sky component of the magnetic field changes along distance for most of the lines of sight. Our study demonstrates that starlight polarization data coupled to distance measures have the power to reveal the great complexity of the dusty magnetized ISM in 3D and, in particular, to provide local measurements of the plane-of-sky component of the magnetic field in dusty regions. This demonstrates that the inversion of large data volumes, as expected from the P ASIPHAE survey, will provide the necessary means to move forward in the modeling of the Galactic magnetic field and of the dusty magnetized ISM as a contaminant in observations of the cosmic microwave background polarization.

The Wide-Area Linear Optical Polarimeters (WALOPs)(12) are 2 instruments - WALOPNorth and WALOP-South - that will be installed at the Skinakas and South African Astronomical Observatories respectively. Their main goal is to work towards a polarimetric map of the Galaxy, for the needs of the PASIPHAE collaboration(3). The WALOP instruments, to be able to operate smoothly, require custom-made software to fit their (and the survey's) specifications. We will present said software's specifications and the methods and technologies used to meet these requirements.

We present the first multi-wavelength study of Mrk 501 including very-high-energy (VHE) gamma-ray observations simultaneous to X-ray polarization measurements from the Imaging X-ray Polarimetry Explorer (IXPE). We use radio-to-VHE data from a multi-wavelength campaign organized between 2022-03-01 and 2022-07-19. The observations were performed by MAGIC, Fermi-LAT, NuSTAR, Swift (XRT and UVOT), and several instruments covering the optical and radio bands. During the IXPE pointings, the VHE state is close to the average behavior with a 0.2-1 TeV flux of 20%-50% the emission of the Crab Nebula. Despite the average VHE activity, an extreme X-ray behavior is measured for the first two IXPE pointings in March 2022 with a synchrotron peak frequency >1 keV. For the third IXPE pointing in July 2022, the synchrotron peak shifts towards lower energies and the optical/X-ray polarization degrees drop. The X-ray polarization is systematically higher than at lower energies, suggesting an energy-stratification of the jet. While during the IXPE epochs the polarization angle in the X-ray, optical and radio bands align well, we find a clear discrepancy in the optical and radio polarization angles in the middle of the campaign. We model the broad-band spectra simultaneous to the IXPE pointings assuming a compact zone dominating in the X-rays and VHE, and an extended zone stretching further downstream the jet dominating the emission at lower energies. NuSTAR data allow us to precisely constrain the synchrotron peak and therefore the underlying electron distribution. The change between the different states observed in the three IXPE pointings can be explained by a change of magnetization and/or emission region size, which directly connects the shift of the synchrotron peak to lower energies with the drop in polarization degree.

Aims.We aim to probe the magnetic field geometry and particle acceleration mechanism in the relativistic jets of supermassive black holes.Methods.We conducted a polarimetry campaign from radio to X-ray wavelengths of the high-synchrotron-peak (HSP) blazar Mrk 421, including Imaging X-ray Polarimetry Explorer (IXPE) measurements from 2022 December 6–8. During the IXPE observation, we also monitored Mrk 421 usingSwift-XRT and obtained a single observation withXMM-Newtonto improve the X-ray spectral analysis. The time-averaged X-ray polarization was determined consistently using the event-by-event Stokes parameter analysis, spectropolarimetric fit, and maximum likelihood methods. We examined the polarization variability over both time and energy, the former via analysis of IXPE data obtained over a time span of 7 months.Results.We detected X-ray polarization of Mrk 421 with a degree of ΠX = 14 ± 1% and an electric-vector position angleψX = 107 ± 3° in the 2–8 keV band. From the time variability analysis, we find a significant episodic variation inψX. During the 7 months from the first IXPE pointing of Mrk 421 in 2022 May,ψXvaried in the range 0° to 180°, while ΠXremained relatively constant within ∼10–15%. Furthermore, a swing inψXin 2022 June was accompanied by simultaneous spectral variations. The results of the multiwavelength polarimetry show that ΠXwas generally ∼2–3 times greater than Π at longer wavelengths, whileψfluctuated. Additionally, based on radio, infrared, and optical polarimetry, we find that the rotation ofψoccurred in the opposite direction with respect to the rotation ofψXand over longer timescales at similar epochs.Conclusions.The polarization behavior observed across multiple wavelengths is consistent with previous IXPE findings for HSP blazars. This result favors the energy-stratified shock model developed to explain variable emission in relativistic jets. We considered two versions of the model, one with linear and the other with radial stratification geometry, to explain the rotation ofψX. The accompanying spectral variation during theψXrotation can be explained by a fluctuation in the physical conditions, for example in the energy distribution of relativistic electrons. The opposite rotation direction ofψbetween the X-ray and longer wavelength polarization accentuates the conclusion that the X-ray emitting region is spatially separated from that at longer wavelengths. Moreover, we identify a highly polarized knot of radio emission moving down the parsec-scale jet during the episode ofψXrotation, although it is unclear whether there is any connection between the two events.

The Wide Area Linear Optical Polarimeter North (WALOP-North) is an optical polarimeter designed for the needs of the PASIPHAE survey. It will be installed on the 1.3m telescope at the Skinakas Observatory in Crete, Greece. After commissioning, it will measure the polarization of millions of stars at high Galactic latitude, aiming to measure hundreds of stars per deg^2. The astronomical filter used in the instrument is a modified, polarimetrically-neutral broadband SDSS-r. This instrument will be pioneering one due to its large field-of-view (FoV) of 30× 30 arcmin^2 and high accuracy polarimetry measurements. The accuracy and sensitivity of the instrument in polarization fraction will be at the 0.1% and 0.05% level, respectively. Four separate 4k×4k CCDs will be used as the instrument detectors, each imaging one of the 0, 45, 90 and 135 polarized FoV separately, therefore making the instrument a four-channel, one-shot polarimeter. Here, we present the overall optical design of the instrument, emphasizing on the aspects of the instrument that are different from WALOP-South. We also present a novel design of filters appropriate for polarimetry along with details on the management of the instrument size and its polarimetric calibration.

ABSTRACT Synchrotron emission is one of few observable tracers of galactic magnetic fields (B) and cosmic rays (CRs). Much of our understanding of B in galaxies comes from utilizing synchrotron observations in conjunction with several simplifying assumptions of equipartition models, however, it remains unclear how well these assumptions hold, and what B these estimates physically represent. Using Feedback in Realistic Environments project simulations which self-consistently evolve CR proton, electron, and positron spectra from MeV to TeV energies, we present the first synthetic synchrotron emission predictions from simulated L* galaxies with ‘live’ spectrally resolved CR-magnetohydrodynamic. We find that synchrotron emission can be dominated by relatively cool and dense gas, resulting in equipartition estimates of B with fiducial assumptions underestimating the ‘true’ B in the gas that contributes the most emission by factors of 2–3 due to small volume-filling factors. Motivated by our results, we present an analytical framework that expands upon equipartition models for estimating B in a multiphase medium. Comparing our spectrally resolved synchrotron predictions to simpler spectral assumptions used in galaxy simulations with CRs, we find that spectral evolution can be crucial for accurate synchrotron calculations towards galactic centres, where loss terms are large.