Hiroshi Akitaya

We present DragonflyPol, a wide-field optical linear polarimetry capability implemented on the Dragonfly Telephoto Array. DragonflyPol leverages Dragonfly’s modular, multi-lens architecture to obtain simultaneous measurements in four linear polarization orientations (0°, 45°, 90°, and 135°) across a ∼5 deg ² field of view, distributed across 44 polarized lens–detector units. Four additional units serve as unpolarized reference channels. We describe a broad range of science goals enabled by this capability, including magnetic field mapping and tomography, dust grain properties, CMB foreground characterization, and the three-dimensional structure of diffuse interstellar clouds. We integrate Canon polarizers and Baader Sloan r ′ bandpass filters into the drop-in filter holders of the Canon lenses, and conduct a three-phase laboratory characterization program to select optimal polarimetric components, measure contrast ratios and transmission efficiencies, and determine and mark the transmission axis of each polarizer with sub-degree repeatability. Laboratory measurements across all 44 deployed polarizers yield a mean noise-subtracted contrast ratio of 1228 ± 104 and a single-polarizer transmission efficiency of ∼33% in the r ′ band. On-sky commissioning, including twilight flat-field characterization and twilight-sky polarization measurements, confirms throughput stability across polarization groups and successful recovery of the expected Rayleigh scattering signal. DragonflyPol achieved first polarimetric light in 2025 September.

We report on a nine-month monitoring campaign of the blazar PG 1553+113, relying on three observations carried out in 2025 with the Imaging X-ray Polarimetry Explorer (IXPE) and supported by multiwavelength facilities. The source displayed pronounced variability across the electromagnetic spectrum, with X-ray flux changes by up to a factor of ∼5 and complex evolution of the optical polarization properties, including one of the largest (exceeding 150°) and fastest rotations in the electric vector position angle (EVPA) ever recorded. This swing of the EVPA was also accompanied by a temporary drop of the optical polarization degree to nearly zero. Significant X-ray polarization was observed during the third IXPE pointing, with a polarization degree Π _X = (18.4 ± 5.8)% and Ψ _X = 74° ± 9° in the 2–8 keV band, while only upper limits were obtained in the first two epochs. The optical data show that the second IXPE observation occurred shortly after a dramatic optical polarization event characterized by a rapid EVPA swing and strong depolarization. Two possible scenarios may explain the broadband polarimetric behavior: (i) the superposition of two emitting regions with nearly orthogonal magnetic field configurations and variable relative contributions, and (ii) the interaction of a single emitting region with a shock that temporarily reorders the magnetic field. In both cases, the data support a picture in which the X-ray and optical emissions arise from closely related but not strictly cospatial regions within a dynamically evolving, magnetically structured jet.

Multiwavelength polarimetry offers a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. Sources with synchrotron emission that spans a broad frequency range, from radio to X-rays, such as High Synchrotron Peak (HSP) type BL Lac objects, are particularly interesting. Previous measurements including radio, optical, and X-ray data show a clear trend, with the degree of polarization increasing with frequency. Here we report radio, optical, and X-ray observations ( Swift , Nustar , and IXPE ) of 1ES 1101-232 and RGB J0710+591, two blazars belonging to the puzzling subclass of extreme BL Lacs (EHBL). For 1ES 1101-232, we find a strong frequency dependence of the degree of polarization, with a ratio Π _X /Π _O ≃ 5.2. For RGB J0710+591, IXPE derives a 1 σ upper limit Π _X < 11.6%, comparable to the measured optical degree of polarization (average Π _O ∼ 12%). We discuss the results in the framework of current interpretations and, in particular, we report an improved version of the stratified shock model that reproduces the observed data of both sources.

We present independent polarimetric observations of the interstellar object 3I/ATLAS, including the first near-infrared polarimetric measurements. Using imaging polarimeters, we measured the degree of linear polarization from the visible R _C band (0.64 μ m) to the near-infrared K _s band (2.25 μ m), and investigated its dependence on solar phase angle (polarization phase curve, PPC) and wavelength (polarization color curve, PCC). We confirm that the PPC of 3I/ATLAS differs significantly from those of typical solar system comets, showing an unusually large polarization amplitude. This PPC shows no significant change in the R _C band across perihelion passage, despite the perihelion lying within the water snow line. This indicates that the unusual polarimetric behavior of 3I/ATLAS is unlikely to be driven by transient volatile activity, but instead reflects intrinsic optical properties of refractory dust particles. The PCC increases with wavelength over 0.6–1.2 μ m and peaks at 1.5–2.0 μ m, suggesting that the dominant scattering units are dust aggregates composed of submicron-sized monomers, broadly consistent with interstellar dust and solar-system cometary aggregates. Taken together, our results indicate that 3I/ATLAS preserves polarimetric properties characteristic of a primitive cometary planetesimal formed in another planetary system, with a refractory dust composition that differs from that typically observed among solar system comets, despite sharing a similar size scale of the aggregate building blocks.

Mechanical properties of small bodies such as dust, pebbles, and planetesimals are key to better understanding the formation of planetary systems against their collisional destruction. We present a simple mathematical formulation to determine the static tensile strength, elastic modulus, fracture energy, fracture toughness, and sound velocity for solid particles of any composition, size, structure, and porosity. This formulation is based on the Johnson–Kendall–Roberts theory of contact mechanics, the Griffith theory of fracture mechanics, and the Weibull theory of flaw statistics, in which both aggregate particles and monolithic particles are treated as agglomerates of small constituent grains and molecules, respectively. We show that our formulae are consistent with the results of laboratory experiments and molecular dynamics simulations for porous and nonporous particles of silica or ice, irrespective of the sintering process and the internal heterogeneity. This provides us with a great opportunity to infer the internal structure of asteroids and comets from observational data on their tensile strengths.

The tensile strength of solid particles is the key parameter that controls not only the formation of planetesimals in protoplanetary disks but also outburst and splitting of comets and rotational bursting of asteroids in planetary systems. It is, therefore, highly desirable to find a mathematical model that properly provides the tensile strength of porous and nonporous solid particles as functions of their known material and structural properties. We demonstrate the applicability of a rigorous theory for the tensile strength of particle assemblies to both porous and nonporous solid particles, by treating the particles as solid media with cross-linked networks of molecules. The practicability of the analytical function for the tensile strength of solid particles provided in this paper is proven to a large extent by the reproduction of laboratory experiments and molecular dynamics simulations of water ice, silica, and carbon specimens, irrespective of their crystallinities. While there is still room for improvement, specifically, on the choice of material-dependent parameters, we may conclude that our formulation is a major step forward in the inference of small-bodies’ material and structural properties from observational data on their tensile strengths.

Polarimetric properties of blazars allow us to put constraints on the acceleration mechanisms that fuel their powerful jets. By studying the multiwavelength polarimetric behaviour of high synchrotron peaked (HSP) and low synchrotron peaked (LSP) blazars, we aim to explore differences in their emission mechanisms and magnetic field structure in the acceleration region. In this study, we take advantage of several X-ray polarisation observations of HSP by the IXPE, including four new observations of Mrk 501, and optical polarisation observations of LSP from RoboPol and many others. We find that the polarisation degree (PD) distribution of HSP in X-rays is systematically higher than in optical and mm-radio wavelengths, as reported in previous IXPE publications. The distribution of the X-ray electric vector position angles (PA) is centered around the jet axis with most of the observations consistent with zero difference within uncertainties. In fact, the distribution of the offset of the PA from the jet axis is consistent between the LSP and HSP populations (with PA measured in optical for the first, X-ray for the latter), suggesting a common magnetic field structure close to the acceleration region, in strong support of the emerging energy stratified picture of particle acceleration followed by energy loss in blazar jets.

The High-z Gamma-ray Bursts for Unraveling the Dark Ages Mission (HiZ-GUNDAM) is dedicated to time-domain and multi-messenger astronomy, focusing on the observation of high-energy astronomical transient events, such as gamma-ray bursts (GRBs). The mission concept of HiZ-GUNDAM has been selected as a candidate for the competitively chosen medium-class mission of the Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency. Conceptual studies of the satellite and its onboard instruments are currently ongoing, with a target launch in the 2030s. HiZ-GUNDAM is designed to deliver rapid alerts of high-redshift GRBs through the use of its wide-field X-ray monitor, the Exploration of Ancient GRBs with Lobster Eye (EAGLE), and a co-mounted 30-cm Multiband Optical and Near-infrared Simultaneous Telescope for Efficient Response (MONSTER), which performs immediate photometric follow-up observations. Upon detecting a transient object, the HiZ-GUNDAM satellite is going to autonomously adjust its attitude toward detected transient events, initiate follow-up observations using MONSTER, and transmit alert information, detailing the object’s position, apparent magnitude, and photometric redshift within 1 h of detection. The MONSTER instrument achieves simultaneous five-band observations in the 0.5 to 2.5 μm range using a beam splitter and a Kösters prism. The incoming light is divided into visible (0.5 to 0.9 μm) and near-infrared components by the beam splitter, with the visible light directed to a visible light detector. The near-infrared light is further divided into four bands (0.9 to 1.3 μm, 1.3 to 1.7 μm, 1.7 to 2.1 μm, and 2.1 to 2.5 μm) by the Kösters prism and detected by a near-infrared detector. To ensure optimal performance, the telescope, beam splitter, Kösters prism, and visible-light detector are cooled to temperatures below 200 K, whereas the near-infrared detector is further cooled to below 120 K through radiative cooling. In addition, all mirrors and structural components of the telescope are constructed from the same aluminum alloy to minimize alignment errors during the cooling process. We provide an overview of the current status of the conceptual study of MONSTER onboard the HiZ-GUNDAM satellite.