Yasuo Fukui

We present a study of the interstellar molecular and atomic gas associated with the gamma-ray supernova remnant (SNR) RCW 103 using Mopra ¹² CO( J = 1–0), Atacama Large Millimeter/submillimeter Array ¹³ CO( J = 1–0), and Australia Telescope Compact Array and Parkes H i data. We find that CO clouds in the velocity range from −58.7 to −43.5 km s ⁻¹ show a clear spatial correspondence with the X-ray shell of RCW 103, particularly extending from the northwestern to the southeastern regions. The position–velocity diagram of the CO emission reveals an expanding gas motion with an expansion velocity of Δ V ∼ 7.5 km s ⁻¹ , which may have been produced by shock waves and/or stellar winds from the progenitor system. We estimate the total cosmic-ray proton energy to be ∼ 3 . 0 − 0.6 + 1.1 × 1 0 47 erg, adopting a total interstellar proton density of ∼810 cm ⁻³ . This value is more than an order of magnitude lower than those inferred for other gamma-ray-emitting SNRs of comparable age. Given that a substantial amount of dense gas remains within the interior of RCW 103, where thermal X-ray emission is dominant, our results possibly suggest that a significant fraction of the shock energy may have been converted into thermal energy via gas heating, thereby reducing the efficiency of cosmic-ray acceleration.

The Double Helix Nebula (DHN) is a peculiar infrared nebula that exhibits two intertwined helical strands at b ∼ 0 . ° 7 . Since two candidates for its molecular counterparts at 0 and −35 km s ⁻¹ extend from the central ∼10 pc of the Galactic center, and magnetic field lines aligned with the infrared DHN have been measured, a scenario in which a magnetic tower anchored to the circumnuclear disk has been proposed. In this paper, we present results from new observations of CO( J = 1–0) and ¹² CO( J = 3–2) obtained with the Nobeyama Radio Observatory 45 m telescope and the Atacama Submillimeter Telescope Experiment. We find robust evidence that the 0 km s ⁻¹ component is associated with the DHN and is located in the Galactic center. In contrast, the −35 km s ⁻¹ component exhibits line intensity ratios typically seen in foreground molecular clouds. The favored molecular counterpart is resolved into 11 dense, elongated clumps whose typical sizes, line widths, and column densities are 1–2 pc, ∼2 km s ⁻¹ , and 10 ²¹ –10 ²² cm ⁻² , respectively. Within the framework of the magnetic tower model, we discuss how these clumps are formed through interactions between dense H I blobs and associated bow shocks.

We report the first identification of molecular clumps directly associated with the rebrightening regions of the large-scale X-ray jets of SS 433, based on ¹² CO ( J = 1–0) observations with the Nobeyama 45 m Radio Telescope. Multiple clumps are detected toward the eastern and western jet heads, showing clear spatial correlation with the X-ray emission. The X-ray emission peaks immediately downstream of the molecular clumps, while the hardness ratio is enhanced at their surfaces, indicating that the observed structures cannot be explained by absorption effects. These results provide direct evidence for shock–cloud interactions between the jets and the surrounding interstellar medium. We suggest that turbulence generated at the jet–cloud interface amplifies magnetic fields, producing the observed nonthermal X-ray emission. Our findings highlight the importance of jet–interstellar medium interactions in shaping the X-ray properties of microquasar jets.

The detection of gravitational waves (GWs) from a binary neutron star (BNS) merger by Advanced LIGO and Advanced Virgo (GW170817), together with its electromagnetic counterpart, the short gamma-ray burst GRB 170817A, heralded the birth of multimessenger astronomy. The detection of TeV emission from GRBs motivates follow-up observations with the Cherenkov Telescope Array Observatory (CTAO), which is ideal for detecting such signals due to its unprecedented sensitivity, rapid response, and wide-field survey capabilities. The aim of this work is to evaluate GeV–TeV GW follow-up strategies for CTAO using a multistep simulation pipeline and to estimate the expected rate of joint GW–GRB detections during observing run O5. Using a simulated sample of BNS systems with corresponding GW detections, gamma-ray emission is simulated through phenomenological prescriptions based on the observed population of short GRBs, including off-axis jet scenarios. CTAO observations are simulated to account for instrument response, sky tiling strategies, integration times, and varying observing conditions. Strategies with variable and constant integration times are investigated. We find that, via an optimized follow-up strategy, about 5% of simulated GW-associated short GRBs produce GeV–TeV radiation detectable by CTAO. Detectability is strongly influenced by the jet opening angle and viewing angle, suggesting that even rough estimates of the viewing angle in GW alerts could enhance targeting. This framework motivates future follow-ups of GW-detectable events, including neutron star–black hole mergers, and further supports the development of advanced strategies incorporating galaxy distributions and synergies with future detectors such as the Einstein Telescope.

We report CO and H i studies of molecular and atomic gas toward the TeV gamma-ray source HESS J1646−458, widely considered to be associated with the young massive cluster Westerlund 1 (Wd1). We found that molecular clouds at V _LSR ∼ −32 km s ⁻¹ coincide with the arc-like structures seen at 8 μ m, likely illuminated by strong far-UV radiation from Wd1. ¹² CO( J = 3–2) emission at the same velocity reveals a cavity-like structure with an expansion velocity of ∼5 km s ⁻¹ toward the central region of Wd1, suggesting a recently formed wind-blown bubble driven by the cluster. We also identify a complementary spatial distribution between the V _LSR ∼ −55 and ∼−32 km s ⁻¹ clouds, connected by an intermediate-velocity component at V _LSR ∼ −44 km s ⁻¹ . These characteristics are consistent with signatures of triggered star formation through a cloud–cloud collision and imply that both clouds are physically associated with Wd1. On larger scales, the total interstellar proton column density at V _LSR ∼ −36 to −23 km s ⁻¹ shows a moderate spatial correspondence with the TeV gamma-ray shell. Together with this correlation, a substantial gas mass of ∼1.6 × 10 ⁶ M _⊙ , and the absence of bright synchrotron X-rays, the TeV gamma-ray emission surrounding Wd1 is consistent with a hadronic origin. The present finding allows us to calculate the total energy of accelerated cosmic-ray protons to be ∼6 × 10 ⁴⁹ erg.

The Large Magellanic Cloud (LMC) exhibits vigorous high-mass star formation, including the H ii regions of 30 Dor, which is the most active site of star formation in the Local Group. This paper focuses on the Giant Molecular Cloud (GMC) in the H ii region N113 in the central part of the LMC. Based on the ¹² CO( J = 2–1) and ¹³ CO( J = 2–1) data at a resolution of ~0.2 pc obtained by the Atacama Millimeter/submillimeter Array and the Atacama Pathfinder Experiment, we reveal that the GMC consists of two filamentary structures, each ~10 pc in length, forming a V-shaped pattern with a vertex angle of 90°. The filamentary structures host high-mass young stellar objects in gravitationally bound dense gas. Large-scale H i gas data covering 100 pc reveal two distinct velocity components separated by more than 40 km s ⁻¹ that correspond to the low-velocity (L-) and disk (D-) H i components of the LMC. The L-component appears to be located in a cavity-like distribution of the D-component, and the CO filaments are positioned at the cavity’s edge. We find evidence for the L-component to fit the cavity by a 53 pc displacement, and suggest that collisional compression of the H i gas during the last 1.3 Myr triggered the GMC formation and the high-mass star formation. This lends support to the idea that the large-scale collision driven by the tidal interaction is playing a role in the evolution of the interstellar medium in N113.

Infall and outflows, coupled with magnetic fields, rapidly structure the gas around newborn protostars. Shocks from interacting components encode the temperature and density distribution, offering a direct probe of the earliest evolution history. However, interferometric observations characterizing warm envelopes using high-excitation lines remain scarce. We present Atacama Large Millimeter/submillimeter Array Band 9 observations of the Taurus dense core MC 27/L1521F, which hosts a Class 0 protostar, targeting the CO( J = 6–5) line at an angular resolution of ∼2″ (≈300 au). We detect an off-centered ringlike structure with a diameter of ∼1000 au that was not identifiable in previous low- J CO data, where emission close to the systemic velocity is strongly affected by optical depth. The ring shows a typical peak brightness temperature of ∼3 K at our resolution. Excitation considerations indicate that the detected CO( J = 6–5) emission likely arises from relatively warm ( T ≳ 20 K) and dense ( n (H ₂ ) ≳ 10 ⁵ cm ⁻³ ) gas embedded within the surrounding cold, dense core. The morphology and kinematics suggest an energetic and localized shock-heating event, potentially linked to dynamical gas–magnetic-field interactions in the earliest protostellar phase. Our results demonstrate that high- J CO observations provide a powerful new window on warm and dense gas components, enabling a more direct view of the physical processes operating at the onset of star formation.

We present a study of the interstellar medium associated with the two middle-aged supernova remnants (SNRs) W41 and G22.7–0.2, both detected in TeV gamma rays. Using high-angular-resolution ¹² CO( J = 1–0) data from the Nobeyama 45 m telescope and H i data from the Very Large Array, we investigated the spatial and kinematic properties of molecular and atomic gas that interact with the SNRs. We identified associated clouds in the velocity ranges of +50 to +80 km s ⁻¹ for W41 and +76 to +110 km s ⁻¹ for G22.7–0.2. Column density analysis indicates that target protons are dominated by molecular hydrogen, while atomic hydrogen contributes less than ∼10%–15% even after correction for self-absorption. The mean proton densities are ∼1.2 × 10 ³ cm ⁻³ for W41 and ∼5.3 × 10 ² cm ⁻³ for G22.7–0.2. From the gamma-ray luminosities, we estimate the total energy of accelerated cosmic-ray protons as W _p ∼ 3 × 10 ⁴⁷ erg for W41 and ∼1 × 10 ⁴⁸ erg for G22.7–0.2, corresponding to 0.03%–0.1% of the canonical supernova explosion energy. These W _p values agree with the decreasing trend in W _p observed in the middle-aged SNRs within the previously reported SNR age– W _p relation.