Shanghai Astronomical Observatory

Abstract We report a large misalignment between the continuous jet and the discrete ejecta in GRS 1915+105, detected in 2023 April with the East Asian VLBI network (EAVN). Two-sided ejecta are shown at 6.7 GHz images, and central continuous jets are resolved at 43 GHz by EAVN quasi-simultaneously. While the continuous jet was aligned with the long-standing jet position angle (PA) of ∼147 ∘ , the discrete ejecta appeared at a markedly different PA ∼188 ∘ , with the lowest intrinsic velocity ∼0.35 c ever reported. A similar misalignment of PA between discrete ejecta and continuous jet was independently detected in late September of 2023 during a consecutive flare event. The pronounced and recurrent angular deviations suggest a time-variable jet launching geometry, which, in conjunction with the observed X-ray obscuration, can be attributable to a warped accretion disk. Our result could offer new insight into the fundamental differences between continuous jet and discrete ejecta, and broadly provides a clue to understand the phenomena for transient black hole X-ray binaries and changing-look active galactic nuclei during the X-ray-obscured phase.

Abstract We investigate the potential birthplace of Galactic binary neutron star (BNS) systems through a kinematic analysis. Using high-precision astrometry from Gaia DR3, updated pulsar distances, and Monte Carlo sampling of astrometric errors, we integrate the past trajectories of 11 Galactic BNSs and 167 globular clusters plus 2967 open clusters, to search for past encounters. Our results suggest that BNS origin in globular clusters is unlikely, with low encounter probabilities (e.g., ≲0.5% for NGC 5139) and requiring excessive ejection velocities. Conversely, our analysis indicates that open clusters are a nonnegligible formation channel. Specifically, the double pulsar J0737−3039 shows a 13.9% (5.4%) probability of originating from the young cluster OC 0450 (Theia 58). Based on encounter proximity and time, we argue that Theia 58 is its more plausible birthplace. Our work provides kinematic evidence consistent with an open-cluster origin for a subset of field BNSs.

Abstract Previous numerical simulations have shown that cold clumps can form within hot accretion flows, offering insights into the detailed processes of the state transition in black hole X-ray binaries. However, the evolution of the cold clumps has not been investigated in detail yet. In this paper, we conduct hydrodynamic simulations to investigate the evolution of the cold clumps. In addition to the previous result that when the accretion rate is high enough, the cold clumps emerge within the hot accretion flow, we found that instead of directly moving toward to the black hole, the clumps move outward when they initially form. The reason should be the combination of viscous torque and the condensation of hot gas from larger radii, which leads to the slightly super-Keplerian angular momentum of the clumps. After reaching the equilibrium position, the clumps begin to fragment at the inner edge, with each fragment moving inward sequentially. Generally, the azimuthal movements of the clumps are quasi-Keplerian, being closer to the outer detached Keplerian cold disk rather than the surrounding sub-Keplerian hot accretion flow, which agrees well with the semianalytical results for the weak coupling case in Wang et al.

Abstract The structure of the lunar crust preserves key records of the Moon's origin and long‐term evolution. Using the latest high‐resolution gravity field from the Gravity Recovery and Interior Laboratory (GRAIL) mission together with Lunar Orbiter Laser Altimeter (LOLA) topography, we investigate the shallow crustal structure of the lunar farside. Applying the effective density spectrum technique with a two‐layer formulation, we constrain the densities of the upper and lower layers and the depth of their interface. Our results reveal a thin (average 4.7 km), low‐density (average 2,111 kg ) surficial layer, likely representing a highly porous megaregolith formed by basin ejecta and impact fragmentation, overlying a higher density unit (average 2,739 kg ) consistent with a less porous, anorthositic crust. From the inferred upper‐layer density, we estimate crustal porosity and find that the porosity decreases systematically with increasing interface depth, while exhibiting substantial lateral variability at a given depth. This variability is closely linked to differences in cumulative impact modification. Residual porosity below ∼2 km shows a statistically significant negative correlation with N(20), indicating that repeated small impacts have progressively compacted the shallow crust and reduced its pore space.

Abstract Chinese Space Station Telescope (CSST), which will begin its scientific operations around 2027, is going to survey the sky area of the median-to-high Galactic latitude and median-to-high ecliptic latitude. The high astrometric precision of the CSST Survey Camera for faint objects enables the detection of a number of giant planets and brown dwarfs around M dwarfs and brown dwarfs via differential astrometry in its optical survey. In this paper, we predict the number of giant planets and brown dwarfs around stars and brown dwarfs detectable with CSST astrometry. We generate synthetic samples of CSST stellar and substellar sources, and carry out companion injection-recovery simulations in the samples using different occurrence rates for FGK dwarfs, M dwarfs, and brown dwarfs. We calculate companion yields based on CSST astrometric precision. Our analysis reveals that over its 10 yr mission, the CSST Survey Camera could barely discover giant planets and low-mass BDs around FGK dwarfs, but is projected to detect 20−170 giant planets and low-mass brown dwarfs around M dwarfs within 300 pc, and 300−570 brown dwarf binaries within 600 pc. Therefore, CSST astrometry is likely to significantly increase the current sample of substellar companions around M dwarfs and brown dwarfs. This sample will deepen our understanding of planet formation and evolution around low-mass stars and brown dwarfs.

We present a multiwavelength study of the symbiotic recurrent nova T Coronae Borealis (T CrB), using the Swift Burst Alert Telescope (BAT)/ X-Ray Telescope (XRT) / UltraViolet Optical Telescope (UVOT) and American Association of Variable Stars Observers (AAVSO) observations from 2005 to 2025. Our analysis spans quiescent, high, and pre-eruption dip states. We find that brightening amplitudes increase toward shorter wavelengths in both the optical and UV bands, while the UV and X-ray fluxes are generally anticorrelated throughout all phases. During the 2023-2024 pre-eruption dip, soft and hard X-rays increased as optical and UV brightness declined, consistent with a transition from an optically thick to an optically thin boundary layer driven by a reduction in the accretion rate. We also report, for the first time, a second, lower-amplitude dip occurring between September 2024 and February 2025 following the primary 2023–2024 pre-eruption dip. The observed variability supports an accretion-variation scenario as a unifying explanation for both the high and dip states and may signal an imminent nova eruption.

Aims . Filamentary infrared dark clouds (IRDCs) are believed to represent the initial conditions for massive star and cluster formation. Methods . We investigated the IRDC G035.39-00.33 using SiO, H 13 CO + , CH 3 OH, and CS emission observed with ALMA at 3.5″ resolution (∼0.05 pc). The analysis of the SiO emission provides a record of shock activity within the cloud, offering insights into both the current level of star formation and the cloud’s formation mechanisms. Results . We identify several regions with broad SiO emission clearly associated with outflows, pinpointing the locations of ongoing star formation across the cloud. The ALMA images also reveal a series of spatially extended SiO emission spots with narrow line profiles aligned along an arc-like path that is also seen in CS and CH 3 OH emission. While the broad SiO emission is mainly associated with the main cloud filament, as seen in visual extinction, the narrow SiO arch is located at the edge of the cloud, far from the identified sites of star formation activity. The presence of these arc-like morphologies suggests that large-scale shocks may have compressed the gas in the surroundings of the G035.39-00.33 cloud, shaping its filamentary structure. By inspecting the large-scale radio continuum emission around G035.39-00.33, we find that this IRDC is part of a larger star-forming complex where the densest and coolest material appears at the interacting regions between a supernova remnant (SNR) and an expanding HII region. In particular, we hypothesise that this IRDC may be spatially coincident with the ionised expanding gas associated with the previously identified SNR G35.6-0.4. Conclusions . We suggest that collisions between giant molecular clouds and expanding gas flows from interacting SNRs and HII regions may be responsible for the observed arc-like structures. Such shock compressions could play an important role in the formation of IRDCs and in the potential triggering of star formation.

Unveiling the 3D shape of the Milky Way's dark-matter halo is critical to understanding its formation history. We created an innovative dynamical model that makes minimal assumptions about the internal dynamical structures and accommodates a highly flexible triaxial DM halo. By applying the method to 6D phase-space data of K-giant stars from LAMOST + Gaia, we robustly determined the 3D dark-matter distribution of the Milky Way out to approximately 50 kpc. We discovered a triaxial, nearly oblate dark-matter halo with q_ ̊m DM = Z/X= 0.92 = Y/X= 0.8 > p_ p_ ̊m DM on average within 50 kpc, where the Z axis is defined perpendicular to the stellar disc. The axes ratio q_ ̊m DM ̊m DM is strongly preferred; the long-intermediate axis plane of the dark-matter halo is unexpectedly vertical to the Galactic disc, yet aligned with the `plane of satellites'. This striking configuration suggests that the Galactic disc (and the inner halo) has flipped, likely torqued by minor mergers, from an original alignment with the outer dark-matter halo and satellite plane, as is supported by Milky Way analogues from Auriga and TNG50. By allowing q_ ̊m DM (r) and p_ ̊m DM (r) to vary with radii, we find tentative evidence that the dark-matter halo is twisted. This agrees alignment with the disc in the inner regions and transitions to a vertical orientation at r≳ 20 kpc, supporting the disc flip scenario prediction. Such disc reorientation is non-trivial, yet its physical mechanism is straightforward to comprehend and naturally originates a vertical satellite plane. Our findings offer a unified framework that links dark-matter halo orientation, satellite alignment, and disc evolution, reinforcing the internal consistency of the Milky Way in the Λ cold dark matter model.