High Energy Astrophysical Phenomena
Variable opacity and accretion-column regimes of magnetized neutron stars
Galina Lipunova and Pavel Abolmasov
Radiation-supported
accretion columns of magnetized
neutron stars can operate in several distinct regimes. As the
accretion rate increases, magnetospheric
accretion may proceed through hot-spot emission, efficiently cooling radiative shocks, and advective sinking columns. The structure of the
accretion flow is determined by the global parameters such as
magnetic field strength and mass
accretion rate, but is also affected by opacity variations in strong
magnetic fields. A decrease of opacity for
photons with energy below the cyclotron energy is able to substantially augment the classical critical
accretion rate separating efficiently cooling solutions from advective solutions. We propose an approach to determine the critical
accretion rate above which the Basko-Sunyaev sinking solution becomes unavoidable. We also use the Basko-Sunyaev sinking solution to estimate the
plasma temperature and compare the characteristic
photon energy with the local cyclotron energy. For a combination of moderate
magnetic fields and high
accretion rates, the characteristic
photon energy in advective columns typically exceeds the cyclotron energy. In this case, the opacity is not reduced by the
magnetic field, and tall
accretion columns are formed. Strong
magnetic-field systems may retain short, efficiently cooling
accretion columns over a wide range of super-Eddington
accretion rates. We identify a region in
accretion rate and
magnetic-field strength where both efficiently cooling and advective solutions may coexist. In this region, the column may switch or oscillate between the regimes and geometries. The typical time of such oscillations are close to the replenishment time of the column, or to the thermal time near its bottom. The quasi-periodic oscillations, observed in some super-Eddington objects in the 1-100 mHz frequency range, may be related to such relaxation cycles.
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High Energy Astrophysical Phenomena
Shock-free super-Eddington accretion onto magnetized neutron star
Galina Lipunova and Pavel Abolmasov
Strong
magnetic fields allow accreting
neutron stars to form magnetically controlled funnel flows. At sufficiently high
accretion rates, such flows become radiation-pressure-supported. In the conventional picture, the magnetospheric flow contains a nearly free-falling transonic outer region, a radiation-mediated shock, and a subsonic
accretion column. We investigate the regime in which the height of the column is expected to exceed the magnetospheric radius. In this case a self-consistent transonic solution with a standing shock is no longer possible, and the entire magnetospheric flow remains subsonic. We construct a semi-analytical hydrodynamical solution for a fully subsonic, radiation-supported magnetospheric flow in the low-Mach-number limit. The
magnetic field is treated in the force-free approximation, while the flow structure is determined by mass conservation, energy conservation, and the momentum equation along the field lines. We test the solution using one-dimensional time-dependent simulations with the code HACol. The shock-free solution is highly advective. Its enthalpy profile is close to the adiabatic limit, while the energy density is regulated by leakage of mass and heat through the sides of the
accretion channel. Most of the observable power is expected to be released by
plasma leaving the magnetospheric flow above the
neutron-
star surface. Such systems should be associated with super-Eddington
accretion discs and low pulse fractions due to visibility selection and scattering in the disc wind. We suggest that many non-pulsating ultraluminous X-ray sources containing
neutron stars may accrete in this shock-free regime.
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High Energy Astrophysical Phenomena
First X-ray detection of symbiotic nova CN Cha with eROSITA/SRG
Sara Saeedi, Jaroslav Merc, Manami Sasaki et al. (+2 more)
Aims. We report on the first X-ray detection of the symbiotic
star CN Cha with eROSITA (extended Roentgen Survey with an Imaging
Telescope Array) onboard Spektrum-Roentgen Gamma (Spektr-RG). Methods. We analyse the eROSITA observations from eRASS2 to eRASS5 individually, as well as the combined eRASS:5 data of all five surveys in order to investigate the X-ray spectral and timing properties of CN Cha throughout the eRASS. Results. Although CN Cha was not detected during the first eROSITA All-Sky Survey (eRASS1), it was detected for the first time in the second eRASS2. Our analysis shows that the X-ray emission of CN Cha gradually increased from eRASS2 to eRASS5. The X-ray spectrum of the source, with the main emission at <2.4 keV, allows us to classify it as a e̱ta}-type symbiotic
star. However, a more detailed analysis suggests that there is very soft component, which may originate either from thermonuclear burning on the white dwarf, or from the wind collisions. Additional X-ray observations are required to better understand the origin and emission mechanisms of the source.
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All authors Sara Saeedi, Jaroslav Merc, Manami Sasaki, Lorenzo Ducci, Michael Freyberg
Solar and Stellar Astrophysics
Nature of small-scale Ca II H brightenings associated with weak fields in a quiet Sun region
Xiang Li, H. N. Smitha, S. K. Solanki et al. (+3 more)
Context. Chromospheric emission observed in the Ca II H and K resonance lines is generally positively correlated with photospheric
magnetic fields. However, a fraction of strong Ca II H&K brightenings are found to occur over regions of low photospheric
magnetic flux density. Aims. We aim to understand the origin of these chromospheric brightenings associated with weak fields in quiet Sun regions. Methods. We use Ca II H observations taken by the SuFI along with the photospheric magnetograms recorded by the IMaX, instruments onboard the first flight of the balloon-borne observatory Sunrise. We also use the atmospheric physical parameters inverted from IMaX data by the SPINOR code. Results. 217 Ca II H brightenings associated with weak fields are identified. 111 of these are located at the peripheries of strong field regions, in the
magnetic canopies formed by the expansion of the kG flux tube. The other brightenings are associated with either weak unipolar
magnetic patches, with interactions between weak mixed-polarity
magnetic fields, or at locations with no clear
magnetic features. For the latter two situations, the Ca II H intensity varies with a period of 3-5 mins, accompanied by variations in the line-of-sight velocity, the temperature, and the
magnetic flux with a similar period. Conclusions. About half of the strong Ca II H brightenings associated with weak fields are located in
magnetic canopies overlying very weak photospheric fields. About 10-12% are associated with
magnetic bright points where the
magnetic field is probably not spatially resolved. The remainder appears to be Ca II H grains, which are overwhelmingly associated with
magnetic emergence or cancellation events. We conclude that although Ca II grains are mainly driven by (magneto-)acoustic shock waves, interactions between opposite-polarity
magnetic fields also contribute to the formation of the brightest peaks.
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All authors Xiang Li, H. N. Smitha, S. K. Solanki, T. L. Riethmüller, L. P. Chitta, S. Jafarzadeh
Astrophysics of Galaxies
Detection of metal absorption lines in quasar spectra: a neural network approach using U-Net
Elena Sofia Mangola, Francesco Pistis, Michele Fumagalli et al. (+7 more)
Current and future large spectroscopic surveys are significantly enhancing the volume and resolution of
quasar spectra that are observed, which requires the creation of efficient and precise automated techniques to detect absorption features. This study focuses on the detection of metal absorption features using a novel U-Net model on WEAVE-like mock spectra in the
quasar rest-frame wavelength interval $1230\,\mathring{\mathrm{A}} \leq \lambda_{\rm RF} \leq 3095\,\mathring{\mathrm{A}}$. We test the network performance for absorption detection both on ideal data and after simulating the continuum fitting step as applied on real data. The performance of these architectures is evaluated by the completeness, purity, and F1 score reached in bins of signal-to-noise ($\mathrm{S}/ \mathrm{N}_\mathrm{line} $) for the absorption lines and with the absolute fractional flux error for the continuum. The ability to recover the correct line centers is also studied. The U-Net reaches scores of $\approx 90\%$ for all metrics (completeness, purity, and F1 score) at $\mathrm{S}/ \mathrm{N}_\mathrm{line} \approx 4$. All false positive detections with $\mathrm{S}/ \mathrm{N}_\mathrm{line} \geq 5$ fall in the tails of the broad Ly$\alpha$ absorbers distribution of damped Ly$\alpha$ systems. The combination of continuum fitting and line detections has negligible effects on the detection performance at $\mathrm{S}/ \mathrm{N}_\mathrm{line} \geq 4$. Our proposed U-Net architecture offers a competitive tool for the analysis of absorption lines in current and upcoming large spectroscopic surveys and is well-suited to the identification of any absorption line feature.
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All authors Elena Sofia Mangola, Francesco Pistis, Michele Fumagalli, Matteo Fossati, Ting-Yun Cheng, Ryan J. Cooke, Rajeshwari Dutta, Ignasi Pérez-Ràfols, Matthew Pieri, Emanuel Gafton
Solar and Stellar Astrophysics
High-Resolution Modelling of Coronae and Winds in Solar-type Stars with Varying Rotation Rates II. Stellar Winds
Yue-Hong Chen, Julián D. Alvarado-Gómez, Xin Cheng et al. (+6 more)
Observations suggest a connection between steady-wind mass loss and coronal X-ray activity in low-mass main-sequence
stars. Interpreting this connection is challenging because the wind is controlled mainly by the large-scale open field, whereas X-ray emission traces heating in small-scale closed fields often unresolved in global wind models. Here we use Space Weather Modelling Framework-Alfvén-Wave Solar Model with global convective dynamo-generated
magnetic maps and solar magnetograms. We model Alfvén-wave-heated winds for four solar-type
stars plus the Sun, spanning rotation rates of $1.0$–$23.3$ times the solar rate, and
magnetic field strengths of $6.0$–$1200$ G. Our models show that faster rotation yields a more tightly wound spiral, a larger Alfvén surface, higher terminal wind speeds, and a harsher wind-pressure environment for orbiting
exoplanets, different from that of the present-day Sun. We estimate the mass- and angular-momentum-loss rates and find systematic differences from Zeeman-Doppler Imaging-based predictions. Building on the successful reproduction of X-ray coronae in our Paper~I, we obtain the first self-consistent activity–wind relation in a unified modelling framework: the mass-loss rate scales with the surface X-ray flux following a power law with an index of ${\sim}0.67$. We also re-examine
magnetic braking via open-flux magnetisation, finding that the effective Alfvénic lever arm depends on the magnetisation parameter with a power-law index of ${\sim}0.35$. Finally, we quantify the stellar wind pressure at the orbits of several super-Earths. Together with our Paper~I, the series of results shows the distinct roles of multi-scale
magnetic fields and provides physically grounded inputs for assessing habitable-zone space weather.
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All authors Yue-Hong Chen, Julián D. Alvarado-Gómez, Xin Cheng, Victor See, Yu Dai, Maarit J. Korpi-Lagg, Jörn Warnecke, Chen Xing, Mingde Ding
High Energy Astrophysical Phenomena
Akihiro Inoue, Yudai Suwa, Ryuichiro Akaho et al. (+3 more)
Young isolated
neutron stars exhibit a wide range of
magnetic field strengths. To understand the origin of this diversity, it is important to observationally constrain
magnetic fields during the proto-
neutron star (PNS) phase. We propose a novel diagnostic of PNS
magnetic fields using
neutrinos associated with
supernova fallback. By performing one-dimensional general-relativistic magnetohydrodynamic simulations of the
supernova fallback onto a magnetized PNS, we investigate how the energy spectra of the fallback
neutrinos depend on the PNS
magnetic field strength and the fallback
accretion rate. We find that stronger
magnetic fields and lower
accretion rates produce softer spectra because the fallback
neutrinos are emitted mainly near the magnetospheric radius, which becomes larger under these conditions. To quantify this spectral softening, we define a hardness ratio based on event rates above and below 30 MeV and introduce a hardness-event-rate diagram. Using this diagram, we identify the parameter region in which our diagnostic is applicable. Its applicability depends on the PNS
magnetic field strength, fallback
accretion rate, onset time of the fallback phase, and
neutrino mass ordering. We also investigate the dependence on the PNS radius and find that a larger radius produces softer spectra, indicating a degeneracy between the
magnetic field and PNS compactness. An independent constraint on the PNS compactness would therefore enable us to better constrain the
magnetic field. These results suggest that future multimessenger observations of galactic
supernovae can provide a new window into the NS diversity.
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All authors Akihiro Inoue, Yudai Suwa, Ryuichiro Akaho, Shinsuke Takasao, Kazumi Kashiyama, Hiroyuki R. Takahashi
Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc
Neural Network Guided Parameter Space Constraints for Gravitational Wave Searches from Binary Black Holes
Chetan Verma, Amit Reza, Gurudatt Gaur et al. (+2 more)
The detection of gravitational waves (GWs) from compact binary coalescences (CBCs) using matched filtering is computationally demanding because detector data must be correlated with many template waveforms spanning a high-dimensional intrinsic parameter space. In our previous work, we showed that a convolutional neural network (CNN) can classify noisy signals against pure noise and constrain the intrinsic parameter space of a true non-spinning binary black hole (BBH) signal, enabling a narrower matched-filter search region and reducing computational cost. Here, we extend this framework to aligned-
spin BBH systems and investigate how different parameter-space representations affect CNN-based patch identification. Using IMRPhenomD waveforms over the aligned-
spin BBH parameter space and Advanced LIGO design sensitivity, we show that a CNN trained solely on the aligned-
spin template bank achieves more than 99.8% signal-noise classification accuracy on independently generated uniformly sampled BBH signals, indicating that additional uniformly sampled training data are unnecessary. We partition the template bank into four approximately balanced patches using a Principal Component Analysis (PCA)-based
quantile scheme and evaluate five parameter-space representations for patch identification. The chirp mass-duration representation achieves the highest average accuracy (93.2%), followed by chirp mass (91.6%) and component masses (90.5%). The post-Newtonian coordinates tau_0-tau_3 and theta_0-theta_3-theta_3s yield substantially lower accuracies. These results show that the existing template bank is sufficient for training and high-accuracy signal detection, while the choice of parameter-space representation is critical for constraining the parameters of the true signal.
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All authors Chetan Verma, Amit Reza, Gurudatt Gaur, Dilip Krishnaswamy, Sarah Caudill
Astrophysics of Galaxies
The Neutral Hydrogen Gas Content of Star-forming and Quiescent S0 Galaxies
D. A. A. Lee, Karen L. Masters, David V. Stark et al. (+1 more)
Lenticular (or S0)
galaxies that remain actively
star-forming (SFS0s) are a unique transition population among
galaxies. We investigated the HI (21cm) neutral hydrogen content of a sample of lenticular
galaxies identified from within the Mapping Nearby
Galaxies at Apache Point Observatory (MaNGA) survey and which have been classified as
star-forming or quiescent (qS0s). We compared these populations to each other and to a sample of
star-forming spiral
galaxies in MaNGA. We found that the HI Mass fraction and HI Deficiency of the SFS0s were intermediate between the quiescent S0s and the
star-forming spirals, but most similar to the spirals. The HI depletion times of the quiescent S0's is very much longer than that of the SFS0 and spirals, which have roughly similar times. We also compared the molecular gas fractions of the three groups which shows that the lowest fraction of the three populations, followed by the
star-forming S0s and then the
star-forming spirals. A comparison of total baryonic mass comparisons indicates that the
star-forming S0s have significantly lower masses than the qS0s in the sample. This suggests that the
star-forming S0s must form from lower-mass progenitors (unobserved by MaNGA) which either accrete external gas, or loose spiral arms. The more massive quiescent S0s are a completely separate population, most likely to be formed either from fading
star-forming spirals or from medium- to high-mass spirals undergoing mergers. The SFS0s in this sample cannot form from the quiescent S0s in this sample that receive in-falling gas, nor can they fade to create these qS0s.
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All authors D. A. A. Lee, Karen L. Masters, David V. Stark, Z. Z. Abidin
Solar and Stellar Astrophysics · Cross-listed from cs.LG
PISCES: Physics-Informed Solar-wind Convolutional autoEncoder for Space-weather Anomaly Detection and Early Warning
Kevin Lee and Alison J. March
Space weather early warning depends on detecting solar wind transients in in-situ measurements at the first Sun-Earth Lagrange point (L1), before they reach Earth. Fixed thresholds can miss combined
magnetic and
plasma structure, and many learning methods provide a single anomaly score. We present the Physics-Informed Solar-wind Convolutional autoEncoder for Space-weather (PISCES), a convolutional autoencoder trained without catalog labels on OMNI solar wind measurements under physics constraints. Its loss includes
magnetic field consistency, an empirical relation between temperature and velocity, the Parker spiral angle, and penalties on changes between consecutive one-minute samples in derived quantities calculated from the reconstruction. At inference, PISCES separates the anomaly score into
magnetic and
plasma reconstruction errors, physics relations, and residual corrections, and reports the magnitude of each contribution. Attenuation of the skip connections, selected on validation data, improves average precision for the trained models, while the untrained scores remain nearly the same. The trained models also give a more consistent ordering of these physical contributions. After smoothing with a trailing median, the alarms can precede independently observed sudden commencements, including positive sudden impulses.
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High Energy Astrophysical Phenomena
Semi-Analytic Solutions For Warped, Super-Eddington Accretion Disks with Outflows
Gabriel Wohlfarth, Matthew Middleton, Omer Blaes et al. (+1 more)
The discovery of billion solar mass black holes at high
redshift challenges any standard Eddington-limited growth model. Previous work has shown that an advective
accretion disk can account for these early black holes with episodic
accretion. It has also been shown that if the
accretion disk of a black hole is misaligned from the
spin axis of the black hole, the disk can become advective within a certain radius. While warped and misaligned
accretion disks have been explored in numerical simulations, there is no fully or semi-analytic framework that self-consistently connects disk warp, mass inflow, winds, and energy transport in the super-Eddington regime. In this work, we develop a first-principles semi-analytic model of a radiation-pressure-dominated, warped
accretion disk, starting from the coupled conservation equations for mass, energy, and angular momentum. We find profiles for the radial velocity and surface density that explicitly account for disk warping and mass loss and determine the radius at which the disk becomes advective. We explore changes with black hole
spin, misalignment angle and viscosity, reproducing relevant results from numerical studies.
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All authors Gabriel Wohlfarth, Matthew Middleton, Omer Blaes, P. Chris Fragile
Solar and Stellar Astrophysics
Stellar flares with Ariel: detection prospects and impacts on exoplanet spectroscopy
K. Vida, B. Seli and B. Edwards
Stellar
flares are important tracers of
magnetic activity and can significantly influence the characterization of
exoplanet host
stars by introducing time-dependent astrophysical variability. ESA's Ariel mission, designed for atmospheric characterization of
exoplanets, will include the VISPhot instrument, providing high-cadence optical photometry offering an opportunity to study stellar
flares while identifying and mitigating their impact on transit spectroscopy. We discuss Ariel's potential for
flare studies, emphasizing the importance of short-cadence observations for resolving
flare morphology and estimating
flare energies. We identify potential flaring targets within the Ariel Mission Candidate Sample by cross-matching it with a TESS-based
flare catalog, identifying 44
stars in the sample of 3321 host
stars with
flare rates spanning 0.0012-1.05
flares per day yielding a lower limit. The majority of these active
stars are expected to
flare infrequently during typical Ariel observations, but for a few of the most active targets there is a significant chance of flaring during a typical Ariel observing window. We further investigate the detectability of stellar
flares in Ariel's wavelength range using synthetic spectra based on time-dependent RADYN
flare models propagated through the Ariel radiometric simulator. Our simulations indicate that strong
flares on cool dwarfs can produce measurable infrared signatures, with flux variations reaching several percent, substantially exceeding the amplitudes of typical
exoplanet atmospheric features. These results demonstrate stellar
flares represent both a valuable ancillary science case and a potential source of contamination for Ariel observations, highlighting the need for improved
flare models and mitigation strategies to maximize the mission's scientific return.
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Solar and Stellar Astrophysics
ALMA's sharpest view of IRAS 08544-4431: Unveiling the dust distribution asymmetries in the circumbinary disk
Maksym Mohorian, Devika Kamath, Elizabeth Cappellazzo et al. (+10 more)
The interactions between post-asymptotic giant branch (post-AGB) binaries and their circumbinary disks play a central role in shaping the
evolution of these systems. Circumbinary disks around post-AGB binaries are stable, though relatively short-lived structures ($10^4-10^5$ years) and their physical properties and
evolution remain poorly understood. In this study, we target IRAS 08544-4431, one of the best-studied post-AGB binaries. This system has well-constrained stellar and orbital parameters, exhibits well-defined characteristic refractory-depleted photospheric chemistry, and hosts a stable, sub-Keplerian circumbinary disk extending from a well-resolved sublimation rim at approximately 8 au to outer radii of approximately 1 000 au. We probe the disk layers close to the midplane using ALMA Band 7 continuum and $^{12}$CO $J=3-2$ observations of IRAS 08544-4431 obtained with baselines up to 16.2 km. These data provide the finest angular resolution yet achieved with ALMA for dust-continuum observations of post-AGB binaries with a (almost) face-on disk (15 au; 10 mas). The resulting map of dust continuum enabled resolving disk inner rim, a largely smooth radial profile, and a prominent azimuthal asymmetry at a radius of $\sim$45 mas. Notably, the location of this dust asymmetry coincides with the forward-scattering peak reported in SPHERE/VLT observations of IRAS 08544-4431. The $^{12}$CO emission is more extended than the dust continuum and confirms the disk rotation in this system. However, the global $^{12}$CO map shows indications of a spatial offset from the dust asymmetry, suggesting some degree of gas-dust decoupling. Together, these observations establish IRAS 08544-4431 as a key laboratory for studying disk and dust
evolution in a parameter space distinct from that of young
stars with protoplanetary disks.
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All authors Maksym Mohorian, Devika Kamath, Elizabeth Cappellazzo, Kateryna Andrych, Jacques Kluska, Hans van Winckel, Daniel Price, Toon De Prins, Arancha Castro-Carrizo, Javier Alcolea, Nicolas Cuello, Matthias Fabry, Valentin Bujarrabal
High Energy Astrophysical Phenomena · Cross-listed from physics.plasm-ph
Anisotropy and energy distribution of leptons and photons in radiative relativistic Alfvénic turbulence
Peng Liu, Xiaochun Wei and Zheng Gong
The anisotropic
evolution of relativistic particles in magnetically dominated Alfvénic
turbulence is a fundamental process governing nonthermal emissions in high-energy astrophysical environments. In this work, we develop a theoretical framework by deriving scaling equations that characterize the pitch-angle dynamics of leptons and
photons across both weak- and strong-radiative cooling regimes. Our model reveals a universal coupling between particle energy and angular distribution, arising from the interplay of field-aligned accelerations, perpendicular drift motions, and radiation reactions. The model can uniquely identify “turning-point energies" that exhibit a nonlinear dependence on the local
magnetic field and the energy-injection scale. The validity of this scaling model is confirmed through radiative particle-in-
cell simulations. We show that the electron pitch-angle distribution fundamentally dictates
photon emission characteristics, further revealing a pronounced statistical correlation between high-energy
photon emission and turbulent
vortices in the reconnection regions. By bridging the gap between small-scale kinetic interactions and broad-band spectral characteristics, this work provides a quantitative basis for interpreting the hardening of radiation spectra and complex polarization signatures in sources such as
pulsar wind nebulae and gamma-ray bursts.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph
Dark Transport Erases Baryon Inhomogeneity from Supercooled Phase Transitions
Sudhakantha Girmohanta, Kohei Kamada, Yuichiro Nakai et al. (+1 more)
Supercooled phase transitions can generate observable gravitational waves, but their bubble dynamics may also imprint large spatial inhomogeneities in the baryon asymmetry. For phase transitions below the TeV scale, ordinary baryon
diffusion can be too slow to erase such inhomogeneities before Big Bang Nucleosynthesis, potentially leading to stringent constraints from the observed light-element abundances. We show that this problem can be naturally avoided in darkogenesis scenarios, where the phase transition first generates an asymmetry in a dark-sector particle that is subsequently transferred to visible baryons. The dark-sector particle can travel over much larger distances than ordinary baryons before decaying, erasing the inhomogeneity before the baryon asymmetry is established. We derive the conditions for efficient erasure in both the diffusive and collisionless transport regimes and translate them into constraints on the dark-sector interactions. For a representative dark phase transition with a reheating temperature of $\mathcal{O}(1) \,\mathrm{GeV}$, $\beta/H \simeq 10$, and a dark-sector particle lifetime of $0.1\,\mathrm{s}$, we find that the inhomogeneity is efficiently erased for a viable range of the interactions. Importantly, baryogenesis before the phase transition is challenged both by dilution from strong supercooling and by the subsequent imprinting of spatial inhomogeneities. This points toward darkogenesis associated with the phase transition as a consistent framework, in which dark-sector transport erases the inhomogeneity before it is transferred to visible baryons.
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All authors Sudhakantha Girmohanta, Kohei Kamada, Yuichiro Nakai, Fumio Uchida
Earth and Planetary Astrophysics
Nordic-based observations of binary asteroid (163693) Atira reveal a hydrated CM chondrite composition
Eric M. MacLennan, Grigori Fedorets, Zuri Gray et al. (+5 more)
We present photometric, spectroscopic, and polarimetric observations of (163693) Atira collected on the night of 05 March 2024. A dual-band lightcurve acquired in the V and R_C filters using the Metsähovi Observatory (L08) in Kirkkonummi, Finland spans nearly 8~hours. From these measurements we estimate a V-R_C = 0.43 +/- 0.07 mag. A visible spectrum and linear polarisation measurement were obtained with the Nordic Optical
Telescope in La Palma, Spain. We employ a semi-empirical model to recover the spectral flux that was lost due to a mis-aligned spectroscopic slit. The corrected model reflectance spectrum indicates a primitive Cgh taxonomic type with a distinct absorption feature from 0.60 to 0.85 um with a depth of ~7.4 %. Direct comparison with laboratory spectra of meteorites indicates close matches with CM carbonaceous chondrites. We detect a large degree of linear polarisation at a phase angle of 83 degrees which is consistent with (3200) Phaethon and (155140) 2005 UD, but lower than (162173) Ryugu and (152679) 1998 KU2. We use a CM carbonaceous chondrite density to estimate its bulk porosity of 51 +/- 31 % from previously estimated bulk density. The uncertainty in bulk density and bulk porosity is dominated by the volume of the primary. Future near-infrared spectroscopic and thermal infrared observations can be used to refine the composition and bulk density. Finally, we discuss implications for ground-based physical characterisation studies of the Atira population.
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All authors Eric M. MacLennan, Grigori Fedorets, Zuri Gray, Teemu Willamo, Tuomas Salo, Anne Virkki, Mikael Granvik, Karri Muinonen
Instrumentation and Methods for Astrophysics
A ready-to-deploy MLOps software platform for satellite and NEO detection at meter-class ground-based observatories
Rafael Carrillo Navarro, Pablo García-Martín, René Duffard et al. (+2 more)
Ground-based astronomical observations frequently contain streaks produced by artificial satellites, space debris, and potentially Near-Earth Objects (NEOs). While machine-learning models can reliably detect these features, their practical adoption in observatory operations is often limited by the lack of integrated tools for visual inspection, validation, workflow management, and structured data storage. This paper presents the StreakMind Workbench, a framework that applies MLOps practices to bridge research-oriented machine-learning pipelines with routine observatory operations. Rather than introducing new detection
algorithms, the Workbench addresses a software-engineering challenge in astronomical computing: maintaining a single authoritative source of scientific processing code while providing astronomers with an operational environment for workflow execution and result inspection. Integrated with the reference StreakMind AI model of Carrillo et al. (2026) and implemented in Python using PyQt5, the Workbench supports the complete workflow from FITS ingestion to database storage, including inference, result inspection, database exploration, training management, and Minor
Planet Center formatted observations. Validation on 273 images from La Sagra Observatory demonstrates successful end-to-end workflows while maintaining consistency with the underlying StreakMind scientific code. The platform facilitates operational use of a research ML pipeline in meter-class observatories and moderate-scale campaigns, supporting Space Situational Awareness and planetary defence.
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All authors Rafael Carrillo Navarro, Pablo García-Martín, René Duffard, Juan Luis Orellana Montes de Oca, Oscar Ortega Ríos
High Energy Astrophysical Phenomena
Energy estimation of direct and reflected cosmic-ray events aboard balloon-borne radio detectors
Sergio Cabana-Freire, Jaime Alvarez-Muñiz and Matias Tueros
Balloon-borne
radio detectors offer a highly effective approach to monitoring immense volumes of the Earth's atmosphere and ice for ultra-high-energy
cosmic rays and
neutrinos. Because these payloads measure
radio emission from a single, highly elevated vantage point, reconstructing the air-shower energy using traditional methods is not possible. In this work, we present a comprehensive, simulation-based energy reconstruction framework tailored to the unique
cosmic-ray event geometries observed by balloon payloads. For the first time, we adapt and apply this methodology to direct atmosphere-skimming
cosmic-ray showers, accounting for their highly asymmetric development in rarefied air. Furthermore, we provide a major update to the energy reconstruction of reflected downward-going showers by incorporating spherical reflection and ray de-focusing mechanics directly into simulations. We rigorously validate the method across different event geometries, demonstrating a baseline energy resolution of $8-11\%$. Finally, we quantify the systematic effects of finite detector pointing resolution, unknown primary mass composition, and signal-to-noise ratio, establishing a robust framework for the analysis of
cosmic-ray events in next-generation balloon missions such as PUEO and POEMMA-Balloon with
Radio.
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High Energy Astrophysical Phenomena
A magnetar-powered jet-wind outflow in gamma-ray burst-associated supernovae: Application to the thermal components of GRB 060218-SN 2006aj
Yun-Peng Li and Da-Bin Lin
A rapidly rotating magnetar formed in the collapse of a massive
star may release its
spin-down power through both a collimated polar jet and a quasi-isotropic magnetar wind. With its long-lasting soft X-ray blackbody component and the double-peaked UV/optical light curves of SN 2006aj, GRB 060218-SN 2006aj provides a useful test case for such a two-component energy-injection scenario. We developed a magnetar-powered jet-wind model in which the two outflows interact with homologously expanding SN ejecta, driving forward shocks that produce shock-breakout emission and subsequent magnetar-powered thermal emission. We first used the jet component to fit the 0.3-10 keV blackbody flux and temperature
evolution of GRB 060218 and subsequently added the wind component to model the UV/optical light curves of SN 2006aj. We find that the soft X-ray blackbody component can be reproduced mainly by shock-breakout emission from the polar ejecta driven by the jet. The double-peaked UV/optical light curves are instead produced by the interaction between the quasi-isotropic magnetar wind and a more massive ejecta component. The first optical peak is dominated by wind-driven shock-breakout emission, whereas the broader and later emission is powered mainly by the subsequent magnetar-powered thermal component. Our results suggest a two-component magnetar-engine picture for GRB 060218-SN 2006aj, in which the X-ray thermal emission and the UV/optical double-peaked
evolution arise from different energy-injection channels of the same central engine.
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High Energy Astrophysical Phenomena
Multi-Band Optical Variability Characteristics of the $\gamma$-ray emitting blazar 1ES 0647+250 during its 2020 flare
K.Subbu Ulaganatha Pandian, C. S. Stalin, S. Muneer et al. (+4 more)
We present a multi-band optical variability study of the $\gamma$-ray emitting high-synchrotron-peaked blazar 1ES~0647+250 during its enhanced optical activity in 2020. The analysis utilizes 107 epochs of high-precision $B$, $V$, and $R$-band photometry obtained with the 75 cm
telescope at the Vainu Bappu Observatory, combined with high-cadence $z_g$ and $z_r$ survey data from the Zwicky Transient Facility. The combined light curves reveal coherent variability across all optical filters, with the source reaching a peak $R$-band magnitude of 15.74 mag. We calculated the variability amplitude, finding the highest intrinsic variability in the B- band. Spectral analysis using color–magnitude diagrams reveals a statistically significant “Bluer-When-Brighter” trend, supported by a Pearson correlation coefficient of $r = 0.68$ ($p < 0.001$) for the (V$-$R) index. Furthermore, the discrete correlation function analysis indicates a near-zero time lag between the $V$ and $R$ bands, suggesting simultaneous emission within the observational cadence. The observed spectral hardening is consistent with synchrotron-dominated optical emission, likely associated with enhanced particle acceleration within the jet during the active phase of 1ES~0647+250.
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All authors K.Subbu Ulaganatha Pandian, C. S. Stalin, S. Muneer, S. Umayal, Aditi Agarwal, Amit Kumar Mandal, B. Natarajan
Earth and Planetary Astrophysics
Probing Exoplanetary Chemistry with Ariel: Scientific Priorities and Observational Strategies
Olivia Venot, Yamila Miguel, Robin Baeyens et al. (+30 more)
Over the past two decades, increasingly precise observations have revealed that
exoplanet atmospheres are chemically diverse and often far from
equilibrium, with processes such as vertical mixing, photochemistry, and atmospheric circulation producing significant departures from thermochemical expectations. As large surveys across a wide range of
planets begin to uncover population-level chemical trends, a coherent interpretation of these patterns remains elusive, particularly for gas giants and Neptune-like
planets where disequilibrium processes likely dominate. The ESA Ariel mission will provide the first homogeneous, statistically significant atmospheric
dataset for nearly a thousand
exoplanets, offering an unprecedented opportunity to investigate the origins of chemical diversity across planetary populations. This white paper highlights central scientific questions for understanding atmospheric chemistry and recognized as priorities for the community. These questions span the mechanisms driving disequilibrium chemistry, the role of sulfur- and phosphorus-bearing species, the influence of stellar activity, the formation of haze precursors, and the chemical
evolution of atmospheres around
stars of different ages and types. Because these topics connect chemistry, physics, and planetary
evolution, they form the core focus of the Ariel Chemistry Working Group, which synthesizes current knowledge and identifies the diagnostics best addressed by Ariel's spectral capabilities. For each theme, we outline observational strategies and representative targets already included in the Mission Candidate Sample, illustrating Ariel's ability to address these questions. By linking large-scale observations to predictive atmospheric models, this work supports refinement of Ariel's target selection and enables population-level studies that will transform our understanding of planetary atmospheres.
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All authors Olivia Venot, Yamila Miguel, Robin Baeyens, Stefano Bellotti, Giuseppe Cassone, Quentin Changeat, Ryan Cloutier, Athena Coustenis, Dwaipayan Dubey, Billy Edwards, Kaustubh Hakim, Eric Hébrard, Christiane Helling, Helgi Rafn Hrodmarsson, Leoni Janssen, Adam Yassin Jaziri, Gaia Lacedelli, Panayotis Lavvas, Jorge Lillo-Box, Amy Louca, Adrien Masson, Zita Martins, Karan Molaverdikhani, Benjamín Montesinos, Harrison Nicholls, Enric Palle, Paul Rimmer, Donna Rodgers-Lee, Jonathan Tennyson, Shang-Min Tsai, Roméo Veillet, Sergey N. Yurchenko, Maria Zamyatina
High Energy Astrophysical Phenomena
Broad-band spectral-timing: simultaneous NICER and HXMT observations reveal an anticorrelation between the softest and hardest X-ray fluxes in MAXI J1820+070
Niek Bollemeijer, Phil Uttley and Bei You
Black hole X-ray binaries (BHXRBs) in the hard state show variable X-ray emission across a wide range of energies. Hard X-rays are thought to be produced through Compton-upscattering of seed
photons from the
accretion disc by the hot X-ray corona, leading to a spectral component roughly shaped like a power-law with a high-energy cut-off. A complete study of the coronal variability requires a full view of the spectrum, from the low-energy cut-off at the seed
photon energy to the high energy cut-off, which may be related to the coronal temperature. We present a detailed spectral-timing study of strictly simultaneous observations by NICER and Insight-HXMT of the bright BHXRB MAXI~J1820+070, covering the 0.3-250 keV energy range. The high data quality allows us to study the lags and coherence across the full X-ray band. We find that at low
Fourier frequencies ($\lesssim0.02$ Hz), the soft X-ray flux below 1 keV and hard X-ray flux above 100 keV are anticorrelated, i.e. phase lags of $|\pi|$ rad. No anticorrelation is observed when comparing energy bands above 1 keV. We consider several explanations for the observed complex behaviour, such as a modulation of the coronal cut-off energy by the disc, power-law pivoting and a contribution from non-thermal electrons. Future, more detailed modelling efforts based on the observed effects may place new constraints on the dynamics of the X-ray emitting regions around accreting black holes.
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Earth and Planetary Astrophysics
Juice-SWI during the Lunar-Earth-Gravity-Assist (LEGA) - Part 2: Instrument operations
T. Cavalié, R. Moreno, L. Rezac et al. (+17 more)
The Jupiter Icy Moons Explorer (Juice) embarked in 2023 on a 8-year interplanetary journey to Jupiter and its icy moons. The Submillimetre Wave Instrument (SWI) is one of the ten science instruments aboard the spacecraft. SWI is a sophisticated and first-of-its-kind payload visiting the outer solar system, featuring dual-band tunable receivers, two independent pointing mechanisms, and spectrometers capable of high resolution (up to a resolving power of 10^{7}). It is designed to support the diverse science objectives of the Juice mission targeting Jupiter's middle atmosphere, icy-moon's exospheres as well as near sub-surface thermophysical properties. For this purpose the Juice mission adopts a complex trajectory tour within the Jovian system, which further necessitates a sophisticated, mission-driven operations concept for SWI. This presents significant planning, operations and commanding challenges which are described in this paper in the context of the Lunar and Earth
Gravity Assist (LEGA). After the development and ground calibration of the instrument, the SWI Team has designed a comprehensive calibration strategy applicable during the Cruise Phase of Juice. Among the various opportunities for calibration, including the Near-Earth Commissioning Phase and more than ten Payload Checkout Windows, the LEGA offers the means not only to improve the calibration of the instrument, but also to validate the operational strategy of future icy moon flybys.
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All authors T. Cavalié, R. Moreno, L. Rezac, F. Herpin, C. Jarchow, P. Hartogh, A. Carrasco-Gallardo, S. Goodyear, P. Mancini, A. Schulz-Ravanbakhsh, B. Dabrowski, Y. Kasai, E. Lellouch, A. Murk, D. Murtagh, M. Olberg, M. Rengel, H. Sagawa, S. Szutowicz, E. Wirström
Earth and Planetary Astrophysics
Europa smooth plains formation: a SPH cryovolcanic model
Bastien Bodin, Daniel Cordier and Ashley Gerard Davies
Cryovolcanism is a fundamental geodynamical process that has been observed on several icy moons. Europa stands out as a primary target for investigating potential cryovolcanic activity and its role in shaping icy moon surfaces. This work characterizes the physical state of ascending cryomagma and evaluates the relevance of terrestrial analogues to explain Europa's surface features. After investigating cryomagma depressurization, we developed CryoPy, a numerical model based on Smoothed-Particle Hydrodynamics (SPH) for computational fluid dynamics and heat transfer. We simulated flow propagation and surface cooling using this model. Our simulations show that snow is a relevant physical analogue for cryolava, as the resulting flows closely match the smooth plains observed by Galileo in both extent and morphology. Additionally, we established the temporal thermal signatures of these events. Such active cryovolcanic events are within the detection limits of, primarily, the E-THEMIS infrared imager, providing a predictive framework for the Europa Clipper mission.
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Cosmology and Nongalactic Astrophysics
Dark Matter-Induced Stellar Oscillations in the de Broglie Regime
Qiuyue Liang and Jeremy Sakstein
We investigate stellar oscillations driven by the time-dependent gravitational potential of ultralight dark matter (ULDM) in the de Broglie regime. The response is controlled by the ratio of the ULDM coherence time to the stellar mode damping time. When the field decoheres faster than the damping time, ULDM acts as a
stochastic source that excites modes below a characteristic de Broglie frequency, while higher-frequency modes are suppressed. In the opposite, long-coherence regime, individual modes can be resonantly driven. Focusing on solar oscillations, we find a signal far below current sensitivity, and estimate that other
stars are similarly unlikely to provide observable signals.
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High Energy Astrophysical Phenomena
Relativistic accretion-disc models of Cyg X-1 in the soft state: a case study
Luciano Ye and Luigi Foschini
The thermal continuum of Cygnus X-1 in the soft state is often modelled with a multicolour disc in a Newtonian spacetime approximation. That choice is convenient for estimating the inner radius, but it neglects relativistic effects near the black hole; these are usually restored through colour-correction and boundary-condition factors. Using Suzaku data from one exemplary observation, we compare this Newtonian description with some fully relativistic disc models, keeping the same treatment of absorption, ionised winds, Comptonization and reflection. With the mass, distance and inclination fixed to the best current dynamical values, the Newtonian inner radius is consistent with a high-
spin Kerr innermost stable circular orbit. The relativistic models do not provide a uniformly better description of the spectrum: some worsen the fit or leave
spin, inclination and hardening unconstrained, while the formally best model requires an inclination much larger than the dynamical one or a smaller
spin. Apparent tensions with the models arise from data degeneracies and from the limited energy band, not from a failure of general relativity.
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Solar and Stellar Astrophysics
White Dwarf Pulsars: an emerging class of compact binaries
David A.H. Buckley
I summarise the expanding class of white dwarf
pulsars for which the prototype, AR Scorpii, was only discovered in 2016. At the time of this meeting, two further systems had been found (eRASSU J191213.9-441044 and SDSS J230641.47+244055.8). These are thought to be detached (or nearly detached) white-dwarf–red-dwarf binaries, where the white dwarf's
magnetic field interacts with the M-
star's corona, resulting in the capture and acceleration of electrons to relativistic velocities in the magnetosphere. This gives rise to non-thermal synchrotron emission across the electromagnetic spectrum. The first discovered, and brightest, system, AR Sco (which is the focus of this paper), shows a white dwarf
spin-down consistent with
magnetic dipole emission in a strongly
magnetic white dwarf. The polarisation properties of the two systems for which polarimetry has been obtained (AR Sco and eRASSU J191213.9-441044) show
spin-modulated polarisation, primarily linear, which remarkably reaches 40% in AR Sco. Furthermore, the Stokes Q and U parameters vary in the same manner as the optical polarisation of the Crab
pulsar, consistent with a rotating
magnetic dipole. These systems are intriguing in that each one seems to show different behaviours (e.g. pulse profiles and light-curve properties), presumably mainly due to different viewing aspect angles. It has recently been postulated that these systems represent a short-lived phase of binary
evolution linking asynchronously rotating intermediate polars and synchronous polars.
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Solar and Stellar Astrophysics
Tracing magnetic flux rope footpoints in AR 12975 with coronal dimmings and data-driven magnetofrictional simulations
Andreas Wagner, Amaia Razquin, Astrid M. Veronig et al. (+3 more)
Coronal mass ejections (CMEs) are driven by the eruption of
magnetic flux ropes (MFRs) whose footpoints are often observed indirectly as coronal dimming regions. However, the relationship between observed dimmings and true MFR footpoint locations remains uncertain due to limitations in both detection methods and coronal
magnetic field modelling caused by the complexity of eruptive events. We aim to investigate the M4
flare and CME event on 28 March 2022 in AR 12975 to determine how well observed coronal dimmings trace the footpoints of an erupting MFR. We also assess how these signatures compare with the MFR
evolution from a data-driven magnetofrictional simulation. We combined multiwavelength SDO/AIA observations with a time-dependent data-driven magnetofrictional simulation of the AR. We identified coronal dimmings using intensity-thresholding and minimum-intensity maps. For challenging regions we used a tailored region growth
algorithm from strong gradients. We extracted MFR structures in the simulation using a combined twist and squashing factor metric. We compared the location of the footpoints and derived
magnetic fluxes from both dimmings and the modelled MFR. The simulation reproduces the formation,
evolution, and eruption of the observed MFR whose footpoints migrate during the eruption. The footpoints match well with the dimming location. The southern dimming shows characteristics of a moving flux rope dimming, while the northern dimming behaves as a shrinking flux rope dimming that is rapidly closed by the
flare ribbons. Quantitative comparisons of area and
magnetic flux show good overall agreement between simulated footpoints and observed dimmings, with discrepancies mainly arising from detection limitations and event complexity. The dimmings at the northern footpoint region is particularly challenging in observations due to pre-existing low intensities and its fast shrinkage.
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All authors Andreas Wagner, Amaia Razquin, Astrid M. Veronig, Emilia K. J. Kilpua, Karin Dissauer, Jens Pomoell
High Energy Astrophysical Phenomena
PANGU I: A Parthenon-Based Framework for Fixed-Spacetime GRMHD
Yuehang Li, Fan Zhou, Mingyang Luan et al. (+2 more)
Existing general-relativistic magnetohydrodynamics (GRMHD) codes offer complementary capabilities, but these capabilities remain distributed across different frameworks. To unify them, we introduce PANGU–Parthenon-Based Astrophysics for Numerical Relativity and GRMHD in a Unified Framework–a modular computational framework that integrates a shared solver across physical regimes, flexible spacetime geometry, and extensible computational infrastructure within a single architecture. In the first paper of this series, we present and validate the fixed-spacetime GRMHD component of PANGU. The shared solver combines finite-volume Godunov methods with constrained transport on uniform and block-structured, statically refined meshes, and covers hydrodynamics and magnetohydrodynamics in Newtonian, special-relativistic, and general-relativistic regimes. PANGU provides a flexible geometry interface supporting coordinate selection and either on-the-fly or precomputed evaluation of prescribed stationary spacetimes, demonstrated here in Kerr spacetime with Cartesian Kerr-Schild and modified Kerr-Schild coordinates; additional components can be incorporated without restructuring the core solver. Verification covers all three regimes on uniform and statically refined meshes: smooth Newtonian and special-relativistic problems approach second order against their analytic solutions, with deliberately matched double-precision trajectories agreeing with AthenaK to roundoff; in fixed-spacetime GRMHD, the stationary analytic solutions are preserved to second order, and the turbulent
accretion reproduces the same flow and flux scales as BHAC. Together, these results establish the fixed-spacetime matter-sector foundation for the broader PANGU framework. Numerical-relativity and coupled dynamical-spacetime GRMHD capabilities will be presented in subsequent work.
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All authors Yuehang Li, Fan Zhou, Mingyang Luan, Minyong Guo, Bin Chen
Solar and Stellar Astrophysics
H_alpha observations of the Be/X-ray binary MWC 656
R. Zamanov, V. Marchev, K. A. Stoyanov et al. (+8 more)
Context. MWC 656 is a Be/X-ray binary identified as an emission-line object, X-ray and
radio source and might be a weak gamma-ray source, harbouring an enigmatic secondary component. Aims. Our aim here is to study the orbital modulation of the H-alpha emission line, disc truncation and to evaluate the binary parameters. Methods. During the period 2021-2026, we obtained high resolution (30000) optical spectra of MWC 656 and analysed them together with the published data. Results. We find an orbital modulation of the H_alpha emission line with period P orb = 60.43 +/- 0.03 d. By assuming that the Be circumstellar disc is truncated by the companion, we estimate orbital eccentricity e = 0.11 +/- 0.01, semimajor axis a=160 R_sun, and mass of the binary M_1 + M_2 = 15.1 M_sun. Conclusions. The orbital modulation of the H_alpha emission suggests that (1) in this object we have a new type of cyclic variability of a Be disc; (2) the orbit is mildly eccentric; and (3) the secondary component of MWC 656 is a massive object 5 M_sun, more massive than the
neutron stars - a stellar-mass black hole or a massive stripped
star.
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All authors R. Zamanov, V. Marchev, K. A. Stoyanov, M. D. Christova, M. Minev, M. Moyseev, D. Marchev, J. Marti, M. F. Bode, P. Markishki, S. Stefanov