UchUchU

Research Trends

Latest preprints in space engineering and astrophysics.

astro-ph.SR

Current Sheet Fragmentation in the Course of Repeated Eruptive Magnetic Flux Emergence

Vaggelis Karantanis, Loukas Vlahos, Heinz Isliker, Angelos Giannis, Vasilis Archontis

Magnetic reconnection between emerging magnetic flux and the ambient coronal field fundamentally drives solar jets. We use three-dimensional resistive-MHD simulations of eruptive flux emergence to investigate how the large-scale current sheet formed at the interface fragments during recurrent jet activity. The simulations show that jet formation and current-sheet disruption are coupled elements of a single multiscale process. The current structures developing along open field lines and within th…

Read →
astro-ph.SR

Massive stars in the SDSS-V survey: New O2 stars in the Large Magellanic Cloud

A. Roman-Lopes, J. G. Fernández-Trincado, Marina Kounkel, A. A. C. Sander, Carlos G. Román-Zúñiga, A. Z. Lugo-Aranda, Sergio Sánchez-Sanjuán, Abhinna Sundar Samantaray et al.

{O2 stars define the hottest end of the normal O-star sequence, but their identification requires blue-optical nitrogen diagnostics absent from many surveys.} {We reassess three luminous Large Magellanic Cloud (LMC) sources with multi-epoch Sloan Digital Sky Survey V (SDSS-V) spectroscopy obtained with the Baryon Oscillation Spectroscopic Survey (BOSS) spectrographs: \wsi, \lh, and \sk.} {We measure visit-resolved radial velocities and H, He, and N equivalent widths (EWs), compare robust combina…

Read →
astro-ph.GA

PEGASUS (Pfs Emission-line GAlaxy SUrvey with Subaru): I. Three-dimensional large-scale structures at $z > 1$ probed with emission-line galaxies

Mariko Kubo, Tadayuki Kodama, Rhythm Shimakawa, Yusei Koyama, Masato Onodera, Akio K. Inoue, Hisakazu Uchiyama, Tohru Nagao et al.

To comprehensively understand the environmental effects on galaxy evolution, it is crucial to precisely map the distribution and thoroughly examine the properties of galaxies, including low-mass building blocks. Deep narrow-band imaging observations conducted with Hyper Suprime-Cam (HSC) on the Subaru Telescope systematically probed the distribution of low-mass star-forming galaxies as H$α$, [O {\footnotesize II}], and [O {\footnotesize III}] emission-line galaxies at $0.4\leq z\leq1.6$. We perf…

Read →
astro-ph.GAastro-ph.CO

CHASE Spectral Survey I: Measuring Contamination in the Color-selected J-region Asymptotic Giant Branch Distance Indicator

Siyang Li, Alexei V. Filippenko, Arief Ahmad, Adam G. Riess, Stefano Casertano, Thomas G. Brink, Richard Anderson, Caroline D. Huang et al.

The J-region asymptotic giant branch (JAGB) method for distance measurements uses the near uniform luminosity of carbon-rich AGB stars as a standard candle. Its application rests on an assumed but rarely examined premise: the stars contained in the JAGB color-magnitude selection box are primarily carbon-rich AGB stars. We investigate the purity of the photometric selection in the Milky Way with an optical spectroscopic survey (~3590-7150 $\mathring{A}$) called CHASE (Carbon stars, Hubble tension…

Read →
gr-qcastro-ph.HE

A Systematic Study of Resonance-Driven Flux Modifications in Extreme-Mass-Ratio Inspirals

Edoardo Levati, Lennox S. Keeble, Alejandro Cárdenas-Avendaño

Transient orbital resonances can introduce phase-dependent corrections to the evolution of extreme-mass-ratio inspirals (EMRIs), potentially altering their long-term dynamics and emitted gravitational-wave signals. In this work, we quantify the resonance-induced modifications to the energy, axial angular momentum, and Carter constant fluxes and compute the corresponding resonance coefficients across a broad region of the orbital parameter space. Using the publicly available \texttt{pybhpt} code,…

Read →
astro-ph.GA

SALVAGE II: no systematic molecular gas depletion in the central 2 kpc of AGN hosts

Scott Wilkinson, Chiara Circosta, Toby Brown, Sara L. Ellison, Shoshannah Byrne-Mamahit, Almudena Alonso-Herrero, Santiago Garcia-Burillo, Federico Esposito et al.

In cosmological simulations, active galactic nuclei (AGN) feedback is found to be necessary for the shutdown of galaxy-wide star-formation by heating and/or removing molecular gas. However, at low-redshift, there is little evidence of systematic large-scale gas depletion. Some recent observations have found molecular gas depletion in AGN hosts, but limited to the central regions of galaxies. In this work, we use the SDSS-ALMA Legacy-Value Archival Gas Exploration (SALVAGE) dataset, which provide…

Read →
eess.SPphysics.space-phstat.AP

Beyond Legacy L1 C/A: Characterizing Advanced Multi-Frequency Spoofing Against Multi-Frequency GPS Receivers for Autonomous Navigation

Minhaj Uddin Ahmad, Muhammad Sami Irfan, Sagar Dasgupta, Mizanur Rahman, Thejesh N. Bandi

Autonomous vehicles increasingly rely on high-end multi-frequency GNSS receivers that track modernized civil signals, such as L2C, L5, and L1C, alongside legacy GPS L1~C/A. Although multi-frequency operation improves robustness against natural error sources, its resilience against synchronous multi-band spoofing attacks remains insufficiently characterized. This paper presents a method for quantifying the inter-band alignment accuracy required for a coherent spoofer to evade cross-frequency cons…

Read →
astro-ph.GA

Calcium Triplet Absorption is Common around Little Red Dots

Jenny E. Greene, Hanpu Liu, Raphael E. Hviding, Rohan P. Naidu, David J. Setton, Bingjie Wang, Ivo Labbe, Vasily Kokorev et al.

We present new evidence for an optically thick atmosphere surrounding Little Red Dots (LRDs) in the form of Ca absorption at rest-frame 8500A, the calcium triplet (CaT). Building on the detection of CaT absorption in one local LRD analog (the "Egg", Lin et. al. 2025), we investigate the region around rest-frame 8500A for an archival sample of 17 LRDs ($2 4500$ K. Alternatively, the distribution can be matched at a low photospheric gas density $ρ_{\rm ph}<10^{-11}{\rm~g~cm^{-3}}$ that requires a…

Read →
astro-ph.IMastro-ph.HE

Convolutional Neural Network for Extraction of $n = 1$ Photon Ring of Black Holes

Courtney Duong, Frank Myhre, Joseph R. Farah

Very-long-baseline interferometry (VLBI) may enable direct imaging of fine, event-horizon scale black hole structures such as the photon ring. In particular, the $n=1$ photon subring encodes information about black hole spin in its radial profile and azimuthal brightness modulation, making it a key proxy for spacetime properties. However, the $n=1$ subring overlaps with emission from the $n=0$ subring in observations, requiring isolation of the $n=1$ subring for measurements. We present a convol…

Read →
astro-ph.SR

Asteroseismology and interferometry of the F7V spectroscopic binary $χ$ Draconis A in the TESS CVZ

Jonatan Rudrasingam, Mikkel N. Lund, Frank Grundahl, Arnab Chowhan, Timothy R. Bedding, Poul E. Nissen, Aldo Serenelli, Joel M. J. Ong et al.

We present a detailed analysis of the asteroseismic main-sequence benchmark star $χ$ Dra A in the TESS northern CVZ. We aim to derive stellar mass and radius from asteroseismic modelling of individual mode frequencies and test the asteroseismic mass and radius against an independent dynamical mass and interferometric radius measurement. We determined the dynamical mass of $χ$ Dra using 618 radial velocity measurements obtained with the SONG telescope at Tenerife, and 53 relative astrometric meas…

Read →
astro-ph.HE

Multi-scale variability in the optical afterglow of GRB 251013C from high-cadence LAST observations

R. Konno, E. O. Ofek, S. Garrappa, O. Teboul, E. Waxman, S. Ben-Ami, D. Kovaleva, A. Krassilchtchikov et al.

Context. Optical afterglows of gamma-ray bursts (GRBs) are often described by smooth power-law decays, but deviations such as flares and rebrightenings provide important constraints on the underlying emission processes. Aims. We investigate the temporal variability of the optical afterglow of GRB 251013C using high-cadence observations, and assess the origin of the observed variability. Methods. We analyze continuous optical observations obtained with the Large Array Survey Telescope (LAST), com…

Read →
astro-ph.EPastro-ph.IM

A Very Fast Symplectic Integrator for Planetary Systems: WHFast512 in x86 Assembly

Rishit Dagli, Hanno Rein

Long-term integrations of planetary systems are used in a variety of astrophysical applications, for example to determine a system's stability. In this paper, we describe significant speed and accuracy improvements to the WHFast512 integrator. By completely rewriting WHFast512 in x86 assembly, we gain fine-grained control over CPU instructions and are able to keep the entire simulation state in CPU registers, only writing to memory when an output is required. We also reduce the number of branche…

Read →
astro-ph.SR

TIC 366240660 - A Dynamically Active Triple With a Very Short Outer Period of 30.5 Days

S. A. Rappaport, T. Borkovits, G. Torres, H. -L. Chen, P. Zasche, D. Vokrouhlický, B. Deme, M. Mašek et al.

Methods: We analyze TESS photometric data, radial velocity measurements, and eclipse timing variation curves for the triple star system, TIC 366240660, with a photodynamical code to infer the properties of all three stars and orbital motions. Results: With an outer orbital period of only 30.5 d, this is the second most compact triple reported in the literature. We have determined the masses and $T_{\rm eff}$ values of the three stars with an accuracy of about 1% and the radii to about 2%. The in…

Read →
astro-ph.IM

Emerging Technologies for Astrophysics Missions: Workshop Summary Report

Brendan P Crill, Nicholas Siegler, Dan Sirbu, Luc Peterson, Sanaz Vahidinia, Ryan Watkins

NASA's Astrophysics Division convened the Emerging Technologies for Astrophysics Missions (ETAM) Workshop in March 2025 at the Ames Research Center in Silicon Valley, California. The workshop's goal was to identify emerging technologies that could critically impact or enable future space missions-including concepts previously considered unachievable. Four key technology areas were examined: Astrophotonics, Artificial Intelligence and Machine Learning, Advanced Materials, and Quantum Sensors. App…

Read →
astro-ph.SRastro-ph.GAastro-ph.HEastro-ph.IM

Using Machine Learning to Model Stellar Collisions in our Galactic Center

Tristan C. Sand, Sanaea C. Rose, Elena González Prieto, James C. Lombardi,, Charles F. A. Gibson, Fulya Kıroğlu, Kyle Kremer, Christopher E. O'Connor et al.

Direct collisions in the inner pc of the Galactic center can alter the orbits and properties of stars. The outcome of a collision depends on a number of parameters, including the masses and ages of the stars, the impact parameter, and the initial relative velocity. We utilize the newly developed $\verb|collAIder|$, a machine learning tool developed to predict the outcomes of stellar collisions, to bridge 3D hydrodynamic simulations of stellar collisions with a dynamical model of the Galactic cen…

Read →
astro-ph.GA

A radial basis of massless potential-density pairs

E. V. Polyachenko, I. G. Shukhman

In the matrix method for linear perturbations of spherical stellar systems, the perturbed potential and density are expanded over a set of potential-density pairs. For radial perturbations, mass conservation makes the coefficient of $1/r$ in the perturbed potential vanish, so that the potential decays faster than $1/r$. Starting from the $\ell=0$ Hernquist--Ostriker family, we construct in closed form a set of potential-density pairs whose potentials decay as $r^{-p}$ with a prescribed $p\ge2$,…

Read →
astro-ph.GA

First Trigonometric Parallax Measurements with the KVN and VERA Array (KaVA)

Nobuyuki Sakai, Daisuke Sakai, Noriyuki Kawaguchi, Shuangjing Xu, Bo Zhang, Taehyun Jung, Chungsik Oh, Jeong-Sook Kim et al.

To demonstrate the astrometric capability of the combined Korean VLBI (Very Long Baseline Interferometry) Network (KVN) and VLBI Exploration of Radio Astrometry (VERA) Array (KaVA), we conducted six-epoch VLBI observations of 22-GHz H$_{2}$O masers associated with the star-forming region W3(OH). Two atmospheric calibration methods, (1) GPS and (2) JMA (Japan Meteorological Agency) mesoscale analysis data, and two phase-reference sources were independently applied to the astrometric analysis. Tri…

Read →
astro-ph.EP

Inside the Eirenesphere: The Interplay of Porosity, Heat Flux and Mineralogy in Exoplanetary Aquable and Habitable Layers

Santiago A. Orjuela, Jorge I. Zuluaga

The classical circumstellar habitable zone restricts the search for life to planetary surfaces where stellar irradiation sustains liquid water, overlooking vast subsurface environments. Here, we present a geophysical model integrating internal radial structure, mineralogy, radiogenic heat, and pressure-dependent porosity to quantify the three-dimensional habitable volume-the eirenesphere-of rocky exoplanets. We distinguish between aquability (thermodynamic water stability) and habitability, whic…

Read →
astro-ph.HE

A Magnetic Unification of Merging Supermassive Black Hole Binaries

Mark J. Avara, David O'Neill, Zoltán Haiman

The detection of an electromagnetic (EM) transient from a supermassive black hole binary (SMBHB) merging within a gaseous environment will constitute a multi-messenger milestone. Numerical simulations of these systems have mostly been performed in 2D with a constant viscosity parameter representing turbulent and magnetic stresses. These simulations predict a steep drop in accretion rate and X-ray luminosity at merger, followed by a slow recovery. However, 3D-GRMHD and 3D-MHD simulations have sho…

Read →
astro-ph.GA

Giant Low-Surface Brightness Galaxies in the Global Picture of Galaxy Formation and Evolution

Fedor M. Kolganov, Igor V. Chilingarian, Anna S. Saburova, Damir Gasymov, Evgenii V. Rubtsov, Vladislav I. Klochkov, Kirill A. Grishin, Mariia Demianenko et al.

Giant low-surface brightness galaxies (gLSBGs) challenge standard merger-driven galaxy assembly models due to their large, dynamically cold disks, with radii often larger than 100~kpc and dynamical masses exceeding ${\sim}10^{12}$~\Ms. We identify $60$ gLSBGs, including a volume-complete subsample of $35$ galaxies out to $z=0.1$ in the 120~sq.deg. area. Using new and archival photometric and spectroscopic data, we homogeneously analyze their structural properties and derive central velocity disp…

Read →
gr-qcastro-ph.HEhep-thphysics.plasm-ph

Reconnection of Gravitational Fields

Luca Comisso, Felipe A. Asenjo

In a tetrad formulation of general relativity, an ideal Ohm-type condition for the gravitational field implies the conservation of gravitational fluxes and the preservation of the connectivity of gravitational field sheets. We investigate the consequences of departures from ideal gravitational evolution by developing a theory of gravitational reconnection, defined as a change in the connectivity of gravitational field sheets arising from the breakdown of the ideal frozen-in evolution of the grav…

Read →
gr-qcastro-ph.HEhep-thphysics.plasm-ph

Frozen-In Gravitational Fields

Felipe A. Asenjo, Maricarmen A. Winkler, Luca Comisso

Spacetime can undergo complex nonlinear evolution, as governed by the Einstein field equations, and a central challenge is to understand the geometric structures that arise, persist, and interact throughout its evolution. Using a formulation of Einstein equations that parallels nonlinear electrodynamics of continuous media, we show that general relativity admits gravitational field connections---two-surfaces and associated field lines whose connectivity is maintained by the spacetime dynamics. T…

Read →
astro-ph.SRastro-ph.GA

The oldest low-$α$ thin disc stars

Riano E. Giribaldi, Laura Magrini, Marco Palla, Carlos Viscasillas Vázquez

The formation pathway of the $α$-poor Milky Way thin disc remains under debate. The latest observational data indicates the presence of comparable numbers of old stars in both the low- and high-$α$ disc components, suggesting either coeval or overlapping formation epochs. Using GALAH DR4, we provide evidence that the $α$-poor stars coexist with $α$-rich stars up to $\sim$12.5 Gyr. Stellar ages of 0.5 Gyr precision were re-derived for nearly $30\,000$ subgiant stars using accurate effective tempe…

Read →
astro-ph.EPastro-ph.IM

Probing Exoplanetary Rings with Asterodensity Profiling: A PhotoRing Analysis of Kepler-51

Jorge I. Zuluaga, Sebastian Numpaque, David M. Kipping, Jaime A. Alvarado-Montes

The Kepler-51 system hosts transiting super-puff planets whose extremely low bulk densities challenge structural models, possibly because unresolved rings inflate their inferred radii. We test this hypothesis by combining asterodensity profiling with TTV-aware transit fits of Kepler long-cadence photometry. For Kepler-51b we measure a density ratio $Ψ\equivρ_{\star,\mathrm{obs}}/ρ_{\star,\mathrm{true}} = 1.18\pm0.06$, a $2.9σ$ offset from $Ψ=1$; for Kepler-51d we find $Ψ=1.04\pm0.08$ ($0.6σ$), c…

Read →
astro-ph.HEastro-ph.SR

Early NIR echo and the time variable mass loss history of Type IIn SN 2024kgi

Naveen Dukiya, Kuntal Misra, Andrea Pastorello, Andrea Reguitti, Anjasha Gangopadhyay, Aravind P. Ravi, Brian Hsu, Noah Franz et al.

We present a comprehensive analysis of the long-term photometric and spectroscopic monitoring campaign of the Type IIn Supernova (SN) 2024kgi. The SN reaches at peak an $r$-band absolute magnitude of $-19.81 \pm 0.06$ mag and a bolometric luminosity of $\sim 2 \times 10^{43}$ erg s$^{-1}$. We observe a break in the lightcurve at day $\sim 312$, after which the luminosity decline changes from $t^{-1.0}$ to $t^{-4.5}$, likely due to the shock sweeping the dense CSM. We introduce a semi-analytical…

Read →
gr-qcastro-ph.COastro-ph.HE

Gravitational wave signatures of subsolar neutron star formation in collapsar disks

Vishal Baibhav, Valerio De Luca, Loris Del Grosso, Emanuele Berti, Brian D. Metzger, Lam Hui

Standard models for stellar core collapse predict the birth of neutron stars with masses exceeding a solar mass. In the collapsar model, a rapidly rotating massive star collapses to a black hole surrounded by a dense, neutrino-cooled accretion disk, whose outer regions may in some cases fragment into neutron-rich clumps that collapse to form subsolar-mass neutron stars. These disk-formed objects can then merge either with one another or, following possibly hierarchical evolution, with the centra…

Read →
astro-ph.GA

Gone with the wind? Potential gas inflow in the broad-line region of a QSO at $z=0.287$ traced by time-varying Balmer absorption

Xihan Ji, Dominic J. Walton, Francesco D'Eugenio, Ignas Juodžbalis, Vasily Belokurov, Andrew C. Fabian, Roberto Maiolino, Jiamu Huang et al.

We report spectroscopic observations of a remarkable QSO, J115724.80-000455.56 at $z=0.287$ from the Dark Energy Spectroscopic Instrument (DESI) survey. This QSO exhibits variability in its hydrogen Balmer absorption over a time interval of 10 months in the observed frame between 2021 and 2022. The observed Balmer absorption can be decomposed into two components with distinct kinematics: a non-varying, "stationary" component and a varying, "dynamic" component. Both absorbers have gas column dens…

Read →
astro-ph.GAastro-ph.CO

Dark matter haloes from dwarf to massive galaxies: no systematic inner-density tension with ΛCDM hydrodynamical simulations

Pavel E. Mancera Piña, Justin I. Read, Jorge Sarrato-Alós, Claudia Muni

Two of the most prominent small-scale challenges to the cold dark matter (CDM) paradigm are the cusp-core and diversity-of-rotation-curves problems. The former concerns the shallow inner DM density profiles inferred for many galaxies compared with the cusps predicted by collisionless CDM, while the latter concerns the wider range of inner DM densities and rotation curve shapes inferred observationally than hydrodynamical simulations traditionally reproduce. Robust observational constraints on DM…

Read →
astro-ph.GAastro-ph.CO

The KAGG survey: KCWI Analysis of Gas around Galaxies at z~3-5

Grecco A. Oyarzún, Marc Rafelski, J. Xavier Prochaska, Nissim Kanekar, Marcel Neeleman, Marie Wisz, Michele Fumagalli, Regina A. Jorgenson et al.

We present the KCWI Analysis of Gas around Galaxies (KAGG) survey of Ly$α$ emission around Damped Ly$α$ Absorbers (DLAs) at z~3-5 with the Keck Cosmic Web Imager (KCWI) integral-field spectrograph. The KAGG survey is characterized by the sample of 36 DLAs spanning a wide metallicity range, the wide search area for associated Ly$α$ emission (impact parameter b$~\lesssim 120~$kpc), and the high emission sensitivity, reaching Ly$α$ luminosities $\lesssim10^{42}$ erg s$^{-1}$. We identify 7 DLA Ly$α…

Read →
astro-ph.GAastro-ph.CO

Dark matter haloes from dwarf to massive galaxies: no systematic inner-density tension with ΛCDM hydrodynamical simulations

Pavel E. Mancera Piña, Justin I. Read, Jorge Sarrato-Alós, Claudia Muni

Two of the most prominent small-scale challenges to the cold dark matter (CDM) paradigm are the cusp-core and diversity-of-rotation-curves problems. The former concerns the shallow inner DM density profiles inferred for many galaxies compared with the cusps predicted by collisionless CDM, while the latter concerns the wider range of inner DM densities and rotation curve shapes inferred observationally than hydrodynamical simulations traditionally reproduce. Robust observational constraints on DM…

Read →