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Physics Astrophysics

astro-ph Watch Astrophysics

Monday 1 June

Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

Thermal Metastable Strings in One-Scale Models and Gravitational Waves

Arturo de Giorgi, James Ingoldby, Valentin V. Khoze et al. (+1 more)

Metastable cosmic strings provide a cosmological interpretation of the nanohertz stochastic gravitational wave background reported by Pulsar Timing Array (PTA) experiments. We revisit this scenario in a minimal dark-sector gauge theory, in which a complex Higgs doublet breaks $\mathrm{SU}(2)\times\mathrm{U}(1)\to\mathrm{U}(1)_{\rm IR}$ at a single symmetry-breaking scale. This one-scale setup predicts metastable $Z$-strings whose endpoints are monopole-like defects, and whose zero-temperature decay rate is controlled by the gauge couplings and mass ratios. We show that, once the string-forming transition occurs in a thermal plasma, the dominant decay channel is not the zero-temperature monopole nucleation but thermally induced nucleation on the string worldsheet. We determine the nucleation temperature $T_{\rm nuc}$, which we \rev{adopt as the reference string-formation scale}, from the one-loop finite-temperature effective potential with daisy resummation, and use it to evaluate the worldsheet bounce action throughout the model parameter space. \rev{Requiring a PTA-compatible gravitational wave} signal selects a narrow region in the model parameter space, in the $(\sin^2\theta_w,\sqrt{\beta})$ plane, where $\theta_w$ is the dark-sector weak mixing angle and $\beta\equiv M_\Phi^2/M_{Z}^2$ is the squared Higgs-to-$Z$ mass ratio. Thermal effects modify the zero-temperature picture significantly, shifting the PTA-compatible region towards lower values of the dark fine-structure constant $\alpha'$ and larger values of the monopole-to-string-tension ratio $\kappa$.
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All authors Arturo de Giorgi, James Ingoldby, Valentin V. Khoze, Jessica Turner

Astrophysics of Galaxies

NOEMA$^\rm{3D}$: A deep view of cold gas flows in a barred spiral galaxy at $z\sim1$

Stavros Pastras, Reinhard Genzel, Linda J. Tacconi et al. (+35 more)

We present a deep, high-resolution CO(4-3) IRAM-NOEMA observation of a main sequence, barred, spiral galaxy at $z\approx1.12$, with an on-source integration time of $\approx37$ hours and a beam FWHM of $\approx0.\!\!^{\prime\prime}3$. We use the molecular gas data in conjunction with the available deep multi-band JWST and HST imaging, covering restframe UV to near-IR wavelengths, to quantitatively study the gas flows in the disk plane of this cosmic noon barred spiral. We find that this target is a massive ($\log(M_{\rm{baryons}}/M_\odot)\approx10.96$), baryon-dominated ($f_{\rm{dm}}(<R_e)=u^2_{\rm{circ,dm}}(R_e)/u^2_{\rm{circ}}(R_e)\sim4\%$), gas-rich ($f_{\rm{gas}}=M_{\rm{gas}}/(M_{\rm{\star}}+M_{\rm{gas}})\approx40\%$) disk, hosting a long ($a_{\rm{bar}}\approx4.2$ kpc), strong ($Q_{\rm{b}}\approx0.37$), and fast ($\mathcal{R}=R_{\rm{CR}}/a_{\rm{bar}}\approx1.05$) bar, which rotates at an angular speed of $\Omega_{\rm{pattern}}\approx$ 50 km/s/kpc. This bar is driving molecular gas inflows with a net inflow rate of $\dot{M}\sim30$ $M_\odot$/yr, based on three estimates, which is of the same order as the galaxy-integrated star formation rate ($\rm{SFR}\approx36$ $M_\odot$/yr). We additionally identify evidence of a well-defined dust lane shock at the northwestern side of the bar, with gas motions parallel to this feature, in agreement with expectations for an established bar-driven flow. Our study highlights the possible role of bars as key drivers of galaxy evolution for a significant fraction of cosmic noon galaxies, offering a detailed picture of well-defined, bar-driven inflows in a high-$z$ barred spiral.
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All authors Stavros Pastras, Reinhard Genzel, Linda J. Tacconi, Thorsten Naab, Natascha M. Förster Schreiber, Karl Schuster, Roberto Neri, Jianhang Chen, Giulia Tozzi, Jean-Baptiste Jolly, Letizia Scaloni, Capucine Barfety, Andreas Burkert, Yixian Cao, Françoise Combes, Ric Davies, Frank Eisenhauer, Juan M. Espejo Salcedo, Simon Flesch, Santiago García-Burillo, Rodrigo Herrera-Camus, Lilian L. Lee, Minju M. Lee, Daizhong Liu, Dieter Lutz, Giovanni Mazzolari, Amit Nestor Shachar, Meghana Pannikkote, Eleonora Parlanti, Panos A. Patsis, Sedona H. Price, Claudia Pulsoni, Alvio Renzini, Taro T. Shimizu, Amiel Sternberg, Eckhard Sturm, Stijn Wuyts, Hannah Übler

Astrophysics of Galaxies

Euclid: Disky titans - surprisingly high star formation activity and gas content in two brightest group galaxies at z~0.75

F. Gentile, E. Daddi, D. Elbaz et al. (+166 more)

We present the discovery of two disky titans in the first data release of the Euclid satellite. These sources are massive (M>10^11 Msun) star-forming (SFR~20 Msun/yr) discs located in strong over-densities at intermediate redshift (z~0.75). They represent an small fraction of the massive galaxies in over-dense regions (just four candidates in more than 20 deg2 analysed in this study), and their existence is puzzling considering the abundance of passive and bulge-dominated sources commonly found at the centre of groups and clusters at low redshift. Firstly, our analysis shows that these objects are located in massive groups (Mh~10^13.8 Msun), where rapid accretion of cold gas should be prevented from the formation of a static hot halo. Despite this, a millimetre follow-up with NOEMA shows significant cold gas reservoirs (M_h2~10^10.3 Msun) within these sources. Secondly, our morphological analysis shows the presence of a massive and passive bulge in these galaxies, which is expected to stabilise the disc against fragmentation thereby suppressing further star formation. However, these sources lie on the Schmidt-Kennicutt relation or even slightly above. Building on these observations, we propose a scenario where these disky titans are the product of a merger-induced rejuvenation episode, in which the most massive galaxy of a group accretes cold gas from another member and briefly restarts star-formation. Such scenario is supported by a comparison with the TNG300 simulation and easily explains the surviving of star-formation activity in massive galaxies in over-dense environments as temporary stages in a more complex evolution. More in general, our study showcases the ability of Euclid to find rare objects thanks to the unprecedented statistics offered by its surveys and the scientific potential residing in the synergy between Euclid and other facilities observing at longer wavelengths.
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All authors F. Gentile, E. Daddi, D. Elbaz, A. Enia, F. Vito, P.-A. Duc, M. Franco, H. Fu, R. Giuffrida, D. Roberts, F. Shankar, S. Lu, P. Awad, J-B. Billand, M. Baes, L. Bisigello, E. Duran-Camacho, G. Castignani, O. Cucciati, G. De Lucia, C. D'Eugenio, D. Donevski, M. Fossati, M. Fumagalli, R. Gobat, C. Gruppioni, M. Magliocchetti, B. Magnelli, G. Papini, L. Pozzetti, V. Sangalli, J. G. Sorce, L. Spinoglio, V. Strazzullo, M. Tarrasse, G. Toni, G. Zamorani, J. Lewis, B. Altieri, S. Andreon, N. Auricchio, C. Baccigalupi, M. Baldi, S. Bardelli, P. Battaglia, A. Biviano, M. Bolzonella, E. Branchini, M. Brescia, S. Camera, V. Capobianco, C. Carbone, J. Carretero, M. Castellano, S. Cavuoti, A. Cimatti, C. Colodro-Conde, G. Congedo, L. Conversi, Y. Copin, F. Courbin, H. M. Courtois, M. Cropper, H. Degaudenzi, H. Dole, F. Dubath, X. Dupac, M. Farina, R. Farinelli, F. Faustini, S. Ferriol, S. Fotopoulou, M. Frailis, E. Franceschi, M. Fumana, S. Galeotta, K. George, B. Gillis, C. Giocoli, J. Gracia-Carpio, A. Grazian, F. Grupp, S. Gwyn, S. V. H. Haugan, H. Hoekstra, W. Holmes, I. M. Hook, F. Hormuth, A. Hornstrup, M. Huertas-Company, K. Jahnke, M. Jhabvala, B. Joachimi, S. Kermiche, A. Kiessling, B. Kubik, M. Kümmel, M. Kunz, H. Kurki-Suonio, A. M. C. Le Brun, S. Ligori, P. B. Lilje, V. Lindholm, I. Lloro, G. Mainetti, O. Mansutti, O. Marggraf, M. Martinelli, N. Martinet, F. Marulli, R. J. Massey, E. Medinaceli, S. Mei, M. Melchior, M. Meneghetti, E. Merlin, G. Meylan, A. Mora, M. Moresco, L. Moscardini, R. Nakajima, C. Neissner, S.-M. Niemi, C. Padilla, S. Paltani, F. Pasian, K. Pedersen, W. J. Percival, V. Pettorino, G. Polenta, M. Poncet, L. A. Popa, F. Raison, A. Renzi, J. Rhodes, G. Riccio, E. Romelli, M. Roncarelli, H. J. A. Rottgering, B. Rusholme, R. Saglia, Z. Sakr, D. Sapone, P. Schneider, T. Schrabback, A. Secroun, E. Sihvola, P. Simon, C. Sirignano, G. Sirri, J. Skottfelt, L. Stanco, P. Tallada-Crespí, A. N. Taylor, H. I. Teplitz, I. Tereno, S. Toft, R. Toledo-Moreo, F. Torradeflot, I. Tutusaus, J. Valiviita, T. Vassallo, G. Verdoes Kleijn, A. Veropalumbo, Y. Wang, J. Weller, F. M. Zerbi, E. Zucca, M. Sereno

Cosmology and Nongalactic Astrophysics · Cross-listed from hep-th

On-Shell Bootstrap of Loop Inflation Correlators with Spectral Dispersion

Haoyuan Liu, Zhehan Qin, Jiayi Wu et al. (+2 more)

We develop a new bootstrap strategy for cosmological correlators at loop level, which we call spectral dispersion. It is based on two conceptual observations that a correlator can be recovered from its on-shell data, also known as nonlocal signals, by analyticity up to local counterterms, and that the on-shell data for a loop process take the form of a discrete sum over quasinormal modes. Technically, our method combines the dS spectral decomposition with dispersion relations. Using this technique, we bootstrap new results in a simple and intuitive form for 3-point and 4-point correlators with 1-loop massive exchanges of scalar and vector bosons, either directly or derivatively coupled. Applications of this bootstrap technique to higher spins and higher-loop banana graphs with dS covariant dispersions but noncovariant couplings are also straightforward.
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All authors Haoyuan Liu, Zhehan Qin, Jiayi Wu, Zhong-Zhi Xianyu, Hongyu Zhang

Astrophysics of Galaxies

A Steep-Extinction Quasi-stellar Object at z=4.6: JWST Evidence for Abundant Small Dust Grains

Mingyu Li, Zheng Cai, Roberto Maiolino et al. (+9 more)

The rapid accumulation of massive dust reservoirs in the early Universe remains a major challenge in astrophysics. While core-collapse supernovae can inject large dust grains ($a \gtrsim 0.1\,\mu{\rm m}$) on short timescales, explaining the total dust budgets in the early Universe likely requires efficient grain growth in the interstellar medium (ISM). Such growth depends critically on an abundant population of small grains, which maximize the surface area available for accretion and may be generated by rapid dust-processing or dust-formation channels. Here, we report the discovery of a QSO, UDS-27023, at $z=4.556\pm0.003$, identified using JWST/NIRSpec spectroscopy. By quantitatively comparing the spectra to QSO composite templates, we find that UDS-27023 displays an exceptionally steep far-UV extinction curve ($A_{1500}/A_V \approx 8$) but notably lacks the 2175 A bump ($A_\mathrm{bump}/A_V<0.34$ at $3\sigma$), indicating a dominance of small silicate dust grains. We interpret this phenomenology as evidence for active small-grain production and processing in the QSO environment. Mechanical shattering of pre-existing large grains by QSO-driven shocks and outflows provides one natural pathway, while in situ condensation of silicate grains inside dense QSO-driven winds may offer an additional route. Such a population of steep-extinction QSOs (SEQs) may therefore reveal a short-lived phase in which luminous active galactic nuclei generate, process, and redistribute small grains, potentially facilitating rapid ISM grain growth and enriching the circumgalactic medium.
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All authors Mingyu Li, Zheng Cai, Roberto Maiolino, Fengwu Sun, Xihan Ji, Qiao Duan, Bjorn H. C. Emonts, Xiaohui Fan, Ignas Juodžbalis, Xiaojing Lin, Yixiao Liu, Sandro Tacchella

Instrumentation and Methods for Astrophysics

Enabling tomorrow's planetary defence and space resource economy: Autonomous fleet-based asteroid rendezvous missions

Stefania Soldini, Paul A. Abell, Daniel J. Scheeres et al. (+8 more)

Asteroids preserve the solar system's earliest history and pose real threats to Earth. Strengthening the UK's capabilities to detect, track, and characterise Near-Earth Objects (NEOs) is vital for national security, world-leading planetary science, and future space resource opportunities. The UK has been an influential contributor to planetary defence, from establishing the UK NEO Task Force in 2000 to active roles in the International Asteroid Warning Network (IAWN), the Space Mission Planning Advisory Group (SMPAG), and the development of the National Space Operations Centre (NSpOC). UK scientists contribute to major international asteroid missions including NASA's OSIRIS-REx, DART, Lucy, and Psyche; ESA's Hera and RAMSES; and JAXA's Hayabusa2 and MMX. However, the UK currently lacks dedicated funding streams to deliver asteroid missions. Ground-based observations of asteroids cannot definitively determine the physical characteristics of these objects, which are crucial for impact-risk assessment, deflection strategy, and resource evaluation. This white paper proposes UK leadership in autonomous, low-cost asteroid-rendezvous missions, leveraging technologies developed through the UKRI-funded REMORA programme. We outline four priorities: (1) strengthen NEO detection capabilities; (2) reinforce UK participation in international planetary defence missions; (3) develop autonomous rendezvous and in-situ characterisation technologies; and (4) enable the future space resource economy through targeted asteroid exploration. Together, these actions position the UK to lead rapid, affordable deep-space missions and secure a long-term strategic advantage.
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All authors Stefania Soldini, Paul A. Abell, Daniel J. Scheeres, Yuichi Tsuda, Xiaoyu Fu, Nicolò Stronati, Hanjoon Shim, Sui Chen, Anirudh Chhabra, Ricardo Torres, Andrew Jones

Astrophysics of Galaxies

Mergers Matter: Gravothermal Collapse in Dwarf Halos with Self-Interacting Dark Matter

Maya Silverman, Abdelaziz Hussein, Arpit Arora et al. (+8 more)

Self-Interacting Dark Matter (SIDM) models with large cross sections at relative velocities below $\sim100\,{\rm km \, s}^{-1}$ can be tested with dwarf galaxy observations. We analyze six dark-matter-only zoom-in $\sim10^{10}\,{\rm M}_\odot$ halos with diverse assembly histories, adopting a cross section over mass of $\sigma/m = 70\,cm^2 \, g^{-1}$. We find that mergers inject orbital kinetic energy into the halo, altering the heat transport and the gravothermal evolution of the core. Three of the six halos – those with the most quiescent merger histories – show clear signs of core collapse in these simulations. Halos with sustained mergers do not collapse. Furthermore, merger-induced heat transport drives two non-collapsing halos to central densities well below the predictions of the gravothermal fluid model. These findings suggest a novel mechanism for producing dark-matter-deficient galaxies and expanding the diversity of rotation curves beyond what halo concentration alone predicts. Merger histories are thus essential for understanding central density distributions of dwarf galaxy halos in SIDM.
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All authors Maya Silverman, Abdelaziz Hussein, Arpit Arora, Mariangela Lisanti, Manoj Kaplinghat, Lina Necib, Andreas Thoyas, Stephanie O'Neil, Robyn E. Sanderson, Xuejian Shen, Jorge Moreno

Cosmology and Nongalactic Astrophysics

JWST's Little Red Dots as collapsed Supermassive Dark Stars

Cosmin Ilie

The nature of the “Little Red Dots” (LRDs) is one of the most profound mysteries posed by the JWST data. One promising class of models that can reproduce the observed LRDs spectra and morphology are quasi-stars: massive envelopes surrounding accreting black holes formed via the collapse of supermassive stars (SMSs). However, the canonical SMS pathway relies on a highly restricted set of environmental and structural conditions: strong Lyman–Werner (LW) backgrounds to suppress H$_2$ cooling, high and sustained gas inflow rates to enforce entropy stratified envelopes, and assume non-zero rotational support in order to prevent GR instability collapse before $\sim 10^6 M_{\odot}$. Here we show that supermassive dark stars (SMDSs), powered by dark matter (DM) annihilation rather than nuclear burning, naturally satisfy the key structural and energetic requirements for quasi-star (QS) formation while relaxing all of those restrictive conditions listed above. Moreover, quasi-stars formed through the SMDS pathway are born with prompt BH masses ($\gtrsim 10\%$) of the progenitor mass. They therefore enter directly into a late-stage quasi-star regime; subsequently the envelope expands and cools until its photosphere reaches the zero-metallicity opacity limit $(T_{\rm eff}\sim3000$-$6000\,{\rm K}$). Those cool, optically thick, unresolved photospheres can reproduce key features of many JWST LRDs.
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Solar and Stellar Astrophysics

On the Relationship between Solar Spicules and Propagating Coronal Disturbances: The Role of Shocks

Ravi Chaurasiya, Sankalp Srivastava, Piyali Chatterjee et al. (+3 more)

Spicules and propagating coronal disturbances (PCDs) are ubiquitous dynamic features of the solar atmosphere, yet their physical connection remains an open question of paramount importance to the mass and energy transport in the solar atmosphere. Using concurrent multiwavelength high-resolution observations from the Swedish 1-m Solar Telescope and the Solar Dynamics Observatory, supported with two-dimensional radiative magnetohydrodynamic (MHD) simulations, we find that i) shock waves in the chromosphere generated from non-linear wave steepening drive some spicules, ii) in the corona, these shock waves may transition into large amplitude non-linear compressive MHD waves depending on the magnetic field strength and the ambient coronal conditions. In either case, the shocks or the large-amplitude compressive waves in the corona, also transport upward mass flux and produce intensity variations in the form of PCDs in coronal passbands. Further a multi-height wavelet analysis shows dominant $\sim$5 minute periods in the lower chromosphere that evolve into longer periods ($\ge$10 minutes) at higher atmospheric layers, consistent with dispersive propagation in a stratified medium. The observational characteristics together with the numerical simulations, demonstrate that a shock-driven MHD mechanism links spicule formation to coronal disturbances. Finally, mass flux estimates from both the observations and the simulations indicate that these PCDs can also aid in supplying mass to the solar wind.
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All authors Ravi Chaurasiya, Sankalp Srivastava, Piyali Chatterjee, Sahel Dey, Robertus Erdélyi, Ankala Raja Bayanna

Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

Teleocosmology and quantum post-selection

Paul C.W. Davies and João Magueijo

Although cosmic acceleration is well established, its physical origin remains contentious. Most explanations invoke either a non-zero vacuum energy, i.e. a cosmological constant, or new fields. We propose instead a mechanism arising purely from quantum mechanics, without additional constants or local sources. The key point is that quantum theory permits both initial and final boundary conditions on a state, here the wave function of the universe. Pre- and post-selected systems are familiar in laboratory quantum mechanics; we extend the idea to minisuperspace quantum cosmology. As a warm-up, we show that a free non-relativistic particle, initially in a semiclassical wave packet and conditioned on a final quantum state, can have an intermediate peak which accelerates even though the Hamiltonian is free. A semiclassical observer would infer a contrived classical force. We then implement the analogous construction in quantum cosmology using connection variables and unimodular time. A forward semiclassical packet, taken for simplicity to describe pure radiation, is post-selected by a normalizable Chern-Simons soliton. The resulting two-boundary amplitude has a peak which leaves the radiation trajectory and enters an accelerating regime, while the forward Hamiltonian has $\Lambda=0$. A classical model can mimic this trajectory only by introducing a contrived effective component: near the transition it resembles $w\simeq -1$, but when extrapolated it evolves towards strongly phantom behaviour, $w<-1$. The acceleration is therefore more naturally interpreted as a quantum boundary-condition effect than as a local classical source. We also discuss why the Chern-Simons soliton is a clean final state, the small overlap between initial and final states, and possible tell-tale signatures.
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High Energy Astrophysical Phenomena · Cross-listed from gr-qc

Dyonic Quark Stars in Quasi-Topological Electromagnetism

Michael Gammon, Nicola De Kock, Robert B. Mann et al. (+1 more)

In this paper we consider quark star solutions to Liu et al.'s \cite{Liu_2019} quasi-topological electromagnetism (QTEM), a recently proposed form of dark energy. Since the QTEM contribution is trivial for pure electric/magnetic charge, we consider the dyonic case in pure QTEM which does induce (dark) non-trivial dynamics from the non-linear theory. Besides the introduction of a dyonic charge distribution generally pushing the characteristic quark star `hook' shape to larger masses and radii, it also induces a second branch at very large mass and radius for stars with a small dyonic charge ratio. This second set of solutions have a negative pressure envelope surrounding a positive pressure core. As we explore the parameter space these features interact and evolve in interesting ways, with the two branches eventually merging in $M/R$ space before settling into a characteristic `paperclip' shape as the dyonic charge ratio becomes large.
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All authors Michael Gammon, Nicola De Kock, Robert B. Mann, Amos Kubeka

Solar and Stellar Astrophysics

Evaluating Cepheid Metallicity Effect Determinations via IC1613 and Gaia-independent Parallaxes

Daniel Majaess

18 HST parallaxes for Galactic classical Cepheids are unified to establish a Gaia-independent IC1613 distance and evaluate metallicity effect determinations, since the Gaia zeropoint is debated. Recently proposed classical Cepheid metallicity corrections of $\gamma (W_{VI}) \simeq 0, -0.25, -0.50$ mag dex$^{-1}$ are benchmarked, and yield $\mu_{0,{W_{VI}}} \simeq 24.39, 24.16, 23.93$ ($\pm0.07$). Larger corrections are disfavored relative to a weighted mean of IC1613 TRGB and TRGB/JAGB distances of $24.39^{+0.07}_{-0.04}$ (EDD) and $24.36\pm0.06$/$24.45\pm0.11$ (CCHP). A more expansive metallicity baseline is desirable to scrutinize smaller corrections (e.g., $\gamma (W_{VI}) \lesssim -0.1$ mag dex$^{-1}$), while concurrently acquiring additional non-Gaia parallaxes since published concerns exist regarding DR3 (e.g., critical long-period Cepheids S Vul, SV Vul).
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Earth and Planetary Astrophysics

Orbit Refinement of WASP-18 b and Evidence Against the Existence of WASP-18 c

Avinash Salguneswaran Nediyedath, Kyle A. Pearson, Tara Fetherolf et al. (+1 more)

We present an updated transit ephemeris for the exoplanet WASP-18 b and critically examine the existence of a proposed second planet, WASP-18 c. Using 205 transit light curves from TESS, CHEOPS, Exoplanet Watch, Exoplanet Transit Database and previous literature, we derive a refined mid-transit time of 2460933.096346 +/- 0.000022 BJD_TDB and an orbital period of 0.94145252 +/- 1.1 x 10^-8 days for WASP-18 b. Our forward-propagated ephemeris to January 1, 2030, shows a timing uncertainty of 2.41 seconds. This high-precision refinement serves as a robust baseline to test for Transit Timing Variations (TTVs), ensuring that any reported deviations are not artifacts of an insufficiently constrained orbital period. In addition, we analyze 449 radial velocity (RV) measurements from the CORALIE, HARPS, PFS, HIRES and ESPRESSO spectrographs to search for signatures of WASP-18 c, a previously proposed additional planetary companion, and also estimated the k2 love number as 0.62199 +/- 0.0011. However, we do not find significant variations in either transit timing or RV data that support the presence of WASP-18 c. Moreover, the most significantly identified periodicities are not consistently measured across the transit or RV datasets, strongly arguing against the existence of a dynamically relevant second planet in the system. Our results indicate that the claimed WASP-18 c signal is likely spurious in nature. Overall, this work enhances our understanding of the WASP-18 system and provides a valuable resource for future observational campaigns with the refinement of the b planet orbit and falsified status of the previously defined c planet.
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All authors Avinash Salguneswaran Nediyedath, Kyle A. Pearson, Tara Fetherolf, Andre O. Kovacs

Earth and Planetary Astrophysics

Size limits on tidal debris around white dwarfs: the km-size barrier

Jordan K. Steckloff, Dimitri Veras and Kathryn Volk

Compact disks of planetary debris orbiting white dwarfs provide a crucial window into our understanding of evolved planetary systems. The formation of these disks has been widely modeled with tidal fragmentation of minor planets that are rubble piles with no internal strength. However, rubble piles do have non-zero cohesive strength from Van der Waals forces, and here we demonstrate the consequences: breakup of these rubble piles sets a maximum fragment size, and we calculate this size \jks{for water ice, iron, and material densities corresponding to the lunar highlands, Vesta and the Earth}. We find that for typical minimum rubble pile strengths of $\sim$10-1000 Pa, the maximum fragment size is as large as small asteroids (0.1-1 km). This limit – the km-size barrier – also represents the characteristic sizes of tidal fragments. Most of the debris mass is contained in fragments of this size. Consequently, subsequent disk evolution should first feature a prominent dust-forming process, such as collisional grinding, before Poynting-Robertson drag can significantly shape the final disk. \jks{Further, we find that non-zero internal strength more narrowly radially confines the fragments than in the strengthless case.} This correction to previous assumptions adds to the growing evidence of the importance of collisions in the formation and evolution of white dwarf debris disks, while also helping to bound the size distribution in these disks for modeling efforts.
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Solar and Stellar Astrophysics · Cross-listed from physics.space-ph

Counterintuitive Magnetic Connectivity and Energetic Particle Flux Differences among Nearby Spacecraft During the 2023 February 24 Solar Energetic Particle Event

Weihao Liu, Xianyu Liu, David Lario et al. (+4 more)

For solar energetic particles (SEPs), it is generally expected that observers magnetically closer to the eruption source region exhibit higher particle intensities than those poorly connected to the eruption site. However, the 2023 February 24 SEP event departs from this simple picture: Earth and STA, near 1 au, are nominally better connected to the source region, whereas Solar Orbiter (SolO), at 0.77 au but less favorably connected, observed SEP fluxes more than an order of magnitude higher. This difference cannot be simply explained by nominal magnetic connectivity or radial scaling of SEP fluxes alone. To investigate this behavior, we perform a global magnetohydrodynamic simulation of the associated coronal mass ejection (CME) using the Alfvén Wave Solar-atmosphere Model-Realtime (AWSoM-R). The simulation reveals that the CME flux rope originates close to a coronal streamer and as it propagates and expands, the CME-driven shock is effectively distorted, developing into two distinct flanks with different strengths. Although the three spacecraft are separated by only $\lesssim$30$^{\circ}$ in heliolongitude, their magnetic footpoints differ by $\gtrsim$50$^{\circ}$ in longitude because of a nearby stream interaction region. Specifically, Earth and STA connect to a weaker shock region, while SolO connects to the shock nose with a higher compression ratio and more efficient particle acceleration. We further simulate SEPs using the Multiple-Field-Line Advection Model for Particle Acceleration (M-FLAMPA) coupled with AWSoM-R, obtaining results that reproduce the observed flux differences among the three spacecraft, demonstrating that this counterintuitive behavior results from their connections to different regions of the inhomogeneous CME-driven shock.
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All authors Weihao Liu, Xianyu Liu, David Lario, Lulu Zhao, Tamas I. Gombosi, Alexander D. Shane, Igor V. Sokolov

High Energy Astrophysical Phenomena

Regression on Regression: Mapping Data-Driven Binary Black Hole Merger Rate Fits to Progenitor Histories

Emma Blanchet, Aryanna Schiebelbein-Zwack and Maya Fishbach

The binary black hole (BBH) merger rate is governed by the progenitor formation rate and the distribution of delay-times between formation and merger, but these functions remain poorly constrained. We introduce a framework that maps the parameters of physics-driven models directly onto existing data-driven fits of the BBH merger rate. This “regression on regression” approach enables physical interpretation of flexible population models without the computational burden of reanalyzing the underlying gravitational-wave event data. Applying this method to the B-Spline merger-rate posteriors from the Fourth Gravitational-Wave Transient Catalog, we fit the minimum delay time ($\tau_{\text{min}}$), delay-time power-law index ($\alpha$), and progenitor formation parameters controlling the normalization ($\mathcal{A}$), early-time growth ($\gamma$), and late-time decay ($\delta$). Increasing the number of anchoring redshift points from two to four reduces the median sum-squared error (SSE) by a factor of $\approx 4.5$. However, residuals reveal that the physical model does not pass through all four anchors, exposing model misspecification and demonstrating a key strength of the framework: unlike standard inference methods, which preferentially weight compatible curves and mask underlying tensions, our approach exposes BBH posteriors irreconcilable with the model. Despite uncertainties at $z\gtrsim1$, the shape of the progenitor formation rate at low-$z$ is robust and evolves more steeply than the global star formation rate (SFR), supporting a preference for low metallicity environments. Specifically, the log-space slope of the progenitor rate is $\approx 5.3$ times steeper than the SFR between $z=0.1$ and $z=1.0$. Ultimately, a more complex phenomenological model is required to match the B-Spline merger rates.
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Solar and Stellar Astrophysics

Modeling the Thermal Low-Frequency Radio Sun with Ray Tracing

Peijin Zhang, Bin Chen, Gregory Fleishman et al. (+4 more)

Incoherent radio emission at meter–decimeter wavelengths provides a key diagnostic of the coronal thermal plasma, but at frequencies below $\sim$\,1\,GHz coronal refraction can substantially bend ray paths and modify the apparent source size and brightness distribution. We develop a forward-modeling framework that combines refractive ray tracing through a global 3D coronal model with radiative transfer along each ray. The method tracks the ray-tube cross-sectional area $S(s)$ using a step-wise perturbation retracing approach and incorporates a geometric magnification term proportional to $d\ln S/ds$ to enforce flux conservation under focusing/defocusing. Thermal free–free emission and absorption are then computed with the GRFF radiative transfer code to produce synthetic radio maps over 40–800\,MHz. Applying the framework to Carrington rotation 2298, we find that including propagation effects allows the quiet-Sun background spectrum to be well reproduced. However, active region brightness is less accurately modeled, suggesting that additional physical factors should be considered in future work. These results establish a physics-based method for generating low-frequency quiet-Sun synthetic images suitable for quantitative comparison with interferometric observations and for assessing how propagation effects shape the observed morphology.
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All authors Peijin Zhang, Bin Chen, Gregory Fleishman, Alexey Kuznetsov, Cooper Downs, Surajit Mondal, Sijie Yu

Astrophysics of Galaxies

Revisiting ram pressure stripping in Wolf-Lundmark-Melotte: No evidence for stripped HI with LGLBS

Daniel R. Rybarczyk, Eric W. Koch, Fabian Caballero Vargas et al. (+12 more)

We analyze HI 21-cm observations of the Local Group dwarf galaxy Wolf-Lundmark-Melotte (WLM) from the Local Group L-Band Survey to search for evidence of ram pressure stripping. While previous MeerKAT-16 observations of WLM showed evidence for off-galaxy atomic gas emission with a geometry suggestive of ram pressure stripping, our observations find no evidence for this stripped gas. We demonstrate that our observations would be sensitive to the claimed detections and suggest that an uncorrected observational flaw with the MeerKAT data led to the apparent off-galaxy emission. The lack of off-galaxy emission obviates the need for uncharacteristically high values of the density of the intergalactic medium in this region.
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All authors Daniel R. Rybarczyk, Eric W. Koch, Fabian Caballero Vargas, Snezana Stanimirovic, Nickolas M. Pingel, Julianne J. Dalcanton, Adam K. Leroy, Erik W. Rosolowsky, Michael P. Busch, Chang-Goo Kim, Adam Smercina, Elizabeth Tarantino, Vicente Villanueva, Alberto D. Bolatto, Thomas G. Williams

Solar and Stellar Astrophysics

Solar vortex detection methods in MHD simulations: impact of magnetic field and spatial resolution

M. Koll Pistarini, E. Khomenko, T. Felipe et al. (+1 more)

The aim of this paper is to investigate the influence of different magnetic field configurations and spatial resolutions on vortex structures. We analyzed a set of six three-dimensional realistic simulations of the solar atmosphere under three different magnetic field configurations: a small-scale dynamo and two initially vertical implanted magnetic fields of 50 G and 200 G. Three different spatial resolutions have been employed: 20x20x14, 10x10x7 and 5x5x3.5 km^3. We applied two vortex detection methods based on the velocity gradient tensor to all of the models: swirling strength and the SWIRL code. We performed a comparison of vortex locations obtained with both methods, and a statistical analyses of the vortex generation mechanisms, the area covered by vortices, their number and characteristic sizes, and temperature profiles as a function of height. We have confirmed that different magnetic field configurations and spatial resolutions impact the area coverage, number, and sizes of vortices. Likewise, the detection methods impact the statistics obtained. Swirling strength detects vortices with any orientation but a height-dependent threshold is needed. SWIRL only detects vertically-oriented vortices but shows a better agreement with the rotating horizontal velocity field. Simulations with a vertical magnetic field of 50 G support the formation of chromospheric vortices without a photospheric counterpart, while most of the vortices in the 200 G model directly connect the photosphere with the chromosphere. Small-scale dynamo simulations are characterized by a large number of horizontal vortices, with vertical vortices being nearly absent at chromospheric layers. Temperature profiles of vortices confirm that they are hotter than their surroundings, regardless the simulation setup.
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All authors M. Koll Pistarini, E. Khomenko, T. Felipe, M. Modestov

High Energy Astrophysical Phenomena

The First Detection of Sub-Populations in the Delay-Time Distribution of Binary Black Holes in GWTC-4 of LIGO-Virgo-KAGRA

Shaunak Padhyegurjar and Suvodip Mukherjee

The imprint of different formation channels of binary black holes (BBHs) is encoded in the distribution of time delays between BBH mergers and the formation of their progenitor stars, along with their source properties such as component mass, mass-ratio, spin, and more. This makes it possible for the presence of a potential correlation between the delay-time distribution and compact-object source properties. We report the first measurement of this inevitable signature from the fourth gravitational wave (GW) catalog (GWTC-4) of LIGO-Virgo-KAGRA and identified three sub-populations that show distinct merger rate behavior as a consequence of this. We find that the delay-time distribution of the sources above a mass of $45$ M$_\odot$ is significantly different from the ones below and exhibits strong dependence on the mass-ratio and spin, indicating that GW sources close to equal masses and close to zero effective spin are more delayed in comparison to the values otherwise. Our analysis identifies the presence of at least three source property dependent sub-population of merger rates with the merger rate at redshift $z=0$ varying from $\sim 0.6- 12$ Gpc$^{-3}$ yr$^{-1}$ for the three different sub-populations and hence rule out a Universal merger rate for all the BBHs detected using GW.
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Solar and Stellar Astrophysics

The light curve of T Cephei: humps, long-term changes and recent phenomena

Mike Goldsmith

The long-period Mira variable T Cephei has been observed for nearly 150 years, during which more than 130 cycles have been recorded. Using visual observations from the AAVSO database, maxima, minima, pulsation periods, amplitudes and hump characteristics were estimated for 136 cycles from the late nineteenth century to 2026. Most cycles display a temporary slowing of the rise to maximum light ("humps"). These humps show significant cycle-to-cycle persistence in occurrence, duration and brightness, and are more likely to occur during high-amplitude cycles. Hump onset magnitudes exhibit a lower limit near V ~ 9.3, with many humps beginning near V ~ 8. Cycles with larger amplitudes/deeper minima tend to have humps which are dimmer and more gradual. Comparison with published spectroscopy supports the interpretation that the humps are associated with changes in TiO-dominated molecular opacity within the stellar atmosphere. A qualitative model involving secondary pressure waves traversing TiO-rich layers is proposed to account for the observed hump morphology and its variability. The most recent cycles of T Cep have shown unprecedented behaviour. Pulsation amplitudes have fallen to historically low values, the maximum-to-maximum period has shortened substantially, and the light curve has become increasingly asymmetric. In Cycle 139, a substantial hump appeared on the descending branch, a phenomenon never before observed in T Cep. Similarities with the Mira variable T UMi raise the possibility that these recent changes may be due to evolutionary developments.
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High Energy Astrophysical Phenomena

Statistical properties of type-II bursts in Rapid Burster observed with Insight-HXMT

Shi-peng Wang and Long Ji

We report the detection of type-II X-ray bursts in a neutron star low mass X-ray binary MXB 1730-335 (also known as Rapid Burster) observed with Insight- HXMT satellite. We found significant variations in the burst properties along with the decay phases of its 2017 and 2020 outbursts, such as switches between different burst modes.We investigated the statistical distributions of burst parameters (peak flux $F_{\mathrm{peak}}$, fluence $E$, burst duration $t_{\mathrm{dur}}$, recurrence time $\Delta t$) and studied their correlations.We confirm the relaxation oscillator behavior ($E \propto \Delta t^{\alpha}$), and find that for mode-2 bursts the index $\alpha$ is around 1. For mode-1 bursts, $\alpha$ varies with $\Delta t$ and the $E-\Delta t^{\alpha}$ relation can be described by a broken powerlaw model.We also confirm that $F_{\mathrm{peak}}$ is correlated with $E$ below a critical value and is independent of $\Delta t$.
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Astrophysics of Galaxies

The magnetic field of the Milky Way: an observational perspective

Marijke Haverkorn

Magnetic fields are an important and enigmatic component of the Milky Way's ecosystem. Mostly frozen into interstellar plasma, they play key roles in (turbulent) gas dynamics, star formation, energy household, evolution of interstellar objects, and cosmic-ray propagation. This paper reviews recent progress on measuring and characterizing these Galactic magnetic fields, limited to the larger-scale fields in mostly diffuse media, and to an observational perspective. On Galaxy-wide scales, the magnetic field roughly follows the spiral arms in the Galactic disk, and includes an additional component perpendicular to the disk away from the Galactic plane. The field configuration is different in the Galactic disk and the Galactic gaseous halo, qualitatively consistent with different dominating dynamo modes. Deviations from this idealized model are ubiquitously observed and include anomalously high Faraday rotation, variable magnetic field orientations and field reversals on kiloparsec scales. On smaller scales, the magnetic field is turbulent, anisotropic and intermittent. Much used descriptions of the turbulent magnetic field such as power laws and Gaussianity are being replaced by higher-order statistics that better capture the complexities of the field. Magnetic field orientations and possibly strength are correlated with both cold and warm components of the multi-phase interstellar gas, and with the interstellar dust distribution. The near future will bring a large increase in observational data in rotation measure grids, Faraday Tomography data and measurements of interstellar polarization of optical starlight, promising exciting developments in characterizing and understanding magnetic fields in the Milky Way in the next few years.
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Instrumentation and Methods for Astrophysics

AstroSkyFlow: an astronomical sky image flow simulator for time domain survey validation and machine learning

Kexin Li, Yicheng Rui, Fabo Feng et al. (+5 more)

Modern time-domain optical surveys produce massive data volumes that require robust, high-fidelity simulated datasets for developing and validating automated pipelines and machine-learning models. We present AstroSkyFlow, a modular sky-image simulator that generates on-demand, time-dependent flux variations and models the full observing stack, from celestial sources and atmospheric effects to sensor response. Given a simulated observing schedule, AstroSkyFlow produces multi-epoch, time-series images with realistic noise and variability. Compared to real observational data, AstroSkyFlow reproduces noise characteristics and point spread function properties more accurately than the widely used SkyMaker simulator. In addition, AstroSkyFlow successfully recovers injected photometric and motion signals, such as exoplanet transits and asteroid trails. AstroSkyFlow enables the generation of labeled, high-fidelity datasets essential for training machine-learning pipelines and conducting rigorous injection-recovery tests for analysis pipelines for next-generation time-domain surveys.
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All authors Kexin Li, Yicheng Rui, Fabo Feng, Shuyue Zheng, Anton Pomazan, Yiyang Guo, Jie Zheng, Lin-Qiao Jiang

Cosmology and Nongalactic Astrophysics

Redshift Duality with Pantheon+SH0ES in a Planck-anchored Flat $\Lambda$CDM Framework: Implications for Hubble Tension and Observational Inference

Tae-Kyoung Lee

We test an operationally defined redshift duality in which the observed redshift comprises the standard metric-expansion component together with an additional line-of-sight quantum contribution arising from the cumulative conversion of photon energy into effective mass as a function of path length and frequency. Fitting this hybrid model to the Pantheon+SH0ES compilation, we find that the metric-expansion Hubble constant, $H_\Lambda$, is recovered to a value consistent with the Planck baseline of $67.4~\mathrm{km\,s^{-1}\,Mpc^{-1}}$ within $\lesssim 0.33\sigma$. Redshift-binned analyses show that while the flat Lambda cold dark matter ($\Lambda$CDM) model produces an apparent drift in the inferred Hubble parameter across the Hubble flow, the hybrid model restores the constancy of $H_\Lambda$ across redshift bins. The correctional trends of cosmological physical quantities re-inferred under this framework further indicate the potential to alleviate anomalies associated with high-redshift galaxies. These results suggest that redshift duality warrants further consideration in observational processing and inference, while preserving consistency with a Planck-anchored flat $\Lambda$CDM baseline.
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Earth and Planetary Astrophysics

Escape of Water- and Metal-enriched Atmospheres from compact Hot mini-Neptunes with CHAIN

Daria Kubyshkina, Jo Ann Egger and Caroline Piaulet-Ghorayeb

Recent observations reveal that atmospheric compositions of close-in sub-Neptunes are diverse and can differ strongly from pure H/He-dominated. We assess the possibility of modelling metal-rich and water-rich atmospheres with CHAIN model. We evaluate the major differences between the upper atmosphere photochemistry of such atmospheres compared to H/He, and the impact on atmospheric mass loss rates. We employ CHAIN to model upper atmospheres of two warm and hot sub-Neptune-like planets which were suggested to host possibly water-/metal-rich atmospheres: GJ 9827 d and TOI-238 b. For each planet, we consider a range of compositions between H/He atmospheres with solar metallicities to highly metal- and/or water-rich atmospheres and evaluate how our predictions change with increasing metal/water fractions. We find that for considered sub-Neptunes (1) an increase in water/metal abundance leads both to the increase in atmospheric cooling rates (due to the metal line cooling and molecular cooling processes) and the increase in heating (metal line heating in dense atmospheric layers and metals' ions); (2) due to the increase in cooling and the mean particle weight of the atmosphere, the atmospheric mass loss drops significantly at high water/metal fractions (water mass fractions of >=50% or metal enrichment over 100 times solar), while at low enrichment levels mass loss rates are similar to those of H/He atmospheres with solar abundances or slightly higher; (3) for the same atmospheric mean particle weight, the escape from water-rich atmospheres is generally lower. In the context of atmospheric evolution, it implies that the atmospheres with initially high water fraction in the atmosphere are more stable. Furthermore, due to the preferential escape of H, the atmospheric metal/H ratio is expected to increase significantly with time, leading to the earlier cessation of the hydrodynamic escape.
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Astrophysics of Galaxies

Hierarchical assembly in action: a galaxy tail from a disrupting group in the Virgo cluster outskirts

J. Alfonso L. Aguerri, Stefano Zarattini, Virginia Cuomo et al. (+1 more)

Group environments are thought to play a key role in shaping galaxy evolution prior to cluster accretion. However, direct observational evidence linking group–cluster interactions to the transformation of low-mass galaxies remains scarce. We reexamine the nature and origin of the W cloud, located in the southern outskirts of the Virgo cluster, to better understand the dynamical processes driving group accretion and galaxy transformation during cluster assembly. Using the spatial distribution, kinematics, and stellar population properties of galaxies in the W cloud and its surroundings, we characterize the three-dimensional structure and dynamical state of the system. We show that the W cloud is not a large-scale filament seen in projection, but is instead dominated by a compact galaxy group (the W group) currently interacting with Virgo. We also identify a previously unknown, dynamically coherent tail of galaxies (the W tail) connecting the W group to the cluster. The tail exhibits a continuous sequence in velocity, velocity dispersion, and three-dimensional distance. Its low-velocity component is already gravitationally bound to Virgo, whereas higher-velocity galaxies remain associated with the W group and are still infalling. The W tail forms a planar structure aligned with the orbital geometry of the W group, strongly supporting a tidal origin. The stellar masses and colours of its members indicate that the stripped population is dominated by low-mass, star-forming dwarf galaxies that remain in the blue cloud. The W group–W tail system provides a well-resolved example of an ongoing group–cluster interaction, illustrating how low-density groups can deliver largely unprocessed dwarf galaxies into clusters. This system provides important observational constraints on the hierarchical assembly of galaxy clusters and the buildup of their dwarf galaxy populations.
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All authors J. Alfonso L. Aguerri, Stefano Zarattini, Virginia Cuomo, Lorenzo Morelli

Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

COSMOS: A numerical relativity code specialized for PBH formation

Chul-Moon Yoo, Hirotada Okawa, Albert Escrivà et al. (+7 more)

Primordial black holes (PBHs) are black holes generated in the early universe without having gone through stellar evolution. In the standard formation process, PBHs are formed from super-horizon primordial fluctuations with non-linearly large initial amplitude. In order to simulate the non-linear gravitational dynamics of PBH formation, one has to rely on numerical relativity solvers to approximate the solution of the Einstein equations. COSMOS is a C++ package for solving the Einstein equations in 3+1 dimensions, providing simple tools for the simulation of PBH formation. In order to resolve the collapsing region, non-Cartesian scale-up coordinates and a fixed mesh-refinement procedure are implemented. In COSMOS, a massless scalar field and a perfect fluid with a linear equation of state are implemented as matter fields. To achieve a practically acceptable computational speed, OpenMP is used for the parallelization. COSMOS has no other dependencies, which makes for an easier installation.
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All authors Chul-Moon Yoo, Hirotada Okawa, Albert Escrivà, Tomohiro Harada, Hayami Iizuka, Taishi Ikeda, Yasutaka Koga, Daiki Saito, Masaaki Shimada, Koichiro Uehara

Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

Tachyonic particle production: quantum 2PI formalism with momentum exchanging collisions

Kimmo Kainulainen, Sami Nurmi and Olli Väisänen

Oscillating spacetime curvature can drive particle production during reheating, whose accurate modeling requires the use of non-perturbative out-of-equilibrium methods. Tachyonic instabilities have previously been studied using 2-Particle Irreducible (2PI) formalism in the Hartree approximation, which however misses important momentum exchanging interactions. We present a self-consistent approximation scheme for reducing the non-local next-to-leading order 2PI equations of motion to local quantum kinetic equations, which can be solved with standard methods. We pay special attention to interactions involving unstable modes during tachyonic instabilities.
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High Energy Astrophysical Phenomena

Revealing the high redshift host galaxy of the short GRB 061201 with JWST

Yuhan Mao, Hanrui He, Jia Ren et al. (+6 more)

Using deep near-infrared and optical images from JWST and HST, we identify a new host galaxy candidate for GRB 061201. It lies ~2" from the optical afterglow position. Photometric redshift fitting yields z~1.2. We compare the previously proposed host at z=0.111 with the new candidate. The chance-coincidence probability is $P_{cc}=0.18$, above the classical threshold of 0.1 but consistent with a physical association given the extreme depth of JWST imaging. In contrast, evaluated with corresponding JWST observations, the previously claimed host has a lower $P_{cc}=0.11$, which is driven primarily by bright-tail statistics rather than a more plausible association. A high-z origin is favored by three independent lines of evidence. First, for the z=0.111 scenario, the beaming-corrected energy shows GRB 061201 is an outlier of the Ghirlanda ($E_{p,i}-E_\gamma$) relation for short GRBs, while for the z=1.2 scenario, it is well consistent with the Amati relation. Second, deep near-infrared observations rule out a kilonova similar to AT2017gfo at z=0.111. Third, afterglow modeling yields an AIC criterion of $\Delta$AIC=16.35, providing strong evidence for the high-redshift scenario. Assuming the host candidate is the actual host galaxy of GRB 061201, the physical offset is 16.4-16.9 kpc (substantially reduced from ~42 kpc) and the host stellar age is ~2 Gyr, which are consistent with the host population of short GRBs. A low-redshift origin would lead to a very high binary neutron star merger rate of ~1400 Gpc$^{-3}$ yr$^{-1}$, which is contradictory to the gravitational-wave constraint. We suggest that GRB 061201 originates from a moderately high-redshift (z~1.2) host, significantly alleviating this apparent merger rate discrepancy. This case demonstrates the power of deep JWST exposures in revealing the host galaxies of historically hostless GRBs.
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All authors Yuhan Mao, Hanrui He, Jia Ren, Yun Wang, Hao Zhou, Qiuli Wang, Yiming Zhu, Zhiping Jin, Daming Wei