Cosmology and Nongalactic Astrophysics
Testing Matter Diffusion with Late-Time Cosmological Observations
Alireza Zafari, Mohammad-Hossein Namdar, Luis A. Escamilla et al. (+1 more)
We investigate a class of late-time cosmological models derived from the phenomenological framework of variable matter
diffusion. In these scenarios, energy-momentum conservation requires a continuous energy exchange between matter and an effective scalar-field dark-energy component, $\phi$. We consider a baseline constant-
diffusion scenario alongside four non-linear power-law parametrizations, in which the
diffusion coefficient evolves as a function of the scale factor, matter density, scalar-field density, or Hubble expansion rate. To assess their cosmological viability, we implement these models within SimpleMC and constrain them using late-time observations, including
cosmic chronometers,
baryon acoustic oscillation measurements, Type Ia
supernovae with and without the local SH0ES calibration. In the absence of the local calibration, the
diffusion models yield only modest improvements in the fit ($\Delta\chi^2 \approx -4$), performing comparably to the CPL parametrization while exhibiting negative
Bayesian log-evidence differences relative to $\Lambda$CDM. In contrast, including SH0ES leads to a dramatic reduction in the minimum $\chi^2$ ($\Delta\chi^2 \approx -22$) and decisive
Bayesian evidence in favor of the
diffusion framework ($\Delta\ln\mathcal{Z} \approx 9$). Remarkably, the single-parameter constant-
diffusion model accounts for virtually all the statistical improvement, outperforming the CPL parametrization by more than 10 units in $\chi^2$ and more than 8 units in $\Delta\ln\mathcal{Z}$. The four non-linear extensions provide only negligible additional improvements ($\Delta\chi^2 \approx -1$), indicating that current late-time background observations favor the presence of a non-vanishing matter-
diffusion interaction over $\Lambda$CDM, while providing no statistically significant evidence for a specific time-dependent functional form of the
diffusion coefficient.
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All authors Alireza Zafari, Mohammad-Hossein Namdar, Luis A. Escamilla, Eleonora Di Valentino
High Energy Astrophysical Phenomena
A statistical relation of energy injection plateaus in multi-band afterglows of gamma-ray bursts
Xiao-Yan Li, Tong Liu and Bao-Quan Huang
The origin of the plateau phase in gamma-ray burst (GRB) afterglows remains under debate, with the energy injection model being one of the most competitive explanations. If the plateau is truly driven by energy injection, the average X-ray and optical luminosities during the plateau phase, $L_{\rm X, plat, ave}$ and $L_{\rm opt, plat, ave}$, should naturally be correlated. Moreover, under this scenario, the scaling relations between the luminosities during the plateau and the normal decay phase are expected to be consistent since they share the same origin, i.e., synchrotron radiation from the external forward shock. Therefore, simultaneous multi-band observations are essential to verify this mechanism. In this work, we select a sample of 47 GRBs with simultaneous plateaus in both bands. We calculate their time-averaged isotropic luminosities for the plateau and the subsequent normal decay phases. We find a moderate positive correlation $\log L_{\rm X, plat, ave}=m\log L_{\rm opt, plat, ave}+c$ with a slope $m = 0.86 \pm 0.11$ for the plateau phase. This correlation supports the energy injection origin and offers a promising diagnostic approach to test the model. Furthermore, we obtain a similar slope $m = 1.05 \pm 0.05$ for the normal decay phase, which reinforces the idea that both phases share the same physical origin. Notably, the post-plateau data exhibit a systematic downward shift in luminosity, which may indicate the cessation of the central engine.
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Earth and Planetary Astrophysics
A High-Resolution Spectroscopic Survey of Directly Imaged Companion Hosts: III. Characterization of the Cold Imaged Planet Hosts AF Lep A and $\epsilon$ Indi A
Aneesh Baburaj, Quinn M. Konopacky, Christopher A. Theissen et al. (+3 more)
JWST has enabled the measurement of carbon, oxygen, and sulfur abundances in the atmospheres of directly imaged
planets. Interpretation of these abundances from a
planet formation standpoint requires the corresponding abundances for the host
star. In this work, we present detailed characterizations of the cold imaged
planet hosts AF Lep A and $\epsilon$ Indi A using high-resolution Gemini/GHOST spectra. We derive the atmospheric parameters $T_{\rm eff}$ and $\log{g}$ using two different approaches, revealing differences in $T_{\rm eff}$ up to $\sim260\,$K. The derived parameters are subsequently incorporated in measurement of 16 elemental abundances (C, O, Na, Mg, Si, S, K, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn, Y) and several abundance ratios. Utilizing both the spectral fit and the equivalent width methods, we find
solar C/O, C/S and O/S ratios ($<1.5\sigma$) for AF Lep A and $\epsilon$ Indi A. We compare our measured abundances and their ratios with those of the
planets AF Lep b and $\epsilon$ Indi Ab, with the elevated abundances for the
planets relative to their host
stars strongly indicating formation by core-
accretion pathways.
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All authors Aneesh Baburaj, Quinn M. Konopacky, Christopher A. Theissen, Jerry W. Xuan, Roman Gerasimov, Kielan K. W. Hoch
High Energy Astrophysical Phenomena
A NuSTAR Reflection-Spectroscopy Survey of Cygnus X-1
Kshitij Duraphe, Kartik Mandar, Chooda Khanal et al. (+1 more)
Relativistic-reflection measurements of Cygnus X-1 disagree on the extent of disk truncation and commonly infer supersolar iron abundances. We analyze a selected sample of 26 archival NuSTAR observations obtained between 2012 and 2024 using two configurations from one reflection-model family, with posterior modes sampled by preconditioned sequential Monte Carlo. In the baseline recovered modes, disk-surface ionization increases with
photon index (Pearson r = +0.59), with state medians rising from log xi approximately 3.3 in the hard state to approximately 3.9 in the soft state. The corresponding free-emissivity fits give median inner radii of 6.6, 4.4, and 4.0 R_ISCO in the hard, intermediate, and soft states. Fixing q = 3 moves three of eight soft-state observations to the ISCO and one to 2.4 R_ISCO; four fixed-q fits have lower chi^2 than the sampled free-q solutions, showing that those runs missed higher-likelihood regions. The inferred radii are therefore model- and mode-dependent, and the spectra neither require nor exclude an ISCO disk or R_in greater than or approximately 20 R_ISCO. Baseline fitted abundances span A_Fe = 1.6-8.6, with a median of 4.9. Two observations separated by 7.2 hr yield A_Fe = 1.9 +/- 0.2 and 4.5 +/- 1.0, indicating that fitted abundance is not a direct composition measurement. Fixing A_Fe = 1.6 drives some densities toward the grid boundary and worsens the fits relative to A_Fe = 4.5. The wind parameters remain sensitive to the continuum, abundance, and orbital-phase sampling.
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All authors Kshitij Duraphe, Kartik Mandar, Chooda Khanal, Gopal Bhatta
High Energy Astrophysical Phenomena
A Population View of the Cosmic-Ray Knee: The Role of Variance in Supernova Maximum Rigidities
Carmelo Evoli
The broad shape of the
Galactic cosmic-ray knee challenges source models in which all
supernova remnants share a nearly universal, sharp maximum
rigidity. We investigate whether the knee can instead arise as a population effect, produced by source-to-source variations in the maximum energy of
Galactic supernova remnants. We derive the population-averaged spectrum for sources with sharp individual cutoffs and distributed $E_{\max}$, showing that it is given by an underlying propagated power law multiplied by the survival probability of the cutoff distribution. A lognormal distribution of $E_{\max}$ naturally produces a smooth, continuously curving knee, while a power-law tail gives an approximately constant post-knee steepening. We then connect the lognormal width to
supernova-remnant physics through maximum-energy scalings with explosion energy and ambient density, finding that the expected variance is mainly driven by the spread in explosion energies. Fitting the measured
proton spectrum with a two-component lognormal-cutoff model, we find that the PeV component requires $\sigma_{\log_{10}E_{\max}}\simeq 0.24$. This width is substantially smaller than the variance expected for the full
Galactic remnant population, indicating that the PeV component must originate from a more restricted and comparatively homogeneous subset of remnants. Our results show that the knee can be understood as the gradual exhaustion of a heterogeneous population of PeV-capable
supernova remnants, without requiring a universal maximum
rigidity.
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Instrumentation and Methods for Astrophysics
A Social Network Analysis of JWST General Observer Programs: The Emergence of a Decentralized Heterarchy
Christopher Williams
Developing proposals to execute programs on space
telescopes involves networks of astronomers coalescing around ideas and plans for observations. When aggregated, these program level networks allow a collective structure of the overall social network of astronomers using a
telescope to be created and analysed. We do this using program level investigator data over the first five cycles of accepted General Observer (GO) programs on the James Webb Space
Telescope (JWST). The aggregate network contains 5252 unique astronomers with 144740 connections between them based on their program level participation. We apply a modularity class coefficient to visualize the sub communities that evolved within the aggregate network. Ten dominant sub communities emerge that are shown to correlate at various levels with the JWST scientific categories as defined by the Space
Telescope Science Institute (STScI). These sub communities vary in size as well as diversity of countries and institutions represented. Analysis of the research interests of investigators within sub communities reveals that separation along the long axis of the graph is associated with the scale of science performed at community level (AU scale vs kpc Gpc scale). While the social network structures based on institutional affiliation and country of institution are highly centralized, the aggregate network at the investigator level is highly heterarchical and decentralized. It appears to be supportive of cross disciplinary interaction, and institutionalized integration between planetary system and
cosmic structure science that is wide and redundant, rather than mediated by a small set of critical brokers. Results have implications for access policy (time allocation processes on
telescopes leaving behind a legacy of heterarchical social networks) and access strategy (astronomer need to access networks before accessing
telescopes).
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Cosmology and Nongalactic Astrophysics
Redshift dependence and dipolar velocity corrections in cosmographic reconstructions through Type Ia supernova samples
Anna Chiara Alfano, Youri Carloni, Orlando Luongo et al. (+1 more)
We perform a cosmographic analysis of Type Ia
supernovae using the Pantheon+&SH0ES, DES-SNY5 and Union3 compilations. We consider different
redshift intervals, analyzed through a third-order Taylor expansion and a Padé $(1,2)$ approximation of the luminosity distance. First, we first infer the cosmographic parameters directly from the observed
supernova redshifts. Afterwards, we extend the analysis by including a dipole correction associated with the local peculiar velocity field, constraining both its amplitude and direction. For Pantheon+&SH0ES, the Cepheid calibration allows a direct determination of the Hubble constant, whereas for DES-SNY5 and Union3 we fix $H_0$ to set the absolute distance scale. By progressively increasing the maximum
redshift of the sample, we study how the inferred cosmographic parameters depend on the adopted
redshift interval. We find that the Hubble constant obtained from Pantheon+&SH0ES remains consistent with previous determinations for both cosmographic parameterizations. Instead, the agreement of the deceleration $q_0$ and jerk $j_0$ parameters with the $\Lambda$CDM values depends on the adopted compilation and improves mainly for Pantheon+&SH0ES as the
redshift interval is enlarged. Moreover, the reconstructed dipole parameters remain stable across the
redshift intervals considered, with velocity amplitudes of order $300\,\mathrm{km\,s^{-1}}$, for all three
supernova samples. Finally, fixing the dipole parameters with the
cosmic microwave background values leads to a slight shift from the fiducial $\Lambda$CDM values of the deceleration $q_0$ and jerk $j_0$ parameters.
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All authors Anna Chiara Alfano, Youri Carloni, Orlando Luongo, Francesco Pace
Solar and Stellar Astrophysics
Planetary systems in the light of asteroseismology: metallicity threshold for the planetary systems and age-metallicity relation
M. Y{\i}ld{\i}z, A. Demirkol, S. Örtel et al. (+2 more)
We compiled data for 127 hosts (plus six candidates) and used them as constraints to construct interior models of the hosts using the {\small MESA} code. Two significant conclusions emerge from these models. First, except for a few
stars, the hosts'
metallicity ($Z_0$) is greater than 0.007. This suggests a possible suppression of the occurrence of
planets below $Z_0\approx0.007$. Second, it concerns how chemical
evolution unfolds in the
galactic disc. For a given $Z_0$ value, considering the oldest
stars, there is a linear relationship between $Z_0$ and age ($t_9$). This line is around $t_9=13.4$ Gyr at $Z_0=0$, a value consistent with the age of the
Galaxy. The linear relationship continues until around $t_9=6$ Gyr, and the maximum value of $Z_0$ remains constant between $t_9=2-6$ Gyr. We further modelled 12 hosts classified as red clump (RC)
stars in the literature, explicitly accounting for mass loss along the red
giant branch. These models highlight the critical role of mass-loss assumptions in determining the initial masses and ages of RC hosts, and their implications for the survival and
evolution of close-in
planets. Another key outcome of this study is the discovery of the relationship between $Z_0$ and the observed
metallicity ($Z_{\rm s}$) for the hosts. We obtain a useful expression for $Z_0$, the input parameter for the models, as a function of
stellar mass, radius, and $Z_{\rm s}$. This expression can be used to estimate $Z_0$ based on the reduced surface
metallicity due to microscopic
diffusion. We also derive an expression for planetary mass relative to the orbital semimajor axis and host mass. This expression may indicate a mass distribution near the inner disc where these
planets formed, except for hot-Jupiters.
Planet radii appear to depend on the
planet's mass and irradiation energy, as well as the orbital period.
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All authors M. Y{\i}ld{\i}z, A. Demirkol, S. Örtel, T. Çak{\i}r Alsaç, T. Daylan
Astrophysics of Galaxies
The Low-Mass Baryon Cycle in QUEST Dwarf Galaxies I: Sample definition and first results
Ava Polzin, Hsiao-Wen Chen, Zhijie Qu et al. (+3 more)
We introduce the QUEST
Dwarfs program, which connects low-mass
galaxies'
stellar populations, interstellar medium, and circumgalactic medium in a large and coherently analyzed sample using a combination of optical spectroscopy, broadband imaging, and FUV absorption spectra of bright background sources .We present initial results from the first-release sample, comprising 14
galaxies with
stellar mass $M_\mathrm{star} \leq 10^9 M_\odot$ at $z\approx0.001-0.017$, each with at least one CGM absorption probe at projected distances $d_\mathrm{proj}\lesssim 100$ kpc. This representative sample triples the number of available probes within 1/3 of the
halo radius of
dwarf galaxies outside of the Local Group. We find that the total silicon column density declines much more rapidly with projected distance than Hi, implying that chemically enriched cool gas is preferentially concentrated in the inner CGM, while the increasing ionization fraction of hydrogen with radius likely enhances this contrast. Accounting for unobserved silicon in higher ionization stages, we infer total metal masses of $\log M_Z/M_\odot\approx4.8$ and $6.5$ in the cool CGM within $0.3 R_\mathrm{vir}$ for
dwarfs with median $\log M_\mathrm{star}/M_\odot=7.6$ and 8.6, respectively. These reservoirs correspond to $\approx3$% and $\approx16$% of the total metals produced over the
galaxies' lifetimes. More massive
galaxies also exhibit systematically stronger metal absorption, suggesting that projected distance governs the radial decline of metal absorption while
stellar mass sets the normalization of the CGM metal profile. Individual ions reveal a multiphase structure, with low-ionization species concentrated in the inner
halo and higher-ionization species extending farther. The full QUEST
Dwarfs survey will provide the statistical power needed to isolate the dominant drivers of CGM enrichment in low-mass
halos.
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All authors Ava Polzin, Hsiao-Wen Chen, Zhijie Qu, Erin Boettcher, Allison Strom, Tsang Keung Chan
High Energy Astrophysical Phenomena · Cross-listed from gr-qc
Effective metric for bound state in an effective-one-body theory based on the third-post-Minkowskian approximation
Hanjun Zou, Sheng Long, Xiaokai He et al. (+1 more)
The effective-one-body (EOB) framework, originally formulated within the post-Newtonian (PN) expansion, is central to modeling and interpreting gravitational-wave signals. Recent developments have incorporated the post-Minkowskian (PM) expansion into EOB theory. In this work, we focus on the bound state dynamics in the PM approximation up to the third order and construct the core ingredients of EOB: the effective metric. We examine two correspondence strategies, one based on the radial action variable and the other on the precession angle. We demonstrate that the radial action variable provides a consistent correspondence, which is further verified by the correspondence based on the precession angle. Building on these results, we adopt an isotropic gauge with a Schwarzschild-like parametrization to fix the remaining freedom. Within this parametrization, the effective metric coefficients are determined at 3PM order. Our results provide a consistent effective metric for the bound state dynamics.
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All authors Hanjun Zou, Sheng Long, Xiaokai He, Zhoujian Cao
Earth and Planetary Astrophysics
Chantal Hemmann, Simon Müller and Ravit Helled
The interior structure and bulk composition of
giant planets are not directly observable and must be inferred from models. Under the common assumption of a well-mixed, adiabatic envelope, the measured atmospheric
metallicity is taken as a proxy for the
metallicity of the entire envelope, and hence for the
planet's heavy-element budget. JWST now provides precise atmospheric
metallicities for a growing number of warm
giants, allowing this assumption to be tested for the first time. We quantify the difference between envelope and bulk
metallicities of warm
giants to assess the evidence for compositional stratification. We assembled eleven warm
giants with atmospheric
metallicities from published JWST
retrievals, computed tailored interior and thermal
evolution model grids for each, and performed MCMC
retrievals to infer the bulk
metallicity consistent with the measured mass, radius, system age, and atmospheric
metallicity. Envelope
metallicities are smaller than bulk
metallicities throughout the sample, with mixing ratios from about 0.02 to 0.90. Eight of the eleven
planets have mixing ratios below 0.50, and ten are inconsistent with a fully mixed interior to within one sigma. We tentatively identify a significant anti-correlation between planetary mass and envelope
metallicity, but no correlation between envelope and bulk
metallicity, nor between envelope and host-
star metallicity. Atmospheric
metallicity is therefore not a reliable proxy for the bulk composition of warm
giants, and incomplete mixing (possibly composition gradients) appears common among the
planets accessible to JWST. Bulk composition estimates assuming a homogeneous envelope substantially underestimate the total heavy-element mass. That the
solar-system
giants are unremarkable within this sample suggests dilute or partially mixed interiors may be a generic outcome of
giant planet formation.
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High Energy Astrophysical Phenomena
Year-timescale changes in AGN radio luminosity as seen by the ASKAP Variables and Slow Transients Survey
Chloe Klare, Charlotte Ward, Dougal Dobie et al. (+6 more)
A few dozen previously
radio-quiet active
galactic nuclei (AGN) have been observed to transition to
radio-loud at 1-3 GHz frequencies over timescales of more than a decade, and this has has been interpreted to be due to newly launched jets. We identified 101 compact
radio sources out of a sample of 64,972 non-blazar AGN which increased in flux density by 80-1800% over 1-6 years in the 887.5 MHz Australian SKA Pathfinder Variables and Slow Transients survey. We obtained optical spectra and
radio SEDs using new observations and archival survey data. We determined 60 sources were consistent with extrinsic variability due to refractive interstellar scintillation and 41 were variable due to intrinsic causes, with 26 continuously brightening and two transitioning from
radio-quiet to
radio-loud. We concluded that young
radio jets launched by either
tidal disruption events or changes in the
accretion properties were responsible for the continuously brightening AGN. These sources were non-variable at higher frequencies over the same time period, as expected for an expanding emission region. Fourteen sources had inverted or peaked SEDs initially which either flattened below the turnover or evolved into steep SEDs, consistent with young, expanding jets. Twelve sources had non-variable steep or gigahertz-peaked SEDs, which suggested these hosted more slowly evolving jets. We investigated the previously discovered AGN with newly launched jets, and found several have faded at multiple frequencies, which suggested the observed
radio-loudness was temporary, rather than the onset of a sustained period of
radio activity.
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All authors Chloe Klare, Charlotte Ward, Dougal Dobie, David Kaplan, Akash Anumarlapudi, Tara Murphy, Joshua Pritchard, Laura N. Driessen, Igor Andreoni
Earth and Planetary Astrophysics
The Viability of Life in Helium-Dominated Exoplanet Atmospheres
Sara Seager and Janusz J. Petkowski
Helium is the second most abundant element in the Universe, yet helium-dominated atmospheres are rarely considered as environments for life. Two assumptions have contributed to this neglect: helium is expected to escape from rocky
planets together with hydrogen, and Earth itself lacks a substantial helium envelope. Recent observations and atmospheric
evolution models, indicate that rocky
exoplanets can retain helium-dominated atmospheres. Here we make the case for life in such environments by synthesizing a largely overlooked body of laboratory research on life in helium-dominated atmospheres. Every organism studied, including bacteria, fungi, protozoa, microalgae, plants, and animals tolerates helium-dominated atmospheres when supplied with essential metabolic requirements. Across a range of pressures, helium has shown no demonstrated toxicity and no fundamental barrier to metabolism,
cell division, photosynthesis, nitrogen fixation, or coordinated multicellular activity, including in humans. Helium-dominated atmospheres therefore constitute a viable and underrecognized planetary environment for life. Helium is chemically inert and spectroscopically unobtrusive, so the bulk atmosphere would neither contribute to the chemical destruction of biosignature gases nor strongly obscure their spectral features. Helium-dominated atmospheres also have low mean molecular weights and large scale heights, strengthening spectral features in transmission and improving remote detectability.
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Astrophysics of Galaxies
The reddening of NGC 7469 and evidence for variable extinction
C. Martin Gaskell, Ananya R. Kamath, Emily I. Kwan et al. (+1 more)
We estimate the reddening of the broad-line region and continuum of the active
galactic nucleus NGC 7469 during the 1996 International AGN Watch multi-wavelength monitoring campaign using seven different reddening indicators. As was found for NGC 5548, these indicators support velocity-integrated broad hydrogen line ratios being close to Case B values. All the indicators point to substantial reddening of E(B-V) = 0.44 +/-0.03 for NGC 7469 during mid-1996. We find evidence for a gradual increase of about 10% in the reddening during the seven weeks of the 1996 campaign. Decades-long optical monitoring before and after the campaign is also consistent with modest changes in the extinction from year to year.
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All authors C. Martin Gaskell, Ananya R. Kamath, Emily I. Kwan, Divya G. Subramonian
Cosmology and Nongalactic Astrophysics
Modelling the nonlinear matter power spectrum in Hu-Sawicki $f(R)$ gravity with the Web-Halo Model
Xin Wang and Zhiqi Huang
We develop a semi-analytic extension of the Web-
Halo Model (WHM) to Hu-Sawicki $f(R)$
gravity, with the aim of linking nonlinear matter clustering to the successive stages of
cosmic-web collapse. The cylindrical sheet and filament windows of the original WHM are replaced by axisymmetric ellipsoidal top-hat windows, yielding a modest improvement around the perturbative-to-nonlinear transition without introducing additional fitting parameters. Modified
gravity is incorporated through environment-dependent chameleon spherical collapse, from which the collapse threshold and virial overdensity are obtained and propagated through the sheet, filament, and
halo contributions. For $|f_{R0}|=10^{-5}$ and $10^{-6}$, the predicted nonlinear enhancement is broadly consistent with the scale-,
redshift-, and field-strength dependence seen in the e-MANTIS emulator, with closer agreement for the weaker field and at higher
redshift. The component responses show that sheets and filaments contribute substantially around the transition regime, while the
halo response becomes increasingly important at smaller scales. As complementary diagnostics, no displacement of the BAO peak is resolved, whereas tomographic weak-
lensing spectra retain a percent-level response to the modified matter power spectrum. The extended WHM therefore provides a physically interpretable framework for tracing screened modified-
gravity effects across the
cosmic-web collapse hierarchy.
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Astrophysics of Galaxies
Multiphase turbulence as the origin of OH+, H2O+ and H3+ column density scatter in the local ISM
Uri Malamud, Shmuel Bialy, Benjamin Godard et al. (+2 more)
Observations of the reactive ions OH+, H2O+ and H3+ in the
Galactic interstellar medium reveal large sight-line-to-sight-line scatter in their column densities, commonly interpreted as evidence for substantial variations in the
cosmic-ray ionization rate (CRIR). We revisit this interpretation using high-resolution three-dimensional magneto-hydrodynamic simulations of the multiphase ISM with time-dependent chemistry for H, H2, H+ and electrons, building on the fiducial model of Godard et al. (2023). We find that a single CRIR of ~2 10^{-16} s^{-1}, together with standard
Galactic-scale parameters, naturally produces broad column-density distributions for all three tracers in good agreement with the observed medians and percentile widths, with no fine tuning. Reaching this match requires that the post-processing of OH+, H2O+ and H3+ retain the time-dependent H2 field generated by the turbulent flow rather than assume chemical
equilibrium:
turbulence drives long-lived H2 enhancements in the unstable neutral medium where OH+ and H2O+ predominantly reside, and an
equilibrium treatment under-predicts their columns substantially. H3+, which receives most of its column from denser CNM gas closer to
equilibrium, is much less affected. Our results caution against interpreting sight-line-to-sight-line scatter as direct evidence for large CRIR fluctuations, and motivate a shift from independent 1D
equilibrium analyses toward 3D dynamical frameworks when inferring ionization conditions in the ISM.
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All authors Uri Malamud, Shmuel Bialy, Benjamin Godard, David Neufeld, Blakesley Burkhart
Astrophysics of Galaxies · Cross-listed from gr-qc
Nonlinear Density Waves and a Galactic-Scale Estimate of the Graviton Mass
Miroslava Vukcevic
In February 2016 the LIGO and Virgo collaborations reported the discovery of gravitational waves in merging black holes, with the conclusion that their observational data did not show any violation of general relativity. The joint LIGO and Virgo team presented an upper limit on the graviton mass, using tests of modified gravitational dynamics. Yukawa-type gravitational potential corrections are often used as generic finite-range parametrisation in modified
gravity contexts, including massive-
gravity inspired models. Yukawa corrections are derived by semi-empirical approach, in the framework of f(R)
gravity, a type of modified
gravity which generalises Einsteins general relativity. Contrary to this approach, we use a classical Newtonian gravitational potential corrected by nonlinear effects in order to obtain a characteristic wavelength of the nonlinear density wave. This wavelength is then phenomenologically identified with an effective graviton Compton wavelength, leading to a model-dependent estimate of an effective graviton mass within the
galaxy scale. The presented approach is independent from other methods published until now.
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Astrophysics of Galaxies
Theoretical emission lines and metallicity calibrations of H II regions in ASTRID simulation
Yao Yao, Kathryn Grasha, Stuart Wyithe et al. (+5 more)
We present a theoretical framework to derive
redshift-dependent
metallicity calibrations for
galaxies at $z$=2-7. The ionization parameter ($U$) and gas pressure ($P$) in our approach are not assumed, but are predicted self-consistently. By combining the ASTRID cosmological simulation with
stellar population synthesis (SPS) and MAPPINGS V photoionization modeling, we evolve young
star clusters under an analytic wind-driven bubble model. This directly couples
stellar feedback to the local ISM density, allowing \hii{} region properties to emerge from the underlying physics rather than being treated as free parameters. The emission-line predictions are validated against observed
star-formation rate indicators (deviation <0.05 dex) and the \oiii{} luminosity function. We derive calibrations for common optical (e.g. R23, O3N2, N2, O32) and UV (e.g. C3O3, N3O3) diagnostics. We find significant
redshift evolution in these relations, driven primarily by changing ionization conditions. A
Bayesian analysis quantifies calibration performance under varying signal-to-noise, enabling diagnostic recommendations as a function of
redshift and data quality. The R23 calibration performs well at all
redshifts with minimal error in our model, while nitrogen- and carbon-based calibrations are highly sensitive to the abundance enrichment process and should be used with caution. These results provide a practical framework for interpreting JWST spectroscopy and tracing chemical
evolution from
cosmic noon to the epoch of reionization.
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All authors Yao Yao, Kathryn Grasha, Stuart Wyithe, Enci Wang, Nianyi Chen, Patrick Lachance, Tiziana Di Matteo, Yihao Zhou
Cosmology and Nongalactic Astrophysics
Addressing position anomalies in the Strong Gravitational Lensing System HS~0810+2554 through Dark Matter Subhalos
Yuanlin Gong, Lei Wu, Qiang Yuan et al. (+1 more)
Self-bounded dark matter (DM) subhalos are predicted to populate
galactic halos in great abundance in the Cold Dark Matter (CDM) scenario. These substructures can leave observable imprints in strong gravitational
lensing and have shown the ability to account for flux-ratio and position anomalies in multiply imaged systems. In this paper, we utilize the DM subhalos to address the image position anomalies of the two
radio quads of HS 0810+2554 observed with the Very Long Baseline Interferometry. We model the lens using an elliptical power-law macro-lens supplemented by a population of CDM subhalos from numerical simulations and perform a dual-source reconstruction to fit all eight
radio images simultaneously. We find that subhalos below $10^{6}M_\odot$ induce astrometric shifts smaller than the measurement uncertainties, whereas more massive subhalos naturally generate the required milliarcsecond perturbations without significantly altering the global lens configuration. Including CDM subhalos improves the fit from $\chi^2=60.38$ for the pure macro-lens to $\chi^2=1.61$. Our results show that the position anomalies of HS~0810+2554 can be explained within the CDM framework and do not by themselves necessarily require non-standard scenarios like fuzzy DM or angular complexity in the macro-lens. Instead, they provide a sharp and testable manifestation of the subhalo population predicted by CDM.
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All authors Yuanlin Gong, Lei Wu, Qiang Yuan, Tengyuan Zhang
High Energy Astrophysical Phenomena
Strangeness Transport in Binary Neutron Star Mergers
Melvin Storbacka, Jiaxi Wu, Alexander Haber et al. (+5 more)
The presence of hyperons in the cores of
neutron stars opens fast strangeness equilibration channels that can produce bulk-viscous dissipation during binary inspiral. Because these reactions coexist with electron $\beta$-equilibration,
tidal compression can drive the two coupled chemical imbalances far beyond linear response. We construct the first reaction network that self-consistently evolves the electron and strangeness fractions with a four-dimensional strangeness-dependent
chiral mean-field (CMF) equation of state, including nucleonic and hyperonic Urca processes and non-leptonic hyperon reactions. For periodic density perturbations, representative of inspiral oscillations, we find that rapid strangeness conversion can generate a large $\beta$-imbalance, after which slow $\beta$-equilibration bottlenecks strangeness relaxation. Rather than decaying exponentially, the coupled system consequently exhibits dynamically important algebraic decay in a far-from-
equilibrium regime. At the $\rm keV$ temperatures expected during inspiral, this nonlinear response produces a broad enhancement of the effective bulk viscosity, reaching $\sim10^{31}\,\mathrm{g\,cm^{-1}\,s^{-1}}$ for $320$ Hz oscillations. A phenomenological estimate of continuous inspiral dissipation yields gravitational-wave phase shifts up to $\sim0.14$ rad for
neutron stars with hyperonic cores. Self-consistent, far-from-
equilibrium strangeness transport may therefore provide a dynamical probe of hyperons in
neutron-
star interiors.
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All authors Melvin Storbacka, Jiaxi Wu, Alexander Haber, Elias R. Most, Jacquelyn Noronha-Hostler, Mateus Reinke Pelicer, Nikolas Cruz-Camacho, Veronica Dexheimer
Earth and Planetary Astrophysics
Plasma Turbulence in the Lunar Environment Across Solar Wind and Magnetotail Conditions: Observations from Chandrayaan-2 Radio Science experiment
Keshav Aggarwal, R. K. Choudhary, Abhirup Datta et al. (+1 more)
The Moon's transit between the
solar wind and Earth's magnetotail exposes the near-lunar environment to large and rapid variations in
plasma density and flow structure. Two-way coherent S-band
radio occultation measurements from Chandrayaan-2 were used to quantify electron-density fluctuations integrated along the Earth-Moon line of sight. Observed frequencies were processed to remove
geometric Doppler contributions derived from relativistic light-time modeling. The remaining frequency residuals represent the cumulative effect of
plasma irregularities along the ray path. Power spectral densities were computed for 54 intervals from 2022, yielding temporal spectral indices in the range $1.05 \le \alpha \le 2.66$, corresponding to spatial indices $4.05 \le p \le 5.66$ indicating ion-kinetic and dissipation-range scales. Of the 54 intervals, 9 were classified as inside the modeled magnetopause, 17 in the bow shock/magnetosheath, and 28 in the
solar wind. Spectral indices measured inside the magnetopause are marginally higher than those in the bow shock and
solar wind, though the difference is not statistically significant given the limited magnetotail sample. No measurable correlation is found between spectral slope and geomagnetic activity, indicating that the observed variability is dominated by local
plasma structure rather than inner-magnetospheric conditions.
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All authors Keshav Aggarwal, R. K. Choudhary, Abhirup Datta, Anshuman Sharma
High Energy Astrophysical Phenomena
Polarization of GRB standard X-ray afterglow and its detection prospects by eXTP
Jun-Xi Lin, Zelin Ren, Mi-Xiang Lan et al. (+4 more)
The polarization signatures of Gamma-ray Burst (GRB) afterglows serve as a powerful diagnostic tool for studying their environments and jet physics. This work systematically investigates the X-ray (2–8~keV) polarization properties of standard GRB afterglows and assesses their detectability with the Polarimetry Focusing Array aboard the enhanced X-ray Timing and Polarimetry (eXTP) satellite. A Morris global sensitivity analysis is first conducted to identify the dominant parameters, which are then assigned observationally motivated probability distributions. In particular, the isotropic energy, half-opening angle, and initial Lorentz factor are sampled jointly via a Gaussian copula to reproduce the empirical Ghirlanda and Liang correlations. Monte Carlo simulations of $10^{3}$ afterglows are performed and validated against the observed 10~keV flux distributions of a selected
Fermi–Swift sample (K–S $p = 0.29$ at $10^{3}~\mathrm{s}$ and $p = 0.18$ at $10^{4}~\mathrm{s}$). The simulations yield an overall polarization event rate of $\lesssim 1.5\%$ for standard GRB X-ray afterglows with eXTP/PFA, reflecting the intrinsically low polarization produced by a random
magnetic field confined to the shock plane. The optimal detection window occurs near the jet break at late times, when the PD peaks. For exceptionally luminous events such as GRB~221009A, however, the PD remains above the MDP over the full interval $10^{3}$–$10^{6}~\mathrm{s}$, demonstrating that eXTP/PFA can capture nearly the entire polarization
evolution for such rare, bright bursts.
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All authors Jun-Xi Lin, Zelin Ren, Mi-Xiang Lan, Ming-Yu Ge, Zi-Gao Dai, Xue-Feng Wu, Shuang-Nan Zhang
High Energy Astrophysical Phenomena
Jet Power, Bulk Lorentz Factor, Black Hole Spin, and Magnetic Field of Accretion Disk in Jetted Active Galactic Nuclei: A Large Gamma-Ray Emission Sample
Dingrong Xiong, Junhui Fan, Feng Yuan et al. (+14 more)
We present a catalog of physical parameters for powerful jet-
accretion disk-black hole systems in one of the largest samples of gamma-ray emitting jetted active
galactic nuclei (AGNs), including jet kinetic and radiative powers, jet radiative efficiencies, bulk Lorentz factors, black hole
spins,
accretion-disk
magnetic fields and Compton dominance. Comparing jet kinetic power
estimators for blazars, values derived from spectral energy distribution (SED) fitting tend to exceed those estimated via cavity power and other scaling relations. For radiatively efficient AGNs, most sources are inferred to possess high
spins; for radiatively inefficient AGNs, many potentially have high
spins, though some may differ. This indicates that black hole
spin does not effectively distinguish radiatively efficient from inefficient jetted AGNs. Our results suggest
accretion-disk
magnetic field strength as a key discriminator, proposing a tentative dividing value of $\approx 10^{3.9}$ Gauss between radiatively efficient and inefficient populations. Jet power and bulk Lorentz factor exhibit significant correlations with black hole mass in radiatively efficient AGNs, while weak-to-moderate correlations are observed in radiatively inefficient AGNs within narrow
accretion-rate bins. Our analysis reveals that jet power correlates with both disk luminosity and
magnetic field strength. Furthermore, correlations linking Eddington ratio and Compton dominance with jet properties are consistent with the jet-
accretion connection. Finally, jet radiative power and bulk Lorentz factor show a potential dependence on black hole
spin. These results are consistent with the scenario in which jets are powered and accelerated by energy extraction from rapidly spinning black holes via
accretion-disk
magnetic fields.
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All authors Dingrong Xiong, Junhui Fan, Feng Yuan, Jun-Xian Wang, Minfeng Gu, Yongquan Xue, Jirong Mao, Liang Chen, Rui Xue, Xu-Liang Fan, Yongyun Chen, Nan Ding, Fei Guo, Jia-Wen Li, Dahai Yan, Y. G. Zheng, Jinming Bai
High Energy Astrophysical Phenomena
Interpretations of the $10\%$ polarization observed in the early forward-shock afterglow of GRB 091208
Yang liu and Mi-Xiang Lan
The $\sim10\%$ optical polarization observed at the early stage of GRB 091208B comes from the forward shock emission, which is higher than the conventionally predicted value. Polarizations of the forward shock radiation would depend on the observational geometry and the post-shock
magnetic field structure. This
magnetic field could arise either from the compression of a pre-existing
magnetic field (i.e., the
magnetic field in the outer medium) or from the shock-generated instabilities. In this paper, we use a synchrotron radiation model to fit the light curve and polarization observations of GRB 091208B. Two scenarios are considered: one is the case of a slightly off-axis observer, and the other is with a large-scale ordered
magnetic field component in the burst environment. We found both scenarios could interpret the observations of GRB 091208B. For the slightly off-axis observation scenario, the observational angle is restricted to be within the range of (1.02, 1.05) times the jet half-opening angle. For the large-scale ordered
magnetic field component scenario, the ratio between the ordered component to the random component is constrained to be around 1.
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High Energy Astrophysical Phenomena
The detection prospects of the polarizations in the plateau phase of GRB afterglow by eXTP
Shu-Ting Wu, Mi-Xiang Lan, Zelin Ren et al. (+3 more)
Approximately (20-50)$\%$ of the gamma-ray burst (GRB) X-ray afterglows exhibit the shallow decay features. Two popular energy-injection models had been proposed to interpret such observational phenomenons, the relativistic wind bubble (RWB) model with a Poynting-flux injection and the structured ejecta (SE) model with a dynamical energy injection. Polarization predictions of the two models had been investigated and can be used as a test of the two models. However, the impacts of the parameters on the model predictions were not studied and the comparisons with the detection ability of the forthcoming mission, enhanced X-ray Timing and Polarimetry (eXTP), had not been discussed. We considered the above issues and found that influences of the model parameters on the predicted polarizations of the two models are very limited. To perform a feasible polarization detection during the plateau phase, the priority ToO response is required. The detection probability of the GRB plateau phase is about $1/3$ for one pointing under the priority ToO. The polarization detection probability would depend on the ratio between the Poynting-flux injection to the dynamical energy injection, which is unclear currently. The predicted flux density and polarization degree (PD) of the RWB model could be well above the threshold flux and minimal detectable polarization degree of the polarimetry focusing array (PFA) on board eXTP, while the predicted PDs of the SE model would be difficult to be detected by eXTP/PFA. Therefore, a detection of a significant polarization signal during the GRB plateau phase would prefer the RWB model and the injected energy would be in the form of the Poynting flux, while a non detection of the polarized signal would indicate a dynamical energy injection of the SE model.
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All authors Shu-Ting Wu, Mi-Xiang Lan, Zelin Ren, Ming-Yu Ge, Xue-Feng Wu, Zi-Gao Dai
Cosmology and Nongalactic Astrophysics
Observational Manifestations of Primordial Objects in the Early Universe Through the Hydrogen Subordinate Lines
Viktor K. Dubrovich, Yury N. Eroshenko and Stanislav I. Shirokov
A new mechanism for the formation of spectral–spatial distortions in
cosmic microwave background radiation near
primordial massive compact objects (such as
primordial black holes) at
redshifts from $z \sim 1000$ to $\sim$100 is proposed. After hydrogen recombination, the radiation from these objects leads to a significant increase in the population of hydrogen subordinate levels in their surrounding environment. Consequently, this allows for the observation of a Fraunhofer-like absorption spectrum in the
cosmic microwave background. Such a distortion is formed due to the temperature difference between matter and relic radiation at the corresponding epoch. Ultimately, we should observe circular absorption or emission features around the objects with small angular sizes. These subordinate hydrogen lines currently lie in the
radio wavelength range. Estimates indicate that the effect under consideration is accessible for the observations with planned large radiotelescopes. The possibility of the proposed mechanism lies in the ability of an object (e.g.,
accretion disk around black hole) to emit few-eV
photons that populate the $n=2,3,4$ and higher levels of hydrogen, enabling the required excitation.
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