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Physics Astrophysics

astro-ph Watch Astrophysics

Friday 26 June

Cosmology and Nongalactic Astrophysics

HIcosmo: a differentiable JAX-based framework for cosmology inference

Jing-Zhao Qi, Jing-Fei Zhang and Xin Zhang

The Stage IV cosmological surveys, such as Euclid, LSST, DESI, and SKA, will deliver observational data of unprecedented volume, calling for efficient and reliable inference tools. This paper presents HIcosmo (High-performance Inference for Cosmology), an open-source JAX-based framework for cosmology inference. In HIcosmo, the forward model, distance integrals, likelihood evaluations, posterior sampling, and Fisher forecasts are all built from JAX primitives, so that gradients and Hessians of the log-likelihood are obtained directly by automatic differentiation, without any finite-difference approximation. The framework implements the $\Lambda$CDM, $w$CDM, $w_0 w_a$CDM, and interacting dark-energy models, and provides likelihoods for Type Ia supernovae (Pantheon+, DES-SN5YR, Union3), baryon acoustic oscillations (DESI DR1/DR2, SDSS), Planck 2018 distance priors, local $H_0$ measurements, and strong-lensing time delays. Its scope is restricted to background cosmology, with Boltzmann solvers and full perturbation-level likelihoods left to external tools. We validate HIcosmo against the reference implementation of each likelihood and against Cobaya. $\chi^2$ values agree to absolute differences of $10^{-6}$-$10^{-2}$, and the marginalized constraints from the two codes differ by less than $0.2\sigma$ in every analysis tested. Leveraging just-in-time compilation and automatic differentiation, HIcosmo achieves about $8.7\times$ the end-to-end sampling throughput of Cobaya on CPU. As the dataset grows to survey scale, GPU acceleration over CPU reaches up to $20\times$. As applications, we present multi-probe $\Lambda$CDM joint constraints, dark-energy equation-of-state constraints, and Fisher forecasts for six 21 cm intensity-mapping surveys, including SKA1, MeerKAT, BINGO, Tianlai, and CHIME.
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Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc

Gravitational-wave responses and noise correlations in geometric time-delay interferometry

Rui Luo, Pan-Pan Wang, Lin-Lin Yang et al. (+2 more)

Space-based gravitational-wave observations will probe the formation and growth of massive black holes and the evolution of compact binaries in the millihertz band. We develop a geometric-link representation of time-delay interferometry using six round-trip and six non-round-trip two-link blocks. Starting from the one-way Doppler response, we derive the elementary block responses and a dictionary that expresses 45 known second-generation combinations in terms of these blocks and residual one-way measurements. Representative Michelson and Relay observables admit compact forms, from which analytical sky-averaged tensor responses are obtained for a fixed, equal-arm constellation. We also derive the noise covariance of the blocks, retaining cross-spectra between blocks that share measurement streams. The resulting noise spectra agree with direct one-way assembly. We apply this formulation to a narrowband compact-binary signal in an equal-arm LISA instrument model. For the Relay example, neglecting the shared-link cross-spectrum changes the low-frequency noise scaling and overestimates the conditional Fisher uncertainties in amplitude or distance by factors of 10.71 at 1~mHz and 3.37 at 3~mHz.
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All authors Rui Luo, Pan-Pan Wang, Lin-Lin Yang, Xin-Lei Zhao, Cheng-Gang Shao

Cosmology and Nongalactic Astrophysics

Searching for primordial features with radio surveys: synergy between the power spectrum and bispectrum

Dionysios Karagiannis, Mario Ballardini and Roy Maartens

We present a comprehensive forecasting framework to assess the detection of primordial oscillatory features by exploiting the synergy between future neutral hydrogen (HI) intensity mapping (IM) surveys and cosmic microwave background (CMB) measurements. Focusing on next-generation single-dish (SKAO) and interferometric (HIRAX) radio configurations, we perform a joint analysis of the redshift-space power spectrum and bispectrum, consistently incorporating scale-dependent bias, redshift-space distortions, and non-Gaussian covariance. We investigate phenomenological templates with linear and logarithmic primordial oscillations, together with a physically motivated sharp-feature model. We find that including the large-scale structure bispectrum improves marginalised constraints on feature amplitudes by $30$–$40\%$ relative to the power spectrum alone and helps break parameter degeneracies. In several cases, the bispectrum contains more information on the power-spectrum feature parameters than the power spectrum itself. Much of this gain arises from the late-time gravitational contribution, which inherits the oscillatory structure of the primordial feature signal and acts as an independent source of information. While the CMB angular power spectrum is crucial for constraining low oscillation frequencies, joint interferometric IM analyses ($P+B$) outperform CMB amplitude constraints by up to $75\%$ in the linear regime. We also show that, despite non-Gaussian covariance degrading the independent constraining power of the bispectrum by $55$–$90\%$, the combined HI+CMB probe achieves percent-level precision on the frequency of primordial features, providing a powerful test of non-slow-roll inflation.
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Earth and Planetary Astrophysics

Demographics of planet-forming disks with the SKAO

Antonio Garufi, Sebastián Pérez, John D. Ilee et al. (+21 more)

Understanding how solid material in planet-forming disks evolves from micron-sized dust to planetary cores is a central challenge in modern astrophysics. This study has advanced dramatically in the past decade, largely driven by ALMA and high-contrast imaging facilities. However, major uncertainties remain regarding the presence, evolution, and role of centimeter-sized grains (the pebbles) in planet formation. The SKAO will fill this gap by enabling the first large-scale, high-resolution survey of disk emission at centimeter wavelengths. This chapter presents the scientific rationale and observational strategies to detect and characterize pebbles in the planet-forming disks of nearby star-forming regions. By resolving their spatial distribution, spectral properties, and evolutionary trends, SKA will offer essential constraints on dust growth and disk dynamics. This work provides observational strategies, target selection, and predictions on the detectability of hundreds of nearby disks. The chapter also explores SKA's potential to uncover the actual dust mass in disks, protoplanets and their circumplanetary disks, and other aspects of the planet formation. Together, these capabilities will establish SKAO as a cornerstone facility for planet formation science in the coming decade.
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All authors Antonio Garufi, Sebastián Pérez, John D. Ilee, Daniel J. Price, Paola Pinilla, Marion Villenave, Eleonora Bianchi, Luca Cacciapuoti, Greta Guidi, Giovanni Sabatini, Yinhao Wu, Asmita Bhandare, Claudio Codella, Nicolás Cuello, Liton Majumdar, Mayank Narang, Linda Podio, Danae Polychroni, Isaac Radley, Jessica Speedie, Leonardo Testi, Claudia Toci, Diego Turrini, David Wilner

Solar and Stellar Astrophysics

Southern Massive Stars at High Angular Resolution (SMaSH+): Properties of hierarchical massive triples

E. Bordier, H. Sana, A. J. Frost et al. (+9 more)

While massive stars are frequently found in triple architectures, the lack of observed parameter distributions has long remained a bottleneck for statistical models of their evolution. We compile the first representative set of physical and orbital distributions for main-sequence hierarchical massive triples. We present a homogeneous analysis of 26 O-type hierarchical triples identified in the SMaSH+ survey by combining spectroscopic data for inner binaries with interferometric and aperture masking detections of tertiary companions within $\sim$200 au. We derive the distributions of masses, mass ratios, and separations, and investigate their joint probability density functions. We assess the dynamical stability of these systems and estimate the relative importance of secular processes by comparing the von Zeipel-Kozai-Lidov (ZKL) timescale to the general relativistic precession timescale for five systems with well-constrained orbital solutions. Finally, we evaluate the observational completeness. The sample is dominated by strongly hierarchical configurations, consisting primarily of tight inner spectroscopic binaries(a_in< 1 au) and wider tertiaries (a_out/a_in $>$ 70 for most systems). We find no significant correlation between tertiary mass and either inner-binary mass or outer separation, indicating a broad diversity of system architectures. Ten systems host relatively massive tertiaries (q_out>0.5), especially at closer outer separations (a_out$\lesssim$30 au). For two to four systems out of five, general relativistic precession dominates over ZKL oscillations in their current configuration. These results provide the first observationally grounded distributions of key parameters for massive hierarchical triples and offer important constraints for population synthesis and evolutionary models, particularly regarding the role of tertiary companions in shaping binary evolution.
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All authors E. Bordier, H. Sana, A. J. Frost, A.-S. Libert, J. Vrancken, L. Mahy, S. Toonen, F. Tramper, A. de Koter, S. Lacour, J.-B. Le Bouquin, W.-J. de Wit

High Energy Astrophysical Phenomena · Cross-listed from gr-qc

Joint inference of line-of-sight acceleration and orbital eccentricity in neutron-star–black-hole binaries

Lorenzo Pompili, Aldo Gamboa and Alessandra Buonanno

A line-of-sight acceleration (LOSA) of a compact-binary center of mass, imparted for example by a nearby tertiary perturber, imprints a Doppler modulation on the gravitational-wave signal and provides a single-event diagnostic of dynamical formation environments. Waveform-modeling systematics – missing higher-order modes, spin precession, or orbital eccentricity – can mimic or mask a non-zero LOSA, making waveform accuracy a leading concern for LOSA inference. We implement LOSA corrections directly in the time domain as a remap of the strain: the treatment applies uniformly to all mode content, spin precession, and orbital eccentricity, and integrates naturally with time-domain inspiral-merger-ringdown models that best capture these effects. We deploy it in the SEOBNRv6EHM (aligned-spin eccentric) and SEOBNRv5PHM (precessing quasi-circular) models, and validate the implementation on simulated signals; we find that SEOBNRv6EHM recovers LOSA correctly on both eccentric and spin-precessing injections, while SEOBNRv5PHM yields a spurious LOSA measurement on eccentric signals. Motivated by the eccentricity hints in the neutron-star–black-hole (NSBH) event GW200105_162426 and the favorable low-mass regime of these sources, we jointly infer LOSA and orbital eccentricity for the five NSBH events in the LIGO-Virgo-KAGRA catalog, reporting the first LOSA constraints on three of them. All five events are consistent with a vanishing LOSA, $\Gamma \equiv a_\parallel/c = 0$; for GW200105_162426, the joint $(\Gamma, e)$ posterior nonetheless disfavors both $\Gamma$ and $e$ being zero simultaneously at 90% credibility, supporting the eccentricity hints reported in previous analyses.
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Cosmology and Nongalactic Astrophysics

The effect of dark energy on the void-halo perpendicular alignments

Geonwoo Kang and Jounghun Lee

We report a numerical discovery that in a more rapidly accelerating spacetime, the galactic halos on void surfaces develop stronger perpendicular alignments with the directions toward the void centers. We utilize the halo catalogs from the AbacusSummit suite of simulations for $10$ different cosmologies that include one Planck $\Lambda$CDM, four $w$CDM and five $w_{0}w_{a}$CDM, which share the identical initial conditions except for the dark energy equation of state. For each cosmology, we identify the voids and void-surface galactic halos at $z=0.1$ and determine the probability density functions of the cosines of the angles, $p(\cos\theta)$, between the shape axes of void-surface galactic halos and the directions toward the void centers. The numerically obtained $p(\cos\theta)$ is fitted to an analytic single-parameter formula derived through an empirical modification of the linear perturbation theory. Eliminating spurious signals caused by the differences in the mass and sphericity distributions of void-surface galactic halos among different cosmologies, we detect a clear net effect of dark energy on the strengths of the perpendicular alignments of void-surface galactic halos, quantified by the single parameter, $d_{t}$. Noting that $d_{t}$ has higher values in the cosmologies where dark energy has more negative pressure and evolves more rapidly, we put forth a bilinear model for the difference in $d_{t}$ between the two cases of $w=-1$ and $w\ne -1$. Demonstrating that this bilinear relation excellently describes the numerical results, we conclude that the perpendicular alignments of void-surface galactic halos should in principle be a powerful independent indicator of the dynamic nature of dark energy.
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Astrophysics of Galaxies

Deep HI observations of cold gas inflow and outflow

W.J.G. de Blok, D.B. Fisher, K. Tachihara et al. (+8 more)

A major question in galaxy evolution is how galaxies acquire sufficient gas to sustain their star formation rates. HI observations with high angular resolution and sensitivity to very low column densities are some of the important observational ingredients that are currently still missing. Answers to these questions are necessary for a correct interpretation of observations of galaxy evolution in the high-redshift universe and will provide crucial input for the sub-grid physics in hydrodynamical simulations of galaxy evolutions. In this chapter we discuss the progress that has been made over the past years, describe the various processes that lead to inflow and outflow of gas, and discuss how SKA-Mid AA4 observations can contribute to further understanding these important aspects of galaxy evolution using deep observations of nearby individual disk and dwarf galaxies.
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All authors W.J.G. de Blok, D.B. Fisher, K. Tachihara, F.M. Maccagni, J. Wang, R. Enokiya, L. Chemin, T. Hayakawa, D. Kleiner, D.J. Pisano, S.H. Oh

High Energy Astrophysical Phenomena

A statistically robust framework for detecting and classifying hysteresis patterns in astrophysical spectral evolution

Tomislav Terzić

Loop-like patterns between spectral parameters are frequently interpreted as evidence of hysteresis in time-dependent astrophysical emission processes. Such patterns have been reported in hardness-intensity diagrams (HID) of accreting black-hole X-ray binaries during state transitions, in the radio-to-X-ray correlation plane during outbursts, in solar activity indices over the solar cycle, and in the spectral energy distribution of active galactic nuclei during flaring episodes. HID often exhibit apparent loops, whose orientation encodes the relative timescales of particle acceleration and radiative cooling. Visual inspection, however, does not provide a statistically controlled detection method. We develop a statistically robust and empirically calibrated framework for detecting, quantifying, and classifying hysteresis patterns in ordered two-dimensional data with measurement uncertainties. The framework provides the normalised signed area $A_\mathrm{norm}$ enclosed by chronologically ordered points in the plane as the primary detection statistic, computed using the shoelace formula. We define open and closed area estimators, introduce cancellation diagnostics for multi-loop structures, and propagate measurement uncertainties via Monte Carlo sampling. Statistical significance is assessed using null ensembles generated by time-order randomization, physically motivated autoregressive surrogate models, and Fourier phase-randomized surrogates. We validate the method on synthetic blazar flare trajectories, and demonstrate its application to an XMM-Newton observation of Markarian~421 during a December 2023 flaring episode, where we confirm a CCW hysteresis loop with $A_\mathrm{norm} = +0.64$ that is robust against measurement noise but does not reach formal significance against stochastic null models, possibly due to the open trajectory geometry.
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High Energy Astrophysical Phenomena

Transition from Diffusion to Drift-Dominated Cosmic Ray Transport and the Origin of the Knee

Luis Enrique Espinosa Castro, Carmelo Evoli and Pasquale Blasi

In a magnetic field with a complex topology, as can be the Galactic magnetic field, cosmic ray transport cannot simply be described by diffusion parallel and perpendicular to magnetic field lines, because the gradients and curvature of the large-scale magnetic field induce drift motions. These effects become especially important at high energies. Here we revisit the possibility that the competition between diffusion and drifts may lead to a knee in the cosmic ray spectrum. We carry out test-particle simulations of cosmic ray transport in a mock Galactic magnetic field made of a regular large scale component, with a non-trivial topology and a homogeneous and isotropic turbulent magnetic field, with a spectrum that is assumed to be Kolmogorov-like in the basic setup. These simulations are used to infer the escape time and the grammage accumulated by cosmic rays with energy in the TeV–10 PeV energy range. In the case of a large scale magnetic field with a purely azimuthal structure, the drift due to the curvature of magnetic field lines produces a knee in the PeV range, but the model fails to reproduce the grammage, due to the exceedingly low value of the perpendicular diffusion coefficient. If the large scale magnetic field acquires a component perpendicular to the Galactic disc, the parallel diffusion coefficient becomes quickly dominant in terms of particle escape, and drifts are unable to compete. A knee structure does not appear in such a scenario. However, if the parallel diffusion coefficient becomes energy independent at $E\gtrsim 1$ TeV, a knee may arise around PeV energies due to drift dominance. We discuss two cases in which this situation may occur.
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High Energy Astrophysical Phenomena

Science of High Resolution X-ray Imaging

H.L. Marshall, K. Weaver, M. Schattenburg et al. (+34 more)

This report summarizes potential studies of the Hi-ReX Science Analysis Group for an Ultra-High Angular Resolution X-ray Observatory. Many potential science cases are presented, spanning a range of astrophysical topics from solar system planets and exoplanets to supermassive black holes and cosmology. These cases demonstrate the value of X-ray imaging beyond current capabilities with an aim toward milli-arcsecond and micro-arcsecond angular scales. There is an imaging gap between the current capability of the best X-ray imaging telescope, the Chandra X-ray Observatory, compared to that of instruments across the electromagnetic spectrum. Next-generation, ultra-high angular resolution X-ray imaging will close this gap and transform our understanding of the universe and fundamental physics.
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All authors H.L. Marshall, K. Weaver, M. Schattenburg, B. Binder, H.M. Günther, S.J. Wolk, K.G. Stassun, S.J. Gunderson, R. Pandey, L. Valencic, P. Draghis, J. Hare, M. Balakrishnan, T. Boztepe, P. Gandhi, T. Holland-Ashford, T. Maccarone, M. Reynolds, M. Sobolewska, R. Tanner, J. Cann, P. Chakraborty, B. Coleman, S. DiKerby, R. Gamble, J. Irwin, P. Maksym, J. McKaig, E. Perlman, D. Pooley, S. Randall, H. Russell, A. Sarkar, S. Turriziani, K.-W. Wong, K. Whalen, D. Haggard

Instrumentation and Methods for Astrophysics

Differentiable design of the PIAA-ZWFS: a flexible wavefront sensor that approaches the fundamental limit

A. K. Taras, S. Y. Haffert and L. Desdoigts

Extreme adaptive optics (AO) is necessary for high contrast astronomy at scales of the habitable zone of nearby systems. We seek to evaluate wavefront sensors that approach fundamental limits of wavefront sensing, enabling adaptive optics systems to run faster or on fainter targets. We present the phase-induced amplitude apodisation Zernike wavefront sensor (PIAA-ZWFS): an adaptation of the conventional Zernike wavefront sensor (ZWFS) that leverages lossless apodisation of the pupil to concentrate the starlight in the focal plane. We optimise and evaluate the sensor with a differentiable modelling framework, drawing on concepts from Bayesian experimental design to minimise the variance of a maximum likelihood estimator that uses the system in the high Strehl regime. Our architecture shows state-of-the-art performance in simulation for different apertures, bandwidths, photon fluxes and source sizes, closing the gap to the fundamental limit by a factor 10 (2.5) compared to the conventional ZWFS (optimised ZWFS) in a typical photon-limited case. For extended sources, we show that even an ideal point source sensor rapidly becomes sub-optimal, and our system outperforms it for stellar diameters larger than 0.8{\lambda}/D. We verify that these gains do not come at the cost of dynamic range with either linear or non-linear reconstructors. Finally, we present a proof that there must be a trade-off between the information gained about amplitude and phase errors for any wavefront sensor. The PIAA-ZWFS is a viable wavefront sensor operating near the fundamental sensitivity limits.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

Probing the neutrino chemical potential with cosmological observations

Pietro Ghedini, Riccardo Impavido, Stefano Gariazzo et al. (+2 more)

The electron neutrino degeneracy parameter, $\xi_{\nu_\mathrm{e}} = \mu_{\nu_\mathrm{e}} / T$, is tightly constrained by Big Bang Nucleosynthesis (BBN), while the degeneracy parameters of the other neutrino species, $\xi_{\nu_\mathrm{x}}$, remain weakly constrained by cosmological observations alone. In this manuscript we shall compute up-to-date bounds on $\xi_{\nu_\mathrm{e}}$ and $\xi_{\nu_\mathrm{x}}$ assuming that either they are constant free-parameters along the cosmic history or that they are redshift dependent quantities. In the latter case we employ a model-independent reconstruction approach based on the Piecewise Cubic Hermite Interpolating Polynomial (PCHIP) formalism with four nodes, located at $z\simeq$ 10, 100, 1000 and $10^8$. We shall also consider two scenarios for neutrinos, specifically three degenerate neutrinos ($\xi_{\nu_\mathrm{e}}$ = $\xi_{\nu_\mathrm{x}}$) and the case in which we actually differentiate between $\xi_{\nu_\mathrm{e}}$ and $\xi_{\nu_\mathrm{x}}$. We perform a cosmological analysis combining CMB data from Planck, SPT, and ACT with BAO measurements from DESI, showing the impact of including BBN observables from either EMPRESS results, which allow for a non-zero chemical potential, or from LBT observations, compatible with the standard $\xi_\nu$ = 0 prediction. We explicitly show that the BBN data, via the change in neutron-to-proton interconversion rates, mostly constrain $\xi_{\nu_\mathrm{e}}$, parameter for which we observe a preferred non-zero positive value at $95\%$ C.L. in the non-degenerate neutrino case at the BBN period. Since the Hubble constant is correlated with $\xi_{\nu}$, through $N_{\rm eff}$, a larger value of $H_0$ is allowed within these models, making them really interesting scenarios where to test non-standard physics models.
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All authors Pietro Ghedini, Riccardo Impavido, Stefano Gariazzo, Olga Mena, Deng Wang

Astrophysics of Galaxies

A DESI Calibration of the [O II]–[S II] Electron-density Offset in Integrated Star-forming Galaxies

Yu Rong, Shihong Liu and Hu Zou

The [O II]$\lambda\lambda3726,3729$ and [S II]$\lambda\lambda6716,6731$ doublets are widely used as low-ionization electron-density diagnostics in galaxy spectra and are often treated as interchangeable when only one of them is accessible. We test this assumption using the DESI DR1 Emission Line Catalog. For star-forming galaxies with fiducial emission lines, [O II] yields systematically higher electron densities than [S II], with a median offset of 0.228 dex. The binned median calibration is $\log n_e({\rm OII})=(0.752^{+0.182}_{-0.097})\log n_e({\rm SII}) +(0.832^{+0.231}_{-0.422})$. The offset is larger in galaxies with higher stellar mass, H$\alpha$ star-formation rate, dust attenuation, and $N2\equiv\log([{\rm NII}]\lambda6583/{\rm H}\alpha)$, an empirical gas-phase metallicity proxy, and smaller in galaxies with higher \(\log O_{32}\equiv\log\{[{\rm OIII}]\lambda5007/ ([{\rm OII}]\lambda3726+\lambda3729)\}\), an ionization proxy; no significant trend is found with specific star-formation rate. These trends are consistent with [O II] and [S II] sampling different low-ionization gas phases in integrated spectra, with [S II] more strongly weighted toward lower-density diffuse or outer gas. Our results show that [O II]- and [S II]-based densities should not be mixed without empirical calibration in studies of ISM pressure, nebular density, and their evolution across galaxy samples and redshift.
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High Energy Astrophysical Phenomena

A Unified Model for the Emission of Supernova-Associated Fast X-ray Transients: Case Studies of EP240414a, EP250108a, and GRB~171205A

Yu-Fei Li, Da-Bin Lin, Jia Ren et al. (+2 more)

The Einstein Probe (EP) has detected several Fast X-ray Transients (FXTs) associated with broad-lined Type Ic supernovae (SNe), including EP240414a and EP250108a. The observations reveal common features among these FXTs, but the corresponding physical origin remains debated. By comparing the FXTs with low-luminosity gamma-ray bursts (e.g., GRB 171205A), we propose a unified model that explains the common features in these events. In this model, a rapidly spinning magnetar generates a collimated Poynting flux-dominated jet and an isotropic wind. As the jet propagates through the stellar envelope, it generates a hot cocoon. In addition, a pulsar wind nebula (PWN) is formed during the interaction of the wind and the ejecta. As the surrounding cocoon gradually becomes transparent, the emission from the PWN escapes and is observed. This model provides a unified explanation for the observations: (1) Early thermal emission originates from the cocoon; (2) Mid-term non-thermal emission comes from the PWN; (3) Late-term emission originates from SNe driven by $^{56}$Ni radioactive decay and magnetar. (4) The X-ray afterglows originate from the structured jet. Our research thus provides a natural explanation for the observed thermal-to-nonthermal evolution in such FXTs and reveals their shared physical origin with some GRB-SNe.
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All authors Yu-Fei Li, Da-Bin Lin, Jia Ren, Zhi-Lin Chen, En-Wei Liang

Instrumentation and Methods for Astrophysics

In-flight calibration of the Wide-field X-ray Telescope on board the Einstein Probe

Huaqing Cheng, Hai-Wu Pan, Yuan Liu et al. (+14 more)

By utilizing novel lobster-eye optics, the Wide-field X-ray Telescope (WXT) onboard the Einstein Probe (EP) satellite achieves an unprecedented combination of a large instantaneous field-of-view (FoV) and high sensitivity for monitoring the dynamic X-ray sky. In this paper, we present the in-orbit calibration results of the WXT during its first two and a half years of operations. By conducting observations of standard celestial sources–including the Crab Nebula, Scorpius X-1, and Cassiopeia A–we systematically characterized key instrumental properties. Our analysis demonstrates that the in-orbit performance of the WXT agrees with prelaunch ground calibrations well. The spatial resolution, denoted by the full width at half maximum (FWHM) of the focal spot, typically ranges from $3'$ to $6'$ across $\sim$90% of the FoV, with a median of $\sim 4.3'$. The post-calibration source positioning accuracy achieves $1.3'$ (at the 90% confidence level). The in-orbit effective area is consistent with model predictions and ground measurements, exhibiting an overall systematic uncertainty of $\lesssim 10\%$ (90% C.L.) in the 0.5-4 keV band. While the vast majority of the detectors remain highly stable, a noticeable long-term degradation at the low-energy end ($\sim30\%$-$40\%$, 0.4-0.6 keV) is observed in a few specific modules. Furthermore, spectral evaluations using Cas A confirm the stability of the energy scale and spectral resolution of the focal-plane Complementary Metal-Oxide Semiconductor (CMOS) detectors. All derived calibration products have been incorporated into the WXT calibration database (CALDB). These results comprehensively verify the instrumental capabilities of the WXT, providing a solid foundation for the reliable analysis of scientific observations.
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All authors Huaqing Cheng, Hai-Wu Pan, Yuan Liu, Jingwei Hu, Haonan Yang, Donghua Zhao, Zhixing Ling, Yifan Chen, Xiaojin Sun, Longhui Li, Ge Jin, Wenxin Wang, Xue Yang, He-Yang Liu, Chen Zhang, Shuang-Nan Zhang, Weimin Yuan

Cosmology and Nongalactic Astrophysics

Thermal Sunyaev-Zel'dovich cross-correlations with unWISE galaxies: disentangling radio contamination, dust properties, and electron pressure

Guandi Zhao, Alex Krolewski and Niayesh Afshordi

Cross-correlations between the thermal Sunyaev-Zel'dovich (tSZ) effect and galaxy surveys provide a sensitive probe of hot gas in low-mass halos, but on the angular scales of greatest astrophysical interest they are also highly vulnerable to residual foreground contamination. We analyze the cross-correlation of two low-redshift unWISE galaxy samples with Planck PR4 and ACT DR6 microwave temperature sky maps directly in harmonic space, fitting simultaneously for correlated tSZ, cosmic infrared background (CIB), and radio emission. Using the nine Planck bands to perform model selection, we find that a three-component model consisting of tSZ, radio emission, and a CIB amplitude term is strongly preferred over a model that omits radio contamination, with a significance of $9.5\sigma$ for the unWISE Low-z sample and $11\sigma$ for the unWISE Mid-z sample respectively. For the unWISE Low-z sample ($\bar{z}=0.14$) the preferred Planck fit gives an effective CIB emissivity index $\beta_0=1.79\pm0.29$, an effective CIB dust temperature $T_0=22.0\pm5.1 K$, and a radio spectral index $\beta_r=-2.18\pm0.24$; for the unWISE Mid-z sample ($\bar{z}=0.23$) we find $\beta_0=1.41\pm0.13$, $T_0=28.0\pm4.2 K$, and $\beta_r=-2.62\pm0.26$. We apply this radio-inclusive recipe to the ACT+Planck 90, 150, and 220 GHz maps to obtain the best small-scale measurements. This removes the apparent negative galaxy-tSZ cross-correlation seen in the fiducial ACT DR6 NILC reconstruction and in no-radio fits. The cleaned tSZ cross unWISE spectra remain positive to $\ell \simeq 6000$ and are broadly consistent with the Planck-only reconstruction on overlapping scales. We show that these radio-cleaned spectra are well described by a conventional halo model calibrated to the galaxy population and a two-parameter generalized NFW electron pressure profile.
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High Energy Astrophysical Phenomena

A Rare Gamma-ray Flaring episode of the Narrow-Line Seyfert 1 Galaxy 1H 0323+342

Aminabi Thekkoth, Vaidehi S. Paliya and Preeti Kharb

Gamma-ray-emitting narrow-line Seyfert 1 ($\gamma$-NLSy1) galaxies represent an enigmatic class of active galactic nuclei (AGNs) bridging populations of radio-quiet and radio-loud AGNs. Here we report the multi-wavelength investigation of a rare $\gamma$-ray flaring episode of an NLSy1 galaxy, 1H 0323+342 ($z=0.063$), using data from Fermi-Large Area Telescope, Swift, and Nuclear Spectroscopic Telescopic Array. The source exhibited a significant enhancement in $\gamma$-ray flux along with rapid variability on $\sim$sub-hour timescales in both $\gamma$-ray and hard X-ray wavelengths, hinting that the energy dissipation is happening from a compact region close to the central supermassive black-hole. The time-resolved X-ray spectral study revealed a transition between a jet-dominated and a mixed jet+corona emission state. Reproducing the broadband spectral energy distribution with a one-zone leptonic model suggested that the high-energy emission is mainly produced by external Compton scattering of the accretion disc and broad line region photons. The inferred jet power reaches values of the order of $10^{46}$ erg s$^{-1}$ comparable to those of powerful flat-spectrum radio quasars, suggesting that even low black-hole mass systems can occasionally produce powerful $\gamma$-ray outbursts.
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Cosmology and Nongalactic Astrophysics

Dark energy perturbations and the robustness of cosmological neutrino-mass constraints

Yu-Hang Yang, Emmanuel N. Saridakis, Yi-Fu Cai et al. (+1 more)

Cosmological observations are placing increasingly stringent bounds on the sum of neutrino masses, approaching the lower limits implied by neutrino oscillation experiments. Recent studies have suggested that dynamical dark energy may alleviate this apparent tension. However, these conclusions generally rely on the assumption that dark energy remains smooth, neglecting its perturbations. In this work we investigate the robustness of cosmological neutrino-mass constraints by consistently incorporating dark-energy perturbations. Using CMB, BAO, RSD, and supernova data, we show that the commonly reported alleviation of the neutrino-mass tension in dynamical dark-energy models is not generic. While smooth dark energy substantially relaxes the neutrino-mass bounds, allowing dark energy to cluster shifts the preferred neutrino mass toward smaller, and even more negative, effective values. We demonstrate that this behavior originates from a degeneracy between neutrino free-streaming and dark-energy perturbations in structure-growth observables. Different combinations of neutrino mass and dark-energy clustering can provide similarly good fits to current data while yielding significantly different neutrino-mass constraints. Our results show that cosmological neutrino-mass measurements are inherently model dependent and that reliable neutrino-mass inference requires a consistent treatment of dark-energy perturbations.
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All authors Yu-Hang Yang, Emmanuel N. Saridakis, Yi-Fu Cai, Hao-Ran Yu

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

Construction of Sensitivity Curves for Dynamic LISA and Taiji

Hong-Yu Shi and Yong Tang

Space-based gravitational-wave (GW) laser interferometers, including LISA and Taiji, are designed to observe gravitational waves in the millihertz band and are expected to open up a frequency range that is otherwise inaccessible. The sensitivity and response of these instruments are central to their scientific goals, mission design and parameter estimation capabilities. However, they are commonly modeled as static, equilateral triangular constellations, an approximation that neglects both orbital motion and directional dependence. In this work, we systematically examine the direction-dependent response and sensitivity of dynamic LISA-like detectors over an entire year of heliocentric orbit. Based on an analytical, time-dependent heliocentric orbital model and an adiabatic unequal-arm interferometer configuration, we construct direction-dependent sensitivity curves in the Michelson interferometric channel for dynamic LISA and Taiji. We obtain analytic expressions for the angular-dependent sensitivity and demonstrate the emergence of a quadrant-like pattern in sky maps at low frequencies. We show that, relative to the static approximation, the low-frequency sensitivity varies by roughly $20\%$, which in turn produces about a $70\%$ variation in the directional dependence of the number of detectable GW sources, with even larger discrepancies at higher frequencies. Therefore, for accurate predictions of the total GW source counts and reliable parameter inference for binary systems, it is necessary to employ fully dynamic, direction-dependent sensitivity curves.
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High Energy Astrophysical Phenomena

Measuring High-Energy Cosmic Particles with the SKA

Tim Huege, Katharine Mulrey, Sjoerd Bouma et al. (+28 more)

The origin of high-energy cosmic rays remain one of astrophysics' greatest unsolved mysteries. SKA-Low will be able to measure air showers initiated by cosmic rays with unprecedented precision in the PeV - EeV energy range, covering the critical transition region between Galactic and extragalactic sources. SKA-Low's densely instrumented core and broad bandwidth will allow for measurements of individual air showers with a level of detail unmatched by any existing or planned detector. The depth of shower maximum, the primary mass-sensitive observable, will be reconstructed with a resolution of better than 8~g/cm$^2$, a significant improvement over existing methods. Additionally, new reconstruction methods are expected to enable full air shower reconstruction across a wide energy range, down to PeV levels. At these energies, efficient photon/hadron separation may offer an opportunity to measure PeV gamma-ray air showers. Furthermore, SKA-Low opens a window into studying high-energy hadronic interactions, including via the unique channel of anomalous air showers. This combination of measurements provides a unique opportunity to investigate the origins and physics of high-energy cosmic rays. A dedicated particle detector array will provide triggered readout of raw antenna-level voltage buffers, enabling fully commensal cosmic-ray observations alongside regular operations. We outline our science case and discuss the observational strategy, signal properties and detector design underpinning these measurements. We also summarize the accompanying book chapters, which address composition measurements in the Galactic-to-extragalactic transition region, next-generation interferometric reconstruction techniques, hadronic interaction physics through anomalous air showers, the prospects for detecting PeV gamma-rays from Galactic sources, and the related project of imaging lightning using SKA-Low.
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All authors Tim Huege, Katharine Mulrey, Sjoerd Bouma, Justin Bray, Stijn Buitink, Arthur Corstanje, Vital De Henau, Edwin Dickinson, Brian Hare, Haoning He, Jörg Hörandel, Clancy James, Philipp Laub, Xingyu Li, Marten Lourens, Hermann-Josef Mathes, Anna Nelles, Subhadip Saha, Felix Schlüter, Olaf Scholten, Ralph Spencer, Christopher Sterpka, Karen Terveer, Satyendra Thoudam, Gia Trinh, Paulina Turekova, Darko Veberic, Keito Watanabe, Chao Zhang, Pengfei Zhang, Yi Zhang

Instrumentation and Methods for Astrophysics

The performance of the TA$\times$4 surface detector array: 4.3 years of the first-half expansion

Telescope Array Collaboration: R.U. Abbasi, T. Abu-Zayyad, M. Allen et al. (+126 more)

The Telescope Array (TA) experiment aims to reveal the origin of ultra-high-energy cosmic rays (UHECRs) by observing air showers using surface detectors (SDs), which spread over an area of approximately 700 km$^2$, and fluorescence detectors (FDs) viewing the skies above the SD array. The TA experiment has been observing UHECRs since 2008, and has reported an indication of clustering in the arrival directions of cosmic-ray events with energy greater than 57 EeV. To improve the exposure for anisotropy studies of UHECRs, the TA$\times$4 upgrade was designed to expand the observational area by approximately 2,000 km$^2$ with 500 additional SDs. Half of the planned upgrade, consisting of 257 SDs, was completed, and the newly installed array began operation in 2019. In addition to the expanded SD array, two FD stations were constructed for the TA$\times$4 experiment. In this paper, we present a study of the performance of the expanded SD array, including the energy resolution, angular resolution, and effective aperture, over the first 4.3 years of data acquisition. While the effective aperture varied initially due to changing detector states, it has stabilized since June 2023 with more than 90% operational SDs. Furthermore, a new inter-tower trigger system was implemented to connect six new communication towers to form two geographically separated arrays, increasing the effective aperture. The time variation of this effective aperture, the resulting total exposure of approximately 3,500 km$^2$ sr yr, and a comparison with the original TA SD array are presented to demonstrate the performance of the expanded array.
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All authors Telescope Array Collaboration: R.U. Abbasi, T. Abu-Zayyad, M. Allen, J.W. Belz, D.R. Bergman, F. Bradfield, I. Buckland, W. Campbell, B.G. Cheon, K. Endo, A. Fedynitch, T. Fujii, K. Fujisue, K. Fujita, M. Fukushima, G. Furlich, A. Gálvez Ureña, Z. Gerber, N. Globus, T. Hanaoka, W. Hanlon, N. Hayashida, H. He, K. Hibino, R. Higuchi, D. Ikeda, D. Ivanov, S. Jeong, C.C.H. Jui, K. Kadota, F. Kakimoto, O. Kalashev, K. Kasahara, Y. Kawachi, K. Kawata, I. Kharuk, E. Kido, H.B. Kim, J.H. Kim, J.H. Kim, S.W. Kim, R. Kobo, I. Komae, K. Komatsu, K. Komori, A. Korochkin, C. Koyama, M. Kudenko, M. Kuroiwa, Y. Kusumori, M. Kuznetsov, Y.J. Kwon, K.H. Lee, M.J. Lee, B. Lubsandorzhiev, J.P. Lundquist, H. Matsushita, A. Matsuzawa, J.A. Matthews, J.N. Matthews, K. Mizuno, M. Mori, S. Nagataki, K. Nakagawa, M. Nakahara, H. Nakamura, T. Nakamura, T. Nakayama, Y. Nakayama, K. Nakazawa, T. Nonaka, S. Ogio, H. Ohoka, N. Okazaki, M. Onishi, A. Oshima, H. Oshima, S. Ozawa, I.H. Park, K.Y. Park, M. Potts, M. Przybylak, M.S. Pshirkov, J. Remington, C. Rott, G.I. Rubtsov, D. Ryu, H. Sagawa, N. Sakaki, R. Sakamoto, T. Sako, N. Sakurai, S. Sakurai, D. Sato, K. Sekino, T. Shibata, J. Shikita, H. Shimodaira, H.S. Shin, K. Shinozaki, J.D. Smith, P. Sokolsky, B.T. Stokes, T.A. Stroman, H. Tachibana, K. Takahashi, M. Takeda, R. Takeishi, A. Taketa, M. Takita, Y. Tameda, K. Tanaka, M. Tanaka, M. Teramoto, S.B. Thomas, G.B. Thomson, P. Tinyakov, I. Tkachev, T. Tomida, S. Troitsky, Y. Tsunesada, S. Udo, F.R. Urban, M. Vrábel, D. Warren, K. Yamazaki, Y. Zhezher, Z. Zundel, J. Zvirzdin

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

The QCD energy-momentum tensor on the lattice: non-perturbative renormalization with $N_f=3$

Matteo Bresciani, Mattia Dalla Brida, Leonardo Giusti et al. (+3 more)

We construct the traceless components of the energy-momentum tensor on the lattice for QCD with $N_f=3$ flavours, such that their correlation functions satisfy the appropriate Ward identities in the continuum limit. To carry out this program, we define the theory on the lattice by the Wilson-plaquette and the $O(a)$-improved Wilson actions for gluons and quarks respectively. The discretization of the space-time entails that (i) the irreducible nonet representation of the SO($4$) group splits into a triplet and a sextet irreducible representations of the hypercubic group, and (ii) for each multiplet non-perturbative determinations of the the gluonic and fermionic renormalization constants are required. The bare gluonic components of the energy-momentum tensor are defined via the clover discretization of the field strength tensor, while the fermionic ones are discretized by appropriate combinations of symmetric covariant derivatives. Either for the triplet or the sextet representations, the two independent renormalization constants are then fixed non-perturbatively by imposing discretized versions of continuum Ward identities for one-point correlation functions in the presence of shifted boundary conditions and an imaginary chemical potential. The non-perturbative calculation is then carried out by Monte Carlo simulations, and the resulting renormalization constants are determined with a final accuracy of a few percent for values of the bare coupling constant squared in the range $0 \leq g_0^2\leq 0.96$.
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All authors Matteo Bresciani, Mattia Dalla Brida, Leonardo Giusti, Mitsuaki Hirasawa, Michele Pepe, Luca Virzì

Astrophysics of Galaxies

Simulating megaparsec-scale jets of radio galaxies: Magneto-hydrodynamics of jets reaching 5 Mpc

Gourab Giri, Dario Borgogno, Marco Tavani et al. (+9 more)

Extragalactic jets have long prompted the question of how far relativistic outflows can extend, with some radio sources reaching 5 - 7 Mpc in length. These great extents motivate investigations into their ages, propagation dynamics, stability, and impact on the environment. We perform 3D high-resolution numerical simulations of two jet configurations involving continuous injection at different powers propagating in low-density regions of the cosmos (static and laminar), investigating the conditions for jet collimation versus disruption at extreme scales. We show that the combined effects of higher jet thrust (enhanced kinetic power), improved collimation (suppression of transverse distortions), and magnetic stabilization (strengthened poloidal field) can sustain a laterally confined flow, enabling such a jet to reach 5 Mpc in just 15 Myr (injecting a total energy of $2.3 \times 10^{61}$ erg into the environment). In contrast, a jet lacking these conditions dissipates more rapidly, forming lobe-like morphologies and reaching only $\sim 3$ Mpc over $\sim35$ Myr (injecting total energy of $8.1 \times 10^{60}$ erg). Pinch and kink MHD instabilities are identified as the primary drivers of transverse distortions; their suppression allows the persistence of a fast spine alongside a slower, dissipative head (location of maximum environmental interaction). We find that the jet-head propagation shows two regimes: one with speed $\sim0.5 c$; the other with speed from $\sim 0.2 c$ to $\sim 0.05 c$. We consider a proxy of synchrotron emission and find that radiation is concentrated in regions of enhanced compression and magnetic amplification, primarily near the first recollimation shock (producing a bright radio spot) and at the jet-head interaction zone (producing the radio termination lobe). Such jets facilitate the transport of substantial energy and magnetic flux into underdense cosmic regions.
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All authors Gourab Giri, Dario Borgogno, Marco Tavani, Andrea Mignone, Prateek Sharma, Claudio Gheller, Valerio Vittorini, Paul J. Wiita, Bernie Fanaroff, Alessio Suriano, D. J. Saikia, Gianfranco Brunetti

Cosmology and Nongalactic Astrophysics

UV artefacts in ultra-slow-roll models of inflation

Gerald Barnert, Laura Iacconi, Hooshyar Assadullahi et al. (+2 more)

Within single-field inflation, primordial black hole and scalar-induced gravitational wave production from enhanced primordial perturbations typically requires a transient non-attractor phase, such as ultra-slow roll. We investigate the physical consistency of modeling such scenarios through analytical Hubble-flow parametrisation. By reconstructing the underlying scalar field potential, we show that even slow transitions in the slow-roll parameters can hide sharp, localised spikes in higher-order derivatives of the potential at the transition from ultra-slow-roll to slow-roll. These are typically not found in analytic potentials. To evaluate the impact of these structures, we implement a UV-filtering procedure based on discrete Fourier transform to systematically suppress high frequency modes in field space in both classes of models. We find that the filter effectively removes sharp features in Hubble-flow-derived potentials. As a consequence, we show that UV-filtered models typically respect Wands duality invariance as the field evolves back from ultra-slow roll to slow roll. Beyond linear perturbation theory, the introduction of spurious UV effects might affect other observables, such as non-Gaussianity and loop contributions. Our results thereby question the robustness of simple analytical Hubble-flow parametrisation for modeling inflationary models with a transient non-attractor phase.
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All authors Gerald Barnert, Laura Iacconi, Hooshyar Assadullahi, Kazuya Koyama, David Wands

Astrophysics of Galaxies

Formation of Isotopically Heterogeneous Molecular Cloud Cores in Filamentary Molecular Clouds

Yoshiaki Misugi, Shu-ichiro Inutsuka, Taishi Nakamoto et al. (+1 more)

Meteorite analysis shows that the older solids of the solar system, such as the calcium-aluminum-rich inclusions (CAIs), have isotopic inhomogeneity. This indicates that the isotopic inhomogeneity could originate from parental molecular clouds. We investigate the evolution of the isotopically heterogeneous molecular cloud cores formed from filament fragmentation using the smoothed particle hydrodynamics method. We show that the effect of the variation of isotopic ratio along the minor axes of the filament is smaller than that along the longitudinal axis of the filament due to the filament geometry. Our results also suggest that isotopic inhomogeneities remain in the resulting cores, although the amounts of initial inhomogeneities are reduced by a factor of 100 from those over the initial filament length of 1 pc. This fraction corresponds to 1-10% of the maximum isotopic ratio that the core can acquire from the filament in each model. The origin of the isotopic inhomogeneity of the shells could be attributed to the initial difference in the center-of-mass of shells caused by the turbulent velocity field. Our model indicates that the isotopic inhomogeneity could survive even in the circumstellar disk.
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All authors Yoshiaki Misugi, Shu-ichiro Inutsuka, Taishi Nakamoto, Tetsuya Yokoyama

Cosmology and Nongalactic Astrophysics

Large-scale structures of the Universe: physics, phenomenology, statistics

Cora Uhlemann

In this series of lectures, we seek to describe the evolution of the cosmic large-scale structure. We will discover the cosmic web - the large-scale skeleton of matter traced by galaxies. It arises from the interplay of the gravitational pull of dark matter and the expansion driven by dark energy. Major large-scale galaxy surveys map the distribution of matter and galaxies across most of the sky, spanning over 10 billion years of cosmic history. I will guide you through some of the principles and challenges behind predicting the statistical properties of the matter and galaxy distribution in vast cosmic volumes. In particular we discuss the underlying nonlinear physics and resulting non-Gaussian statistics that need to be predicted to extract fundamental physics from observational data.
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Solar and Stellar Astrophysics

Multivariate Statistical Analysis of Low Mass Ratio Contact Binaries: Definition, Dynamical Stability, and Parameter Relationships

A. Poro, R. Poggiani, A. Foroutanfar et al. (+3 more)

This study explores multiple aspects of W Ursae Majoris (W UMa) contact binary systems with low mass ratios, providing empirical insights into their definition, structure, rotational stability, and parameter relationships. We first examined the range of mass ratios that characterize these systems and, based on an analysis of 818 contact binaries, established an empirical threshold of $q \approx 0.27$ to identify low mass ratio systems. To investigate rotational stability, we conducted a Monte Carlo analysis of the squared gyration radii ($k_1^2$ and $k_2^2$) and assessed the resulting spin-to-orbital angular momentum ratio ($J_\mathrm{spin}/J_\mathrm{orb}$), finding that while $k_1$ remains nearly constant, $k_2$ and $J_\mathrm{spin}/J_\mathrm{orb}$ decrease slightly with increasing mass ratio, emphasizing the role of the secondary star's internal structure. Moreover, we compiled a dedicated sample of 115 low mass ratio contact binaries and estimated their absolute parameters using Gaia DR3 parallaxes. From this dataset, we derived empirical parameter relationships for low mass ratio systems, which provide a useful reference for future observational and theoretical studies. The resulting datasets and statistical summaries offer benchmarks for modeling, stability evaluation, and evolutionary studies of W UMa-type binaries with low mass ratios.
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All authors A. Poro, R. Poggiani, A. Foroutanfar, R. Harzandjadidi, N. Kahali Poor, F. Alicavus

Cosmology and Nongalactic Astrophysics

A hidden reionization prior biases cosmological inference

Zihan Wang and Huanyuan Shan

Precision cosmology assumes that cosmic reionization was a single smooth transition. We show that this assumption is in tension with current observations: the Planck optical depth, patchy kinetic Sunyaev–Zel'dovich (PkSZ)limits from SPT and ACT, and the Ly$\alpha$ forest endpoint cannot be simultaneously reproduced by any viable monotonic ionization history. Non-parametric reconstructions and Planck EE polarization provide independent support for an additional ionization component at $z \gtrsim 12$. Incorporating this early phase relaxes the upper bound on the summed neutrino mass to $\sum m_\nu < 0.39$~eV ($95\%$ CL) and shifts $\sigma_8$ toward values preferred by weak-lensing surveys, both consequences of the standard $A_s$–$\tau_e$ degeneracy. These shifts arise from relaxing a hidden prior on reionization shape rather than from new physics, and identify reionization shape as an implicit prior in cosmological inference. Next generation of CMB and 21~cm experiments will be able to test this directly,which will convert what has been an unflagged systematic on $\sigma_8$ and $\sum m_\nu$ into a quantified statistical uncertainty.
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Instrumentation and Methods for Astrophysics

On-sky Fibre-Target-Alignment of the 4MOST instrument: calibration and performance

Roland Winkler, Weijia Sun, Daniel Sablowski et al. (+9 more)

The 4-metre Multi-Object Spectroscopic Telescope (4MOST) is a new wide-field, fibre-fed spectroscopic survey facility for the VISTA telescope at ESOs Paranal Observatory. The instrument enables the simultaneous acquisition of 2436 spectra across a 4.2 square deg field of view, using a tilting spine fibre positioner feeding three dedicated spectrographs. In this paper, we describe the calibration process, and performance verification of the Fibre-Target-Alignment (FTA) process for 4MOST. We show the complete FTA process, including calibration of the individual hardware- and software components. Namely the Metrology camera system, the Fibre Positioner AESOP, a spine based Secondary Guiding System, the sky to focal surface projection software, and residual minimization via raster scans. In total, the FTA system required one special tool, a large calibration target for the focal surface, and approximately 1 month of accumulated calibration work on the telescope. The FTA process reached approx. 24 um (0.4 arc sec) RMS distance between fibres and targets on sky about 3 weeks after installation of the final hardware components of 4MOST, which is when 4MOST had its first light event. By the time of writing this paper, i.e. 6 months later, we reach approx. 16 um (0.27 arc sec) RMS. Currently, we far exceed our requirements in terms of accuracy, and are doing trade-off studies to maximize scientific returns.
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All authors Roland Winkler, Weijia Sun, Daniel Sablowski, Thomas Liebner, Ole Streicher, Steffen Frey, Ingo Stilz, Alexander Pramskiy, Scott Smedley, Thomas Szeifert, Diogo Rio Fernandes, Gerard Zins