arXane › Physics › Astrophysics
Wednesday, 4 November 2026: 15 new papers, 12 cross-lists and 20 replacements in High Energy Astrophysical Phenomena (astro-ph.HE), in arXiv's order, with full abstracts. The same papers as arXiv's own listing, set as a newspaper; every title links to arXiv.
Super-critical accretion onto stellar-mass compact objects is sometimes revealed by ultraluminous X-ray sources (ULXs). In other cases, X-rays are heavily obscured along our line of sight, but a super-Eddington accretion power can be indirectly revealed by multiband studies. We have identified a candidate super-critical source in NGC 5408 (labelled X-2). We argue that: it is powered by accretion; it is among the most powerful in the local universe, despite its apparent X-ray faintness; it has exceptional optical properties (both in the continuum and in the lines). We carried out a multiband imaging and spectral study with the ATCA, the VLA, HST, the VLT and Chandra. We found an unresolved source with: variable radio emission (5-GHz luminosity of a few x 10^{35} erg/s); red optical continuum with absolute mag M_{V,0} = -9.1, M_{I,0} = -10.6; exceptionally broad low-ionization emission lines (FWHM ~ 1700 km/s) with P-Cygni profiles; narrow, photoionized emission lines including strong He II 4686; point-like hard X-rays; marginally resolved soft X-rays. We also found an ~20-pc He^{++} nebula near X-2, associated with candidate Wolf-Rayet stars. The optical continuum declined by 2 mag between 2010 and 2024, as did the intensity of broad and narrow lines. We argue that the system is powered by the radiative and mechanical output of a super-Eddington stellar mass BH, with an accretion power >~10^{40} erg/s. We propose that X-2 is a Compton-thick ULX seen at high inclination, with a flaring jet and X-ray emission emerging from the polar funnel. We argue that the optical continuum and the broad emission lines are signatures of an optically thick outflow, launched either from the accretion disk, or from an LBV-like donor star, or, more speculatively, caused by the elusive phase of common envelope ejection. We suggest that we are looking at a short-lived phase of hyper-Eddington accretion.
We report the results of the spectral timing analysis of the quasi-simultaneous X-ray follow-up with XMM-Newton ($50\,\rm ks$) and NuSTAR ($180\,\rm ks$) of the very high energy flare of the blazar PKS 1725+123 ($z=0.586$) in August of 2025. The X-ray monitoring started $\sim3$ days after the announcement of the VHE flare by LST-1, MAGIC and H.E.S.S. and lasted for almost 5 days. The high sensitivity of XMM-Newton reveals two spectral components, a soft ($\Gamma_{\rm X}>2$) and a hard ($\Gamma_{\rm X}<2$) power law. While the soft component dominates at $E<2\,\rm keV$, the hard component dominates at $E>2\,\rm keV$, likely marking the transition between the synchrotron and the high-energy emission, the "valley" of this blazar's spectral energy distribution. Within the XMM-Newton pointing ($\sim14$ hours) we witness the shift of the synchrotron component out of the X-ray band. In the following $\sim 4$ days, NuSTAR further reveals the brightening by a factor of 1.5 of the hard component, following the shift of the high-energy part of the SED into the X-ray band. Overall, within a week (observed frame) of the VHE flare, PKS 1725+123 went from temporarily being an intermediate synchrotron peaked blazar to transitioning back to its `normal' state of a low synchrotron peaked one. This is the first time that the evolution of a VHE flare from a blazar jet has been followed continuously in X-rays over such an extended period and our results highlight the importance of long-term quasi-simultaneous X-ray follow-up of VHE blazar flares.
The extended X-ray jet (EXJ) of SS 433/W50 shows hard X-ray and very-high-energy gamma-ray emission, indicating strong particle acceleration, likely sensitive to the EXJ magnetic field structure and turbulence. To study this field, we re-analyze the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observation of the eastern EXJ using the publicly available polarization fitting package LeakageLib. Compared with standard PCUBE and spectropolarimetric analyses, this approach improves the polarization constraints and enables spatially resolved polarization measurements. Dividing the EXJ into head, spine, and sheath regions with independent polarization properties, we find significant (>3 sigma) improvement over a single-zone model. The sheath exhibits lower polarization than the head and spine, while the electric vector position angles are consistent across all regions, indicating magnetic fields aligned with the jet axis. The measurements imply enhanced magnetic turbulence at the jet boundary, potentially driven by Kelvin-Helmholtz instabilities.
We derive the basic equations suitable for the coupled fluid--spacetime eigenvalue problem from the second-order action in full general relativity and accurately determine quasi-normal modes of a variety of rapidly rotating stars with outgoing-wave boundary conditions. For MIT-bag strange stars, the spectrum realizes Chandrasekhar's instability picture, with distinct Chandrasekhar--Friedman--Schutz and Jacobi-like neutral conditions and the transition to dynamical bar-mode growth. For neutron stars, we determine the $f$-, $r$-, and other inertial-mode spectra and their radiative damping or growth times. Interactions between $f$- and inertial modes reveal avoided crossings and exchanges for radiative damping or growth rates. We locate quasiradial collapse thresholds through a static stability analysis and independently verify them by tracking the fundamental $F$-mode frequency to zero, clarifying that the central density at the onset of the dynamical instability is always larger than that of the secular instability for uniform rotation. Our formalism and numerical implementation can also resolve highly damped $w$-modes.
Reprocessed X-ray emission from cold material surrounding compact objects carries critical information about the geometry and physical state of the system. In particular, the Fe K$\alpha$ fluorescence line provides a powerful diagnostic through its spectral and timing properties relative to the direct emission. Interpreting these observables requires self-consistent modelling of radiative transfer (RT) in three-dimensional (3D) space. We aim to implement photon arrival time lags in SKIRT, a Monte Carlo RT code, thereby enabling self-consistent modelling of both the spectral and timing properties of reprocessed emission in 3D space. We modified the code to accumulate photon path lengths over their lifetimes and convert them into arrival time lags relative to the direct emission. The implementation is validated using simple geometries and then applied to a realistic model of an active galactic nucleus (AGN), for which we compute the time-lag and energy distribution and the cross-correlation functions (CCFs) between the fluorescence line and broad bands. The simulations reproduce the expected time-lag distributions and their energy dependence for both free-electron scattering and the photoelectric absorption $+$ fluorescence. In the AGN application, the model naturally yields distinct lag components from the inner and outer structures, as well as energy-dependent time lags associated with the fluorescence line. The derived CCFs successfully recover both the peak time lag and its finite width, which arise from the extended geometry and intrinsic variability. In X-rays, this capability provides a robust framework for interpreting high-resolution spectroscopic data in conjunction with timing information from current and future missions such as XRISM and NewAthena. A wide range of applications is foreseen, including reverberation mapping studies of accreting systems.
We analyse 17 years of Fermi-LAT $\gamma$-ray data toward the middle-aged supernova remnant (SNR) G156.2+05.7, supplemented by radio and CO observations and by published X-ray constraints. An extended $\gamma$-ray source is detected, well described by a 2D Gaussian template ($R_{68} \sim 0.84^\circ$). This description is preferred over a foreground CO template, whereas the Gaussian and a uniform disc are statistically indistinguishable under the AIC. The spectrum shows evidence for curvature, with a log-parabola preferred over a power law at the $\approx 3.4\sigma$ level. Multi-wavelength spectral energy distribution (SED) modelling, under the adopted assumptions, suggests a leptonic--hadronic joint scenario consistent with the data, with the Fermi-LAT spectrum favouring a hadronic contribution over a purely leptonic interpretation. However, the proton-dominated alternative remains competitive, so the partition of the GeV emission is model-dependent. In the adopted leptonic--hadronic model, the electron population accounts for the radio synchrotron emission and a leptonic $\gamma$-ray component, while a steep proton population contributes the remaining hadronic $\gamma$-rays. Adopting a reference distance $d = 2.7$ kpc, the fitted electron cutoff ($E_{e,\mathrm{cut}}\approx 1.2$ TeV) is compatible with the thermal-dominated X-ray emission, while the proton cutoff is fixed by assumption at $1$ TeV. The northeast and southwest quadrants show enhanced $\gamma$-ray emission, which may be related to a clumpy or asymmetric surrounding medium. A correspondence with the directions of optical H$\alpha$ filaments is suggested by the quadrant orientation but is not yet established. No significant long-term flux variability is found, consistent with steady emission from an extended source.
We report the discovery of the Seyfert galaxy J234728.73+242745.7 as a recurrent intermediate-type CLAGN with extreme optical/UV variability. Optical spectra spanning nearly three decades reveal Type 1.0$\rightarrow$1.5$\rightarrow$1.2 transitions, accompanied by the disappearance and reappearance of broad He II $\lambda4686$ and a broad-line Balmer decrement evolving from 4.5 to 7.2 and then 2.4. Monitoring reveals coordinated variability from X-rays to the MIR. SED decomposition yields a host-corrected g-band high-to-low-state flux ratio of $5.4\pm0.4$, exceeding the Swift/XRT ratio of $2.7\pm0.8$ across the changing-look transition. This discrepancy may arise from reduced dust extinction, as correcting for the low-state continuum reddening ($A_V^{\rm cont}=0.66\pm0.23$ mag) brings the optical/UV ratios close to the X-ray value, although the correction is model-dependent and intrinsically weaker X-ray variability may also contribute. However, X-ray spectra show no compelling evidence for intrinsic gas absorption or its variability ($N_{\rm H}^{\rm intr}\lesssim5\times10^{20}$ cm$^{-2}$). In contrast, EP/WXT monitoring during the recent high state reveals order-of-magnitude X-ray fluctuations on monthly-to-sub-year timescales, exceeding contemporaneous optical variations. Meanwhile, the radio flux density remains stable at $\sim2$ mJy across all ASKAP observations since mid-2020. Together with optical/UV and MIR variability and the broad He II transition, these results favor intrinsic accretion-state changes as the primary driver, while allowing a secondary contribution from variable obscuration. The source displays a loop-like MIR color--magnitude evolution, likely driven by the delayed response of an extended torus. This recurrent CLAGN offers a laboratory for probing the dynamic response of the broad-line region and circumnuclear dust to accretion-state transitions.
We present a systematic study of the spin evolution of accreting black holes. Using the BHAC code for general relativistic magnetohydrodynamic simulations, we conduct a total of 500 high resolution simulations of magnetized accretion disks around Kerr BHs with spin parameters ranging from $\chi=0$ to $\chi = 0.99$. The baseline configuration follows the default Fishbone--Moncrief torus setup in BHAC. We further explore the effects of: I) introducing multiple poloidal magnetic loops, II) the initial position of the torus, III) magnetic field strength and IV) radial loop modulation. From each simulation we extract the mass flux and angular momentum flux through the horizon to compute the time-averaged spin evolution rate after the system becomes quasi-stationary. The equilibrium spin ranges from $\chi_{\rm eq}\simeq0.88$ in the fiducial model to $\chi_{\rm eq}\simeq0.98$ for weak-flux model. Applied to Sgr A$^*$, even an optimistic estimate for rapid spin evolution gives a timescale longer than $10^{13}$ yr, indicating an earlier growth or merger history. These trends are consistent with the picture that larger net magnetic flux threading the horizon enhances jet-torques and reduces the equilibrium spins, whereas weak net flux yields near-maximal spins. Our results provide a pathway to interpret the high spins reported for X-ray binaries and active galactic nuclei and they sharpen the astrophysical viability of near-horizon high-energy processes that rely on near-extremal black holes.
ALICE experiment at the CERN's Large Hadron Collider, carried out a cosmic data-taking campaign in the period 2015-2018 (LHC Run 2), for a total of 62.5 days of real observation time. In this contribution, the analysis of these data is limited to multimuon events, i.e. events with more than four detected muons. In particular, the muon multiplicity distribution (MMD) is studied in the low-intermediate multiplicity range ($4 < N_{\mu} < 50$), while for multiplicities $N_{\mu} > 100$, dominated by large statistical fluctuations of the data, the rate of the high muon multiplicity events (HMM) is measured. The results are compared with Monte Carlo simulations using three of the main hadronic interaction models describing the air shower development in the atmosphere: QGSJET-II-04, EPOS-LHC, and SIBYLL 2.3d, and assuming two extreme compositions of primary cosmic rays, pure proton and pure iron, representing respectively the lightest and the heaviest composition. Although the models have difficulty precisely describing the trend of primary p and Fe as a function of cosmic ray primary energy, QGSJET-II-04 is the only model that reproduces reasonably well the MMD and the rate of HMM events assuming a heavy composition for the entire energy range studied.
Spherically symmetric general-relativistic simulations show that a first-order hadron--quark phase transition can collapse radially stable neutron stars into black holes, even when a stable hybrid star of the same baryon mass exists. The dynamical threshold lies $0.12\,M_\odot$ in baryon mass below the static one, so black holes can be lighter than the heaviest stable star. The threshold is a Type-I critical point: near it, the star lingers at an unstable hybrid star of equal baryon mass for a time set by that star's unstable mode. The effect persists within a narrower window for finite conversion rates.
We present the long-term rotational evolution and radio emission properties of the magnetar Swift~J1818.0$-$1607, using simultaneous 2.25/8.60~GHz observations obtained with the Shanghai Tian Ma Radio Telescope from MJD~58936 to 59427. The average pulse profiles exhibit significant evolution with time and frequency, attributable to relative changes in the leading, central, and trailing emission components. The magnetar generally spins down with an average rate of $-1.65(4)\times10^{-11}\,\mathrm{s^{-2}}$, with the magnitude of the spin-down rate first increasing and then decreasing over the monitoring span. The overall flux density shows an initial rise followed by a decline, with sporadic variability superimposed; the variations at 2.25 and 8.60~GHz are broadly consistent across most epochs. The two-point spectral index increased from $\alpha\simeq-1.8$ shortly after the outburst to near-zero values around MJD~59030--59050, followed by repeated decreases and increases. Despite this variability, 41 of the 54 two-point spectral-index measurements in the later campaign have $\alpha>-1$. Three component-dependent types of mode changing are identified: Type~I, involving primarily intensity changes in the central component; Type~II, involving central and trailing components; and Type~III, involving leading and central components. All three classes include events showing a corresponding component response in the other observing band, with event-dependent response strengths and directions. Type~I variations are generally more pronounced at 8.60~GHz than at 2.25~GHz.
We study photon dispersion in the presence of a strong external magnetic field for propagation both perpendicular and parallel to the field, considering (i) the magnetized QED vacuum and (ii) a magnetized electron-positron medium at finite density (nonzero chemical potential). The analysis is performed in one-loop QED at finite temperature and density, and we extract polarization-dependent dispersion relations and phase velocities from the eigenmodes of the photon polarization operator. Using these phase velocities, we estimate propagation-induced time delays for photons emitted from neutron stars in the strong-field regime, adopting a simplified magnetosphere model with a toroidal magnetic field geometry and a radially varying chemical potential profile in the degenerate limit. We compare the predicted delays with observational timing constraints and discuss how strong-field QED and plasma corrections can generate measurable departures from the light cone, with potential signatures relevant for magnetars and modern polarimetric and timing observations. It should be noted that the time-delay estimates derived here are based on the phase velocity and serve as indicative order-of-magnitude predictions.
In the hard state, X-ray emission from black hole X-ray binaries is attributed to Comptonization in the corona, a highly magnetized region near the black hole. Relativistic magnetic reconnection drives bulk motions that can upscatter soft photons and reproduce the spectral peak at $\sim 100$ keV for parameters relevant to Cygnus X-1. However, the origin of the seed photons and of the MeV tail above the peak remains unclear. In 3D reconnection, so-called "free" particles escape the flux ropes and accelerate at the maximum rate set by the reconnection electric field. Their synchrotron emission extends to tens of MeV, which enables $\gamma \gamma$ pair production with the Compton-upscattered photons. We present 3D particle-in-cell simulations of relativistic reconnection in pair plasma, in which particles cool by inverse Compton (IC) scattering below the synchrotron self-absorption threshold and by synchrotron radiation above it. Of the incoming magnetic power, $\sim 40 \%$ is converted to IC radiation and $\gtrsim 10$-$20\%$ to synchrotron. Using analytical estimates and radiative calculations for Cygnus X-1, we model how the radiation from reconnection-accelerated particles is reprocessed in the corona. Pair production attenuates the synchrotron emission above $\sim 1$ MeV, and the escaping $\gamma$-rays form an MeV tail consistent with the observations, extending up to at least $\sim 10$ MeV. Most of the absorbed power goes into secondary pairs, whose Thomson optical depth can reach order unity. Their synchrotron emission, peaking at $\sim 0.1 \, \text{keV}$, can supply the seed photons for Comptonization. Our results support reconnection-based synchrotron self-Compton models of the hard-state emission in X-ray binaries.
Context. Blazar jets -- collimated and relativistic plasma outflows viewed at small angles -- are prolific sources of non-thermal radiation. Aims. We develop a numerical framework for calculating the emission from an extended blazar jet where gradual energy dissipation, mediated through magnetic reconnection, takes place across a wide range of distances from the supermassive black hole. Methods. We adapt the open-source radiative transfer code LeHaMoC that self-consistently calculates photon spectra from relativistic charged particles that occupy a spherical blob to treat the emission from an extended jet. The jet is treated as a series of segments that interact with each other through radiative transfer. The effects of radiation spillage are calculated iteratively throughout the jet, accounting for relativistic effects arising from the relative motion between jet segments, until a steady state is reached. Results. We find that this approach produces some distinct differences in the total spectral energy distribution compared to the results obtained by adding the emission escaping from each constituent segment of the jet (optical thin regime), increasing the part of the spectrum affected by inverse Compton scattering by a factor of three. However, the latter is always far less luminous than the synchrotron component, even when enhanced by external radiation fields. Conclusions. Gradual magnetic dissipation in jets from supermassive black holes leads to nearly universal, synchrotron-dominated photon spectra. The numerical framework developed here is readily adaptable to a broad range of jet dissipation scenarios and applications to extended jet emission.
We present the luminous, long-lived Type IIn SN 2019qem and use it to reconstruct how much mass its progenitor lost and when. We combine host-subtracted optical and mid-infrared photometry and 26 optical spectra from -22 to +699 d. We model the bolometric light curve with thin-shell CSM-interaction models in REDBACK, with the shock velocity constrained by the H$\alpha$ line widths, and compare them with an analytic interaction model. SN 2019qem rose in $\approx30$d to $M_I = -20.5$mag and a bolometric peak of $7.0\times10^{43}$ erg s$^{-1}$, and radiated $6.9\times10^{50}$ erg in the optical within 575 d, closely resembling luminous, strongly interacting SNe IIn such as SN 2010jl and SN 2015da. A Heii/Niii flash feature at -22 d and unresolved narrow Balmer lines reveal dense, slow CSM. From +78 to +445 d the H$\alpha$ wings are shaped by electron scattering, and the line stays blueshifted by 400-700 km s$^{-1}$, which points to an optically thick, asymmetric interaction region rather than to newly formed dust. In the preferred model, $\approx7 M_\odot$ of ejecta run into $\approx9 M_\odot$ of gas ejected $\approx2$ yr before explosion and then into a wind of $\approx0.6 M_\odot$ yr$^{-1}$ (for $v_w = 100$ km s$^{-1}$), sweeping up $\approx32 M_\odot$ by +544 d. The mid-infrared excess brightens for two years while the optical fades, tracing dust heated by the ongoing interaction. The host is a low-mass, star-forming galaxy, $\log(M_*/M_\odot) = 9.19$, with a sub-solar metallicity at the explosion site. In its final decades the progenitor of SN 2019qem lost mass far faster than any steady stellar wind, ending with an eruption of $\approx9 M_\odot$ two years before core collapse, consistent with luminous-blue-variable-like eruptive mass loss. Taken at face value, the mass budget implies an initial progenitor mass of $\gtrsim40 M_\odot$.
Cosmic-ray antideuterons and antihelium-3 nuclei are promising probes of dark matter (DM). In light of potential detections of heavy antinuclei at AMS-02, we investigate a recently proposed scenario in which DM annihilates into color-singlet scalars carrying baryon and lepton number, which has been shown to enhance antideuteron and antihelium-3 fluxes relative to antiprotons. Accounting for Galactic propagation and detector sensitivities, we calculate the number of events predicted by our model for parameters that are compatible with the seven-year AMS-02 antiproton data. Broad regions of the DM-mediator mass plane result in observable antideuteron fluxes at AMS-02 and GAPS. While detecting antihelium-3 remains more challenging, potentially detectable fluxes can arise for light mediators and moderately light DM, and in general are enhanced relative to the standard WIMP predictions. Furthermore, we find antihelium-to-antideuteron event ratios of O(0.1), closer to those suggested by the reported AMS-02 candidate events than conventional WIMP predictions.
The strong nitrogen (N) features observed in high-redshift galaxies, such as GN-z11, with JWST point to an efficient source of N production in the early Universe. Very massive stars (VMSs) are promising candidates owing to their strong winds and large convective cores of CNO-processed material. Whether massive-star surfaces in general become N-enriched depends on the interplay of convective-boundary mixing (overshooting), rotation, and mass loss, which remains poorly constrained. Using a grid of MESA evolutionary models, we isolate the overshooting and rotation channels at $40$, $100$, and $200\,M_\odot$ and investigate how these channels bring N-rich material to the surface of massive and very massive stars as a function of initial mass and metallicity. We also construct a synthetic Large Magellanic Cloud (LMC) population based on a larger initial mass grid ranging from $20-300\,M_\odot$ for comparison with the observed N-enriched fraction. VMS surface enrichment is remarkably robust across cosmic metallicity. At lower metallicities, wind stripping is weaker and the overshooting channel shuts off entirely, yet a modest amount of rotation of $\sim90\,\mathrm{km\,s^{-1}}$ alone still enriches the surface. At higher metallicity, stronger winds dominate and uncover the core almost independently of the adopted mixing. The two channels operate through physically distinct mechanisms: overshooting enriches the surface only once winds strip the envelope down into the N-rich layers left by the receding core, giving an abrupt jump. Rotation, by contrast, mixes N outward continuously through the layers above the core, producing a gradual rise. Applied to the LMC, a population with modest rotation drawn from the observed velocity distribution and modest overshooting reproduces the observed N-enriched fraction without large core boundary mixing efficiencies otherwise required by non-rotating models.
Meteoritic inclusions indicate that the early Solar System contained a substantial abundance of the short-lived radionuclide 10Be. Whether it was inherited from the parent molecular cloud or requires local irradiation or late injection remains debated. Here we present the first three-dimensional simulations of a Milky Way-like disc that follow the multi-phase interstellar medium, cosmic rays (CRs), and the production and transport of 10Be by galactic CRs (GCRs) and in supernova remnants (SNRs). Nearly all 10Be is produced by GCR interactions; SNRs contribute at most about ten percent of the 10Be inherited by newly formed stars. The 10Be/9Be ratio traces the CR energy density, declining smoothly with galactocentric radius, and because stars form where supernovae inject CRs, their natal gas is CR-enriched roughly fivefold relative to the mean interstellar medium, which one-zone estimates miss. Accounting for a possibly higher Galactic star formation rate 4.6 Gyr ago, inheritance of GCR-produced 10Be can then explain the early Solar System's 10Be/9Be in our models, without late injection or local irradiation. Even when GCR-induced 10Be production in dense gas is reduced tenfold, the meteoritic value is reached in the innermost disc and missed by only a factor of a few elsewhere, so the main conclusion is robust to uncertain low-energy GCR penetration. The inferred birth radius of the Sun depends strongly on the CR diffusivity, ranging between 3 and 8 kpc, suggesting that improved modelling of low-energy CR transport could sharpen constraints on the birthplace of the Solar System.
The years preceding a supernova (SN) can constrain new electrophilic bosons with MeV masses more than the explosion itself. Although cooler than SNe, the massive progenitors maintain consistently high temperatures much longer, injecting more particles whose decays to positrons contribute to the diffuse Galactic 511-keV emission. For dark photons, the increase is exacerbated by the lower plasma frequency compared to SNe, where MeV excitations are screened. We find preSNe bounds on dark photon couplings up to two orders of magnitude stronger than SNe for masses between 1 and 3~MeV, with similar improvements for other electrophilic species such as axion-like particles.
The Seyfert 1 galaxy H 0557$-$385 (z = 0.034) has long been classified as radio-quiet on the basis of its radio-to-optical flux ratio, yet its radio properties are physically inconsistent with this designation. We present a broadband ($0.076 - 258.8$ GHz) radio spectral analysis combining new Australia Telescope Compact Array (ATCA) observations with archival Murchison Widefield Array (MWA), Australian Square Kilometre Array Pathfinder (ASKAP) and Atacama Large Millimeter/submillimeter Array (ALMA) data. The spectrum requires a two-component model: a compact, self-absorbed component with a stratified gigahertz-peaked (GPS)/high-frequency-peaked (HFP) spectral shape (turnover at $9.4^{+0.6}_{-0.5}$ GHz), superposed on an extended, optically thin power-law lobe ($\alpha_{\rm pl} = -0.78$) that dominates below ~1.6 GHz. The derived parameters $-$ a source size of ~15 pc, an equipartition magnetic field of 2.8 mG, a synchrotron age of $\lesssim1.8\times10^{3}$ yr, and a time-averaged jet power of ${\sim}5\times10^{41}$ $-$ $5\times10^{44}$ erg s$^{-1}$ $-$ are characteristic of compact steep-spectrum and GPS young radio galaxies, not of canonical radio-quiet AGN. The two-component structure admits two interpretations that the present data cannot distinguish: the extended emission may be the growing lobe of a genuinely young, expanding radio galaxy, or relic plasma from an earlier episode of a transient or recurrent jet. Under either, H 0557$-$385 is better understood as an intermediate object: optically a Seyfert, but with the radio-physical properties of a newly ignited or recently re-triggered radio galaxy. It is therefore a candidate missing link between radio-quiet AGN and large-scale radio galaxies. Distinguishing these scenarios will require high-resolution imaging and multi-epoch monitoring
The Laser Interferometer Space Antenna (LISA) is expected to observe gravitational-wave signals from massive black hole binaries, extreme mass ratio inspirals, and more. Despite the significant scientific potential, accurately extracting these signals from the data will only be possible if LISA's instrumental noise is understood: poor noise modelling will result in lost signals, biased parameter estimates, and incorrect astrophysical insights. Modelling LISA's instrumental noise is more challenging than its ground-based counterpart since the data is highly correlated, meaning that the noise covariance matrix can no longer be assumed diagonal in frequency and in the different data streams. In this work, we present an initial approach to perform non-parametric noise inference for LISA that requires no prior knowledge of the LISA observatory. By fitting the full noise covariance matrix with a combination of splines and inverse Lorentzians, we show that our model can estimate the power and cross spectral densities for LISA from 7 days of simulated noise-only data. We find that the median estimate from our non-parametric model whitens the data according to the expected unit normal distribution. We envision that our non-parametric model can be used as part of the global fit, as well as in combination with modelled approaches to identify, quantify, and constrain the frequency dependence of unknown noise artifacts.
We study fully non-linear binary black hole mergers in massive Einstein-scalar-Gauss-Bonnet (EsGB) gravity with a linear Gauss-Bonnet coupling function. In the absence of a well-developed initial data solver for EsGB gravity, we demonstrate that the conventional workaround solution for massless scalar fields results in confined junk radiation that contaminates late-time dynamics in the massive theory. We propose a solution to this problem by suitably modifying the equations of motion at the early stages of the simulation to tame such junk radiation, thereby allowing us to study the dynamics of the scalar field during the process of a black hole merger. This is followed by a preliminary analysis of the frequency content in the ringdown of the scalar and tensor modes. In particular, the quadrupole scalar mode exhibits a rich ringdown structure: while it is dominated by the spin-2 ``contamination'' frequency in the early stages, its late time morphology becomes primarily determined by the scalar mass frequency.
We re-assess effects of Axion Electrodynamics in the presence of non-homogeneous background magnetic fields. The Primakoff effect, which is an ALP-photon mixing induced by a background magnetic field, is generalized to the case of non-homogeneous backgrounds. The production of ALPs from a magnetized plasma thermal medium is also computed. The Eddington's solar standard model is adopted as the phenomenological scenario for our settings, and new bounds on the Axion Electrodynamics parameter space are imposed by considering helioseismological and neutrino observational measurements.
In applied statistics, likelihoods involving high-dimensional integrals are often approximated via Monte Carlo summation. Controlling the resulting numerical variance is crucial for reliable inference. We provide a practical introduction to Latin hypercube sampling (LHS) and its extension based on orthogonal arrays (OA-LHS) as strategies to reduce this variance by stratifying the sampling distribution. LHS asymptotically removes variance contributions from individual coordinates, while strength-2 OA-LHS also removes those from pairwise interactions, yielding orders-of-magnitude improvements over plain Monte Carlo estimators for suitable integrands. As a case study, we investigate their application to the selection function entering hierarchical inference of gravitational-wave populations. At approximately one million injections, we find only a marginal reduction in variance, with low statistical significance. Nevertheless, the computational cost of constructing these designs is negligible relative to evaluating gravitational-wave injections, making even marginal gains worth considering for future applications. This practical introduction to LHS and OA-LHS offers a starting point for other applications in physics and astronomy involving computationally expensive Monte Carlo calculations.
The next generation of an international network of gravitational-wave observatories is now in the advanced planning stages. With Australia's substantial history of contributions in this field, its history of scientific mega-projects like the Square Kilometer Array, and its unique geographic location and conditions, it is ideally placed to host an observatory in the global network. It has already been demonstrated that a gravitational wave detector in Australia plays an important role in minimizing the sky localization uncertainty of gravitational wave events. We study a particular science case for an Australian observatory, focusing on its potential contributions to multimessenger astronomy. We demonstrate that an observatory in Australia can enable the detection of up to eight multimessenger gravitational-wave plus kilonova events per year, with localisations better than $1-10\deg^2$ in both second- and third-generation networks. We show that, in a global network of XG observatories, an Australian observatory with 2G sensitivity would still enable significant advances in multimessenger science. The full case for an Australian observatory must be considered in a much broader context; however, this work indicates it could make substantial contributions to multimessenger astrophysics.
We simulate the propagation and scattering of an incident plane scalar wave in the spacetime of an ultracompact, horizonless constant-density star using a time-domain multipole mode-sum method. By reconstructing the coherently superposed time-domain waveforms, we compare the star's dynamical response with that of a Schwarzschild black hole. We find that the early-time response closely mimics a black-hole ringdown due to wave scattering near the exterior unstable photon sphere. Conversely, the late-time signal develops a sequence of delayed echoes originating from wave trapping near the interior stable light ring and subsequent leakage through the exterior potential barrier. While individual multipoles exhibit regular echoes, their coherent superposition induces interference, producing complex waveforms with irregular profiles that nonetheless retain an underlying temporal organization with alternating peak separations. Finally, as a proof of concept, we perform a signal-to-noise ratio analysis using an assumed strain normalization, demonstrating that these weak, late-time signatures can be resolved by third-generation gravitational-wave detectors.
Large language model (LLM) agents are now applied to astronomical data analysis. They retrieve the literature, reduce observations, and draft conclusions. Under pressure to complete a task, such agents can fabricate results, and the numbers they report may not come from the analysis they ran. We present auto_research, a semi-autonomous multi-agent pipeline that retrieves and analyses raw data from the High Energy Astrophysics Science Archive Research Center (HEASARC) and writes the analysis tools that it needs. We check the work of the agents at two points. A new tool enters the toolkit after it passes automatic checks, including repeated test runs and a scan for phrases that accompany invented values, and a code review by a model from a different provider; a human decides whether to promote it to the trusted branch. A number in a report passes only if a recorded output of a toolkit tool or the request contains it, or if the report gives its location in a cited paper. We test the pipeline with data from the Imaging X-ray Polarimetry Explorer (IXPE). Of 65 defects that we planted in the toolkit, 47 pass the automatic checks, and one passes the code review as well; the review also rejects 40 of 69 correct changes. The pipeline reproduces the published IXPE results for Cyg X-1: all 14 compared quantities agree within 3 sigma, and the polarization degrees are lower than the published values. With weaker models in the main roles, the pipeline recovers 12 of the 14 quantities and reports the other two as not measured. We also used the pipeline for a polarization-stability analysis of six observations, a survey of polarization against hardness in 29 observations, and a search for a time lag between the Stokes parameters q and u.