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The detectability of neutrinos from past Galactic core collapse supernovae (SNe) is investigated in terms of nuclear recoil tracks in ancient minerals, known as paleodetectors. To account for the diversity of core-collapse outcomes, variations in neutron star (NS) masses as well as failed SNe leading to black hole (BH) formation are taken into account. The role of the nuclear equation of state (EOS) is also considered, as it determines NS radii and maximum masses. The enhancement of sensitivity is quantified for models with larger neutrino emission. This emission reflects the gravitational energy released during core collapse and is larger for more compact remnants in the NS-forming case and for EOS with larger maximum masses in the BH-forming case. Furthermore, motivated by the hypothesis that nearby SN activity may have contributed to the snowball Earth events, the sensitivity to SNe occuring in a burst-like manner is also investigated. The results indicate that burst-like SN activity involving several tens of events at a distance of 10 pc would be within the reach of paleodetectors.

In this study, within the framework of <mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mo>(</mml:mo><mml:mi>R</mml:mi><mml:mo>,</mml:mo><mml:mi>T</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:math> modified gravity, we investigate the influence of coupling parameter (λ), magnetic field and anisotropy parameter on the neutron star structure. This work employs an accurate equation of state (EoS), derived from realistic microscopic calculations based on the AV18 nucleon-nucleon potential, to compute the structure of this compact object. Here, determination of Schwarzschild radius, compactness, gravitational surface redshift and Kretschmann scalar within the <mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mo>(</mml:mo><mml:mi>R</mml:mi><mml:mo>,</mml:mo><mml:mi>T</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:math> gravity, confirms that our theoretical results are consistent with the observational constraints. While established physical EoSs within the framework of Einstein gravity have successfully characterized a broad range of compact objects, they remain inadequate in explaining certain massive objects residing within the mass gap (<mml:math><mml:mrow><mml:mn>2.5</mml:mn><mml:mo>−</mml:mo><mml:mn>5</mml:mn><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:mrow></mml:math>). We show that some compact objects residing in the mass gap can be interpreted as candidates of neutron stars within the framework of <mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mo>(</mml:mo><mml:mi>R</mml:mi><mml:mo>,</mml:mo><mml:mi>T</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:math> gravity. Motivated by the observation of objects within this mass range, we employ a framework that simultaneously incorporates <mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mo>(</mml:mo><mml:mi>R</mml:mi><mml:mo>,</mml:mo><mml:mi>T</mml:mi><mml:mo>)</mml:mo><mml:mo>=</mml:mo><mml:mi>R</mml:mi><mml:mo>+</mml:mo><mml:mn>2</mml:mn><mml:mi>λ</mml:mi><mml:mi>T</mml:mi></mml:mrow></mml:math> gravity and Bowers-Liang anisotropy model. Our study concludes that an increase in the star's surface magnetic field (which dominates the softening of the EoS) leads to a reduction in the maximum masses and corresponding radii. Furthermore, at a fixed λ, larger values of the anisotropy parameter β result in increased masses and radii. Conversely, for fixed β, a decrease in λ yields an increasing trend in both masses and radii. Finally, we compare our results with the observational data from LIGO/Virgo/KAGRA and NICER, setting the parameters of the <mml:math><mml:mrow><mml:mi>f</mml:mi><mml:mo>(</mml:mo><mml:mi>R</mml:mi><mml:mo>,</mml:mo><mml:mi>T</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:math> theory and anisotropy to successfully reproduce the masses and radii of the GW170817, PSR J0952-0607 and PSR J0740+6620 and the masses of the secondary components of GW190814 (<mml:math><mml:mrow><mml:msub><mml:mi>m</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mo>=</mml:mo><mml:mn>2</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mn>59</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.09</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.08</mml:mn></mml:mrow></mml:msubsup><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:mrow></mml:math>) and GW200210-092254 (<mml:math><mml:mrow><mml:msub><mml:mi>m</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mo>=</mml:mo><mml:mn>2</mml:mn><mml:mo>.</mml:mo><mml:msubsup><mml:mn>83</mml:mn><mml:mrow><mml:mo>−</mml:mo><mml:mn>0.42</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mn>0.47</mml:mn></mml:mrow></mml:msubsup><mml:msub><mml:mi>M</mml:mi><mml:mo>⊙</mml:mo></mml:msub></mml:mrow></mml:math>).

Fast radio bursts (FRBs) emit broadband radio emission that may, in rare cases, encode atomic hydrogen (HI) absorption signals as they traverse the interstellar medium of their host galaxies. Though considered in the early FRB literature, the demanding observational prerequisites and the rarity of suitable events have meant that no thorough search for HI absorption in FRB spectra has yet been undertaken. Here, we present an updated systematic analysis assessing the likelihood of modern facilities to detect such absorption features. As a proof of concept, we search for absorption in the spectrum of the bright ASKAP-localised FRB 20211127I, finding a <inline-formula> 3 sigma 3σ<tex-math> $3\sigma$ </tex-math> </inline-formula> opacity upper limit of 0.51. While this test case offers little constraining power, we find that narrow FRBs with fluences exceeding 20/70/150 Jy ms observed with MeerKAT/ASKAP/DSA can probe opacities below 0.1 ─ a regime in which absorption detections become physically meaningful. We further highlight that stacking thousands of bursts from hyperactive repeaters with FAST offers a very powerful avenue towards detection. Finally, we discuss the broad scientific potential of such detections, including constraints on extragalactic HI spin temperatures, a means to physically probe the environment surrounding the progenitor, and a path towards disentangling host galaxy contributions to dispersion and scattering.

We investigate the late-time cosmology of the Wald─Gauss─Bonnet (WGB) topological dark energy framework by examining two versions; Model I, a purely geometric scenario, and Model II, which treats the integration constant as a model parameter linked to the Gauss-Bonnet coupling. Using a joint dataset of Cosmic Chronometers, DESI DR2 BAO, Type Ia Supernovae (PN<mml:math><mml:msup><mml:mrow></mml:mrow><mml:mo>+</mml:mo></mml:msup></mml:math>, DES-SN5YR, Union3), and Gamma-Ray Bursts, we constrain the model parameters and reconstruct the expansion history. Both models provide a good fit to the observed H(z) and distance-modulus data and predict a consistent transition from deceleration to acceleration. Model II yields stable correlations involving the Gauss─Bonnet coupling, indicating that the extended parameter space remains observationally well behaved. A tension heatmap constructed for these models shows generally low internal tension across most dataset combinations, with noticeable shifts primarily when the SH0ES prior is included. The WGB framework is able to accommodate higher SH0ES-calibrated values of H<SUB>0</SUB> while remaining consistent with other late-time probes. Information criteria further show that the WGB models, particularly Model II, perform competitively with ΛCDM for the full dataset combination. These results suggest that WGB topological dark energy provides a viable geometric framework for describing late-time cosmic acceleration.

Black holes (BHs) and neutron stars (NSs) with low-mass stellar companions challenge traditional isolated binary evolution models, motivating hierarchical triple evolution as a promising alternative. To search for tertiaries, we perform deep, adaptive optics-assisted, near-infrared imaging of five quiescent BH low-mass X-ray binaries (LMXBs), Gaia BH1, and twelve Gaia NSs. We detect several faint stars previously unresolved in survey imaging, but none are close enough to robustly rule out a chance alignment. To achieve high contrast sensitivity at close separations, we use the reference star differential imaging strategy with the Karhunen─Loéve Image Processing algorithm to model and subtract the point-spread function of each target. We identify tertiary candidates in the speckle-dominated regime, but injection-recovery tests suggest most 5σ detections are likely artifacts. We derive 5σ contrast curves and convert these to limits on the mass of main sequence (MS) tertiaries and the effective temperature of white dwarf (WD) tertiaries consistent with a non-detection. We rule out plausible MS tertiaries and young, hot WD tertiaries at projected separations ≳500 au for the Gaia compact object binaries and ≳2000 au for the more distant BH LMXBs. While the recent discovery of a 1.2 M<SUB>⊙ </SUB>tertiary to V404 Cygni supports triple formation scenarios for BH LMXBs, our results suggest such companions are relatively rare. Our observations remain consistent with intermediate-mass tertiaries that have since evolved into cool WDs, detectable with deeper JWST imaging. Follow-up observations are required to measure proper motions and confirm or rule out physical association of tertiary candidates.