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Abstract:

Fast radio bursts (FRBs) are sufficiently energetic to be detectable from luminosity distances up to at least seven billion parsecs (redshift ). Probing the maximum energies and luminosities of FRBs constrains their emission mechanism and cosmological population. Here, we investigate the maximum energetics of a highly active repeater, FRB 20220912A, using 1500 h of observations. We detect 130 high-energy bursts and find a break in the burst energy distribution, with a flattening of the power-law slope at higher energy – consistent with the behaviour of another highly active repeater, FRB 20201124A. There is a roughly equal split of integrated burst energy between the low- and high-energy regimes. Furthermore, we model the rate of the highest energy bursts and find a turnover at a characteristic spectral energy density of erg Hz. This characteristic maximum energy agrees well with observations of apparently one-off FRBs, suggesting a common physical mechanism for their emission. The extreme burst energies push radiation and source models to their limit: at this burst rate a typical magnetar ( G) would deplete the energy stored in its magnetosphere in 2150 h, assuming a radio efficiency . We find that the high-energy bursts ( erg Hz) play an important role in exhausting the energy budget of the source.

Abstract:

The launch of the Einstein probe (EP) mission has revolutionized the detection and follow-up observations of fast X-ray transients (FXTs) by providing prompt and timely access to their precise localizations. In the first year of its operation, the EP mission reported the discovery of 72 high signal-to-noise FXTs. Subjected to the visibility in the sky and weather conditions, we search for the optical counterparts of 42 EP-discovered FXTs from the Lulin Observatory. We successfully detected the optical counterparts of 12 FXTs, and five of those were first discovered by us from the Lulin Observatory. We find that the optical counterparts are generally faint (r > 20 mag) and decline rapidly (>0.5 mag day−1). We also find that 12 out of 42 FXTs show direct evidence of their association with gamma-ray bursts (GRBs) through significant temporal and spatial overlapping. Furthermore, the luminosities and redshifts of FXTs with confirmed optical counterparts in our observations are fully consistent with the faintest end of the GRB population. However, the nondetection of any associated optical counterpart with a significant fraction of FXTs suggests that EP FXTs are likely a subset of the so-called “dark FXTs,” similar to “dark GRBs.” Additionally, the luminosities of two FXTs with confirmed redshifts are also consistent with jetted tidal disruption events (TDEs). However, we find that the optical luminosities of FXTs differ significantly from typical supernova shock breakout or kilonova emissions. Thus, we conclude that a significant fraction of EP-discovered FXTs are associated with events having relativistic jets; either a GRB or a jetted TDE.

Abstract:

The stellar velocity dispersion ( σ ) of massive elliptical galaxies is a key ingredient in breaking the mass-sheet degeneracy and obtaining precise and accurate cosmography from gravitational time delays. The relative uncertainty on the Hubble constant H 0 is double the relative error on σ . Therefore, time-delay cosmography imposes much more demanding requirements on the precision and accuracy of σ than galaxy studies. While precision can be achieved with an adequate signal-to-noise ratio (S/N), the accuracy critically depends on key factors such as the elemental abundance and temperature of stellar templates, flux calibration, and wavelength ranges. We carried out a detailed study of the problem using multiple sets of galaxy spectra of massive elliptical galaxies with S/N ∼ 30–160 Å −1 , along with state-of-the-art empirical and semi-empirical stellar libraries and stellar population synthesis templates. We show that the choice of stellar library is generally the dominant source of residual systematic errors. We propose a general recipe for mitigating and accounting for residual uncertainties. We show that a sub-percent level of accuracy can be achieved on individual spectra with our data quality, which we subsequently validated with simulated mock datasets. The covariance between velocity dispersions measured for a sample of spectra can also be reduced to sub-percent levels. We recommend this recipe for all applications that require high precision and accurate stellar kinematics. Thus, we have made all the software publicly available to facilitate its implementation. This recipe will also be used in future TDCOSMO collaboration papers.

Abstract:

The central regions of disc galaxies host a rich variety of stellar structures: nuclear discs, bars, bulges, and boxy-peanut bulges. These components are often difficult to disentangle, both photometrically and kinematically, particularly in star-forming galaxies where dust obscuration and complex stellar motions complicate interpretation. In this work, we used data from the GECKOS-MUSE survey to investigate the impact of dust on axisymmetric Jeans Anisotropic Multi-Gaussian Expansion (JAM) models and assess their ability to recover kinematic structures in edge-on disc galaxies. We constructed JAM models for a sample of seven edge-on ( i ⪆ 85°) galaxies that span a range of star formation rates, dust content, and kinematic complexity. We find that when dust is appropriately masked, the disc regions of each galaxy are fit to χ reduced 2 ≤ 5. We analysed 2D residual velocity fields to identify signatures of non-axisymmetric structure. We find that derived dynamical masses are constant within 10% for each galaxy across all dust masking levels. In NGC 3957, a barred boxy galaxy in our sample, we identified velocity residuals that persist even under aggressive dust masking, aligned with bar orbits and 91̽��ed by photometric bar signatures. We extended this analysis to reveal a bar in IC 1711 and a possible side-on bar in NGC 0522. Our results highlight both the capabilities and limitations of JAM in dusty, edge-on systems and attempt to link residual velocities to known non-axisymmetric kinematic structure.