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Kilonovae are approximately thermal transients, produced by the mergers of binary neutron stars (BNSs) and neutron star (NS)–black hole binaries. As the optical counterpart of the gravitational-wave event GW170817, AT2017gfo is the first kilonova detected with smoking-gun evidence. Its observation offers vital information for constraining the Hubble constant, the sources of cosmic r -process enrichment, and the equation of state of NSs. The 2.5 m Wide-Field Survey Telescope (WFST) operates in six bands (u, g, r, i, z, w), spanning from 320 to 925 nm. It will be completed in the first half of 2023, and with a field-of-view diameter of 3°, aims to detect kilonovae in the near future. In this article, considering the influence of the host galaxies and sky brightness, we generate simulated images to investigate WFST's ability to detect AT2017gfo-like kilonovae. Due to their spectra, host galaxies can significantly impact kilonova detection at longer wavelengths. When kilonovae are at peak luminosity, we find that WFST performs better in the g and r bands and can detect 90% (50%) of kilonovae at a luminosity distance of 248 Mpc (338 Mpc) with 30 s exposures. Furthermore, to reflect the actual efficiency under target-of-opportunity observations, we calculate the total time of follow up under various localization areas and distances. We find that if the localization areas of most BNS events detected during the fourth observing (O4) run of LIGO and Virgo are hundreds of deg2, WFST is expected to find ∼30% of kilonovae in the first two nights following the detection of a GW event produced by a BNS during the O4 period. [ABSTRACT FROM AUTHOR]

In this paper, we model the dynamics and radiation physics of the rarity event GRB 221009A afterglow in detail. By introducing a top-hat jet that propagates in an environment dominated by stellar winds, we explain the publicly available observations of afterglow associated with GRB 221009A over the first week. It is predicted that GRB 221009A emits a luminous very high energy afterglow based on the synchrotron self-Compton (SSC) process in our model. We show the broadband spectral energy distribution (SED) analysis results of GRB 221009A and find that the SSC radiation component of GRB 221009A is very bright in the 0.1–10 TeV band. The integrated SED shows that the SSC emission in the TeV band has detection sensitivity significantly higher than that of LHASSO, MAGIC, and CTA. However, since the release of further observations, deviations from the standard wind environment model have gradually shown up in data. For example, the late-time multiband afterglow cannot be consistently explained under the standard wind environment scenario. It may be necessary to consider modeling with a structured jet with complex geometry or a partial revision of the standard model. Furthermore, we find that the inclusion of GeV observations could break the degeneracy between model parameters, highlighting the significance of high-energy observations in determining accurate parameters for GRB afterglows. [ABSTRACT FROM AUTHOR]

Observationally, kilonovae are astrophysical transients powered by the radioactive decay of nuclei heavier than iron, thought to be synthesized in the merger of two compact objects 1-4 . Over the first few days, the kilonova evolution is dominated by a large number of radioactive isotopes contributing to the heating rate 2,5 . On timescales of weeks to months, its behaviour is predicted to differ depending on the ejecta composition and the merger remnant 6-8 . Previous work has shown that the kilonova associated with gamma-ray burst 230307A is similar to kilonova AT2017gfo (ref. 9 ), and mid-infrared spectra revealed an emission line at 2.15 micrometres that was attributed to tellurium. Here we report a multi-wavelength analysis, including publicly available James Webb Space Telescope data 9 and our own Hubble Space Telescope data, for the same gamma-ray burst. We model its evolution up to two months after the burst and show that, at these late times, the recession of the photospheric radius and the rapidly decaying bolometric luminosity (L bol  ∝ t -2.7±0.4 , where t is time) support the recombination of lanthanide-rich ejecta as they cool., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)