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Abstract:

ABSTRACT We present the discovery and multiwavelength follow-up of gamma-ray bursts (GRB) 250818B, detected by the Space Variable Objects Monitor (SVOM) and localized by the Gravitational-wave Optical Transient Observer. We compile X-ray, optical/near-infrared, and radio data into broad-band light curves and spectral energy distributions. Despite being initially reported as short, the public prompt light-curve figure suggests a duration of $\sim 5$ s, arguing against a secure short-duration classification. The afterglow is unusually bright relative to short-GRB samples: in X-rays and optical it is broadly consistent with the long-GRB population, while in the radio it is among the most luminous events in the short-GRB comparison sample and remains within the long-GRB range. MeerKAT detects the source at 3.1 GHz, while the Atacama Large Millimetre/submillimetre Array provides deep higher-frequency limits. Keck/LRIS spectroscopy shows continuum and metal absorption (Fe ii, Mg ii, Mg i), giving $z=1.216$. Synchrotron forward-shock modelling favours a constant-density medium and refreshed (energy-injection) emission, well described by a two-component jet with core isotropic-equivalent kinetic energy $E_{0}\sim 8\times 10^{52}$ erg, ambient number density $n_0\sim 12\, \mathrm{cm^{-3}}$, jet half-opening angle $\theta _j\simeq 0.13$ rad ($\sim 7.4^\circ$), and electron index $p\simeq 1.67$. We do not identify a host conclusively: the data admit either a faint LS DR10 galaxy at $\sim 4\, {\rm arcsec}$ ($\sim 34$ kpc) as an offset candidate, or a fainter, near-coincident uncatalogued host. GRB 250818B highlights the power of rapid counterpart identification in the SVOM era, while uncertain prompt classification and inconclusive host association still limit progenitor interpretation.

Abstract:

We present optical + near-infrared + mid-infrared (MIR) observations of the normal Type Ia supernovae (SNe Ia) 2022aaiq and 2024gy in the nebular phase, continuously spanning 0.35–28 μm. Medium-resolution JWST spectroscopy reveals novel narrow (vFWHM < 1500 km s−1) [Ni ii] 1.94 and 6.64 μm cores in both events. The MIR [Ni ii] 6.64 μm line exhibits a distinct narrow core atop a broader base, indicating a central enhancement of stable Ni. This structure points to high central densities consistent with a near-Chandrasekhar-mass (MCh) progenitor or a high-metallicity sub-MCh progenitor. From detailed line-profile inversions of SN 2024gy, we derive emissivity profiles for stable iron-group elements, radioactive material, and intermediate-mass elements, revealing spatially distinct ejecta zones. The [Ni iii] 7.35 μm line shows a shallow-to-steep slope transition—a “broken-slope” morphology—that matches predictions for delayed detonation explosions with separated deflagration and detonation ashes. We also reanalyze and compare to archival JWST spectra of SN 2021aefx and the subluminous SN 2022xkq. From the stable Ni luminosities, we infer that SN 2024gy produced ∼5–10 times more stable Ni mass than SN 2022xkq, favoring a near-MCh scenario for SN 2024gy and a sub-MCh scenario for SN 2022xkq. These results demonstrate that resolved line profiles, now accessible with JWST, provide powerful diagnostics of explosion geometry, central density, and progenitor mass in SNe Ia.

Abstract:

We report the detection of cyclotron resonant scattering features (CRSFs) in the spectrum of the unique bursting pulsar GRO J1744–28, observed during its recent outburst in 2021 with the Neutron Star Interior Composition Explorer (NICER). Clear pulsations at a frequency of 2.141128 Hz as well as Type II X-ray bursts were observed. The pulse profile exhibits a single-peaked shape in all energy bands, with the pulse fraction showing a positive correlation with energy. We find that the persistent X-ray continuum of the accreting pulsar is well described by typical phenomenological models, and we confirm the presence of the cyclotron line at ∼5 keV as reported in previous studies. In addition, we detect a candidate absorption feature with a centroid energy of 2 keV. If confirmed, this feature could be interpreted as a CRSF, which would correspond to a magnetic field of ∼1.8 × 10 11 G. Pulse-phase-resolved analysis also reveals this absorption line around the peak pulse phases. These NICER observations provide tentative evidence for the cyclotron line candidate, establishing GRO J1744–28 as a key laboratory for studying accretion physics in an intermediate-strength magnetic field.