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Abstract The brightest gamma-ray burst (GRB) ever, GRB 221009A, displays ultralong GRB (ULGRB) characteristics, with a prompt emission duration exceeding 1000 s. To constrain the origin and central engine of this unique burst, we analyze its prompt and afterglow characteristics and compare them to the established set of similar GRBs. To achieve this, we statistically examine a nearly complete sample of Swift-detected GRBs with measured redshifts. We categorize the sample to bronze, silver, and gold by fitting a Gaussian function to the log-normal ofT₉₀duration distribution and considering three subsamples respectively to 1, 2, and 3 times of the standard deviation to the mean value. GRB 221009A falls into the gold subsample. Our analysis of prompt emission and afterglow characteristics aims to identify trends between the three burst groups. Notably, the gold subsample (a higher likelihood of being ULGRB candidates) suggests a collapsar scenario with a hyperaccreting black hole as a potential central engine, while a few GRBs (GRB 060218, GRB 091024A, and GRB 100316D) in our gold subsample favor a magnetar. Late-time near-IR observations from 3.6 m Devasthal Optical Telescope rule out the presence of any bright supernova associated with GRB 221009A in the gold subsample. To further constrain the physical properties of ULGRB progenitors, we employ the toolMESAto simulate the evolution of low-metallicity massive stars with different initial rotations. The outcomes suggest that rotating (Ω ≥ 0.2 Ω_c) massive stars could potentially be the progenitors of ULGRBs within the considered parameters and initial inputs toMESA. 

The bright GRB 210610B was discovered simultaneously by Fermi and Swift missions at redshift 1.13. We utilized broadband Fermi-GBM observations to perform a detailed prompt emission spectral analysis and to understand the radiation physics of the burst. Our analysis displayed that the low energy spectral index (αpt) exceeds boundaries expected from the typical synchrotron emission spectrum (-1.5,-0.67), suggesting additional emission signature. We added an additional thermal model with the typical Band or CPL function and found that CPL + BB function is better fitting to the data, suggesting a hybrid jet composition for the burst. Further, we found that the beaming corrected energy (Eγ,θj = 1.06 × 1051 erg) of the burst is less than the total energy budget of the magnetar. Additionally, the X-ray afterglow light curve of this burst exhibits achromatic plateaus, adding another layer of complexity to the explosion’s behavior. Interestingly, we noted that the X-ray energy release during the plateau phase (EX,iso = 1.94 × 1051 erg) is also less than the total energy budget of the magnetar. Our results indicate the possibility that a magnetar could be the central engine for this burst. 

ABSTRACT Hydrogen-poor superluminous supernovae (SLSNe) are among the most energetic explosions in the universe, reaching luminosities up to 100 times greater than those of normal supernovae. This paper presents the largest compilation of SLSN photospheric spectra to date, encompassing data from the advanced Public ESO Spectroscopic Survey of Transient Objects (ePESSTO+), the Finding Luminous and Exotic Extragalactic Transients (FLEET) search, and all published spectra up to December 2022. The data set includes a total of 974 spectra of 234 SLSNe. By constructing average phase binned spectra, we find SLSNe initially exhibit high temperatures (10 000–11 000 K), with blue continua and weak lines. A rapid transformation follows, as temperatures drop to 5000–6000 K by 40 d post-peak, leading to stronger P-Cygni features. Variance within the data set is slightly reduced when defining the phase of spectra relative to explosion, rather than peak, and normalising to the population’s median e-folding decline time. Principal Component Analysis (PCA) supports this, requiring fewer components to explain the same level of variation when binning data by scaled days from explosion, suggesting a more homogeneous grouping. Using PCA and K-means clustering, we identify outlying objects with unusual spectroscopic evolution and evidence for energy input from interaction, but find no support for groupings of two or more statistically significant subpopulations. We find Fe ii  $$\lambda$$5169 line velocities closely track the radius implied from blackbody fits, indicating formation near the photosphere. We also confirm a correlation between velocity and velocity gradient, which can be explained if all SLSNe are in homologous expansion but with different scale velocities. This behaviour aligns with expectations for an internal powering mechanism. 

Abstract With a small sample of fast X-ray transients (FXTs) with multiwavelength counterparts discovered to date, their progenitors and connections toγ-ray bursts (GRBs) and supernovae (SNe) remain ambiguous. Here, we present photometric and spectroscopic observations of SN 2025kg, the SN counterpart to the FXT EP 250108a. Atz= 0.17641, this is the closest known SN discovered following an Einstein Probe (EP) FXT. We show that SN 2025kg’s optical spectra reveal the hallmark features of a broad-lined Type Ic SN. Its light-curve evolution and expansion velocities are comparable to those of GRB-SNe, including SN 1998bw, and two past FXT-SNe. We present JWST/NIRSpec spectroscopy taken around SN 2025kg’s maximum light, and find weak absorption due to HeI1.0830μm and 2.0581μm and a broad, unidentified emission feature at ∼4–4.5μm. Further, we observe broadened Hαin optical data at 42.5 days that is not detected at other epochs, indicating interaction with H-rich material. From its light curve, we derive a⁵⁶Ni mass of 0.2–0.6M_⊙. Together with our companion Letter, our broadband data are consistent with a trapped or low-energy (≲10⁵¹erg) jet-driven explosion from a collapsar with a zero-age main-sequence mass of 15–30M_⊙. Finally, we show that the sample of EP FXT-SNe supports past estimates that low-luminosity jets seen through FXTs are more common than successful (GRB) jets, and that similar FXT-like signatures are likely present in at least a few percent of the brightest Type Ic-BL SNe.