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1. Characterizing the Ordinary Broad-line Type Ic SN 2023pel from the Energetic GRB 230812B

   https://doi.org/10.3847/2041-8213/ad16e7  

   Srinivasaragavan, Gokul P. ; Swain, Vishwajeet ; O’Connor, Brendan ; Anand, Shreya ; Ahumada, Tomás ; Perley, Daniel ; Stein, Robert ; Sollerman, Jesper ; Fremling, Christoffer ; Cenko, S. Bradley ; et al ( January 2024 , The Astrophysical Journal Letters)

   We report observations of the optical counterpart of the long gamma-ray burst (GRB) GRB 230812B and its associated supernova (SN) SN 2023pel. The proximity (z= 0.36) and high energy (E_γ,iso∼ 10⁵³erg) make it an important event to study as a probe of the connection between massive star core collapse and relativistic jet formation. With a phenomenological power-law model for the optical afterglow, we find a late-time flattening consistent with the presence of an associated SN. SN 2023pel has an absolute peakr-band magnitude ofM_r= −19.46 ± 0.18 mag (about as bright as SN 1998bw) and evolves on quicker timescales. Using a radioactive heating model, we derive a nickel mass powering the SN ofM_Ni= 0.38 ± 0.01M_⊙and a peak bolometric luminosity ofL_bol∼ 1.3 × 10⁴³erg s⁻¹. We confirm SN 2023pel’s classification as a broad-line Type Ic SN with a spectrum taken 15.5 days after its peak in therband and derive a photospheric expansion velocity ofv_ph= 11,300 ± 1600 km s⁻¹at that phase. Extrapolating this velocity to the time of maximum light, we derive the ejecta massM_ej= 1.0 ± 0.6M_⊙and kinetic energy$E_{\mathrm{KE}}={1.3}_{-1.2}^{+3.3}\times {10}^{51}\mathrm{erg}$. We find that GRB 230812B/SN 2023pel has SN properties that are mostly consistent with the overall GRB-SN population. The lack of correlations found in the GRB-SN population between SN brightness andE_γ,isofor their associated GRBs across a broad range of 7 orders of magnitude provides further evidence that the central engine powering the relativistic ejecta is not coupled to the SN powering mechanism in GRB-SN systems.

    

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2. A Collapsar Origin for GRB 211211A Is (Just Barely) Possible

   https://doi.org/10.3847/1538-4357/acc384  

   Barnes, Jennifer ; Metzger, Brian D. ( April 2023 , The Astrophysical Journal)

   Gamma-ray bursts (GRBs) have historically been divided into two classes. Short-duration GRBs are associated with binary neutron star mergers (NSMs), while long-duration bursts are connected to a subset of core-collapse supernovae (SNe). GRB 211211A recently made headlines as the first long-duration burst purportedly generated by an NSM. The evidence for an NSM origin was excess optical and near-infrared emission consistent with the kilonova observed after the gravitational-wave-detected NSM GW170817. Kilonovae derive their unique electromagnetic signatures from the properties of the heavy elements synthesized by rapid neutron capture (ther-process) following the merger. Recent simulations suggest that the “collapsar” SNe that trigger long GRBs may also producer-process elements. While observations of GRB 211211A and its afterglow rule out an SN typical of those that follow long GRBs, an unusual collapsar could explain both the duration of GRB 211211A and ther-process-powered excess in its afterglow. We use semianalytic radiation transport modeling to evaluate low-mass collapsars as the progenitors of GRB 211211A–like events. We compare a suite of collapsar models to the afterglow-subtracted emission that followed GRB 211211A, and find the best agreement for models with high kinetic energies and an unexpected pattern of⁵⁶Ni enrichment. We discuss how core-collapse explosions could produce such ejecta, and how distinct our predictions are from those generated by more straightforward kilonova models. We also show that radio observations can distinguish between kilonovae and the more massive collapsar ejecta we consider here.

    

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3. A Sensitive Search for Supernova Emission Associated with the Extremely Energetic and Nearby GRB 221009A

   https://doi.org/10.3847/2041-8213/accf97  

   Srinivasaragavan, Gokul P. ; O’Connor, Brendan ; Cenko, S. Bradley ; Dittmann, Alexander J. ; Yang, Sheng ; Sollerman, Jesper ; Anupama, G. C. ; Barway, Sudhanshu ; Bhalerao, Varun ; Kumar, Harsh ; et al ( June 2023 , The Astrophysical Journal Letters)

   We report observations of the optical counterpart of the long gamma-ray burst GRB 221009A. Due to the extreme rarity of being both nearby (z= 0.151) and highly energetic (E_γ,iso≥ 10⁵⁴erg), GRB 221009A offers a unique opportunity to probe the connection between massive star core collapse and relativistic jet formation across a very broad range ofγ-ray properties. Adopting a phenomenological power-law model for the afterglow and host galaxy estimates from high-resolution Hubble Space Telescope imaging, we use Bayesian model comparison techniques to determine the likelihood of an associated supernova (SN) contributing excess flux to the optical light curve. Though not conclusive, we find moderate evidence (K_Bayes= 10^1.2) for the presence of an additional component arising from an associated SN, SN 2022xiw, and find that it must be substantially fainter (<67% as bright at the 99% confidence interval) than SN 1998bw. Given the large and uncertain line-of-sight extinction, we attempt to constrain the SN parameters (M_Ni,M_ej, andE_KE) under several different assumptions with respect to the host galaxy’s extinction. We find properties that are broadly consistent with previous GRB-associated SNe:M_Ni= 0.05–0.25M_⊙,M_ej= 3.5–11.1M_⊙, andE_KE= (1.6–5.2) × 10⁵²erg. We note that these properties are weakly constrained due to the faintness of the SN with respect to the afterglow and host emission, but we do find a robust upper limit onM_NiofM_Ni< 0.36M_⊙. Given the tremendous range in isotropic gamma-ray energy release exhibited by GRBs (seven orders of magnitude), the SN emission appears to be decoupled from the central engine in these systems.

    

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4. Production of Very Light Elements and Strontium in the Early Ejecta of Neutron Star Mergers

   https://doi.org/10.3847/1538-4357/ac3751  

   Perego, Albino ; Vescovi, Diego ; Fiore, Achille ; Chiesa, Leonardo ; Vogl, Christian ; Benetti, Stefano ; Bernuzzi, Sebastiano ; Branchesi, Marica ; Cappellaro, Enrico ; Cristallo, Sergio ; et al ( January 2022 , The Astrophysical Journal)

   We study the production of very light elements (Z< 20) in the dynamical and spiral-wave wind ejecta of binary neutron star mergers by combining detailed nucleosynthesis calculations with the outcome of numerical relativity merger simulations. All our models are targeted to GW170817 and include neutrino radiation. We explore different finite-temperature, composition-dependent nuclear equations of state, and binary mass ratios, and find that hydrogen and helium are the most abundant light elements. For both elements, the decay of free neutrons is the driving nuclear reaction. In particular, ∼0.5–2 × 10⁻⁶M_⊙of hydrogen are produced in the fast expanding tail of the dynamical ejecta, while ∼1.5–11 × 10⁻⁶M_⊙of helium are synthesized in the bulk of the dynamical ejecta, usually in association with heavyr-process elements. By computing synthetic spectra, we find that the possibility of detecting hydrogen and helium features in kilonova spectra is very unlikely for fiducial masses and luminosities, even when including nonlocal thermodynamic equilibrium effects. The latter could be crucial to observe helium lines a few days after merger for faint kilonovae or for luminous kilonovae ejecting large masses of helium. Finally, we compute the amount of strontium synthesized in the dynamical and spiral-wave wind ejecta, and find that it is consistent with (or even larger than, in the case of a long-lived remnant) the one required to explain early spectral features in the kilonova of GW170817.

    

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5. Revisiting the Spectral Energy Correlations of GRBs with Fermi Data. I. Model-wise Properties

   https://doi.org/10.3847/1538-4365/acc867  

   Li, Liang ( June 2023 , The Astrophysical Journal Supplement Series)

   Abstract Gamma-ray bursts (GRBs) exhibit a diversity of spectra. Several spectral models (e.g., Band, cutoff power law (CPL), and blackbody) and their hybrid versions (e.g., Band+blackbody) have been widely used to fit the observed GRB spectra. Here, we attempt to collect all the bursts detected by Fermi/GBM with known redshifts from 2008 July to 2022 May, having been motivated to (i) provide a parameter catalog independent of the official Fermi/GBM team and (ii) achieve a “clean” model-based GRB spectral energy correlation analysis. A nearly complete GRB sample is created, containing 153 such bursts (136 long GRBs and 17 short GRBs). Using the sample and by performing detailed spectral analysis and model comparisons, we investigate two GRB spectral energy correlations: the correlation of the cosmological rest-frame peak energy ( E p, z ) of the ν F ν prompt emission spectrum with (i) the isotropic-bolometric-equivalent emission energy E γ ,iso (the Amati relation) and (ii) the isotropic-bolometric-equivalent peak luminosity L p,iso (the Yonetoku relation). From a linear regression analysis, a tight correlation between E p, z and E γ ,iso (and L γ ,iso ) is found for both Band-like and CPL-like bursts (except for CPL-like long burst E p, z – E γ ,iso correlation). More interestingly, CPL-like bursts do not fall on the Band-like burst Amati and Yonetoku correlations, suggesting distinct radiation processes, and pointing to the fact that these spectral energy correlations are tightly reliant on the model-wise properties. 

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