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1. A Cosmological Fireball with 16% Gamma-Ray Radiative Efficiency

   https://doi.org/10.3847/2041-8213/acb99d  

   Li, Liang; Wang, Yu; Ryde, Felix; Pe’er, Asaf; Zhang, Bing; Guiriec, Sylvain; Castro-Tirado, Alberto J.; Kann, D. Alexander; Axelsson, Magnus; Page, Kim; et al (February 2023, The Astrophysical Journal Letters)

   Abstract Gamma-ray bursts (GRBs) are the most powerful explosions in the universe. How efficiently the jet converts its energy to radiation is a long-standing problem, which is poorly constrained. The standard model invokes a relativistic fireball with a bright photosphere emission component. A definitive diagnosis of GRB radiation components and the measurement of GRB radiative efficiency require prompt emission and afterglow data, with high resolution and wide band coverage in time and energy. Here, we present a comprehensive temporal and spectral analysis of the TeV-emitting bright GRB 190114C. Its fluence is one of the highest for all the GRBs that have been detected so far, which allows us to perform a high-resolution study of the prompt emission spectral properties and their temporal evolutions, down to a timescale of about 0.1 s. We observe that each of the initial pulses has a thermal component contributing ∼20% of the total energy and that the corresponding temperature and inferred Lorentz factor of the photosphere evolve following broken power-law shapes. From the observation of the nonthermal spectra and the light curve, the onset of the afterglow corresponding to the deceleration of the fireball is considered to start at ∼6 s. By incorporating the thermal and nonthermal observations, as well as the photosphere and synchrotron radiative mechanisms, we can directly derive the fireball energy budget with little dependence on hypothetical parameters, measuring a ∼16% radiative efficiency for this GRB. With the fireball energy budget derived, the afterglow microphysics parameters can also be constrained directly from the data. 

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2. Time-resolved Spectral Properties of Fermi-GBM Bright Long Gamma-Ray Bursts

   https://doi.org/10.1088/1674-4527/ad16af  

   Wang, Wan-Kai; Xie, Wei; Gao, Zhi-Fu; Xiao, Shuo; Dong, Ai-Jun; Zhang, Bin; Zhi, Qi-Jun (January 2024, Research in Astronomy and Astrophysics)

   Abstract The prompt emission mechanism of gamma-ray bursts (GRBs) is still unclear, and the time-resolved spectral analysis of GRBs is a powerful tool for studying their underlying physical processes. We performed a detailed time-resolved spectral analysis of 78 bright long GRB samples detected by Fermi/Gamma-ray Burst Monitor. A total of 1490 spectra were obtained and their properties were studied using a typical Band-shape model. First, the parameter distributions of the time-resolved spectrum are given as follows: the low-energy spectral indexα∼ − 0.72, high-energy spectral indexβ∼ − 2.42, the peak energyE_p∼ 221.69 keV, and the energy fluxF∼ 7.49 × 10⁻⁶erg cm⁻²s⁻¹. More than 80% of the bursts exhibit the hardest low-energy spectral index ${\alpha }_{\max }$exceeding the synchrotron limit (−2/3). Second, the evolution patterns ofαandE_pwere statistically analyzed. The results show that for multi-pulse GRBs the intensity-tracking pattern is more common than the hard-to-soft pattern in the evolution of bothE_pandα. The hard-to-soft pattern is generally shown in single-pulse GRBs or in the initial pulse of multi-pulse GRBs. Finally, we found a significant positive correlation betweenFandE_p, with half of the samples exhibiting a positive correlation betweenFandα. We discussed the spectral evolution of different radiation models. The diversity of spectral evolution patterns indicates that there may be more than one radiation mechanism occurring in the GRB radiation process, including photospheric radiation and synchrotron radiation. However, it may also involve only one radiation mechanism, but more complicated physical details need to be considered. 

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3. Uniform Modeling of Observed Kilonovae: Implications for Diversity and the Progenitors of Merger-driven Long Gamma-Ray Bursts

   https://doi.org/10.3847/1538-4357/ad9c77  

   Rastinejad, J_C; Fong, W.; Kilpatrick, C_D; Nicholl, M.; Metzger, B_D (January 2025, The Astrophysical Journal)

   Abstract We present uniform modeling of eight kilonovae, five following short gamma-ray bursts (GRBs; including GRB 170817A) and three following long GRBs. We model their broadband afterglows to determine the relative contributions of afterglow and kilonova emission. We fit the kilonovae using a three-component model inMOSFiT, and report population median ejecta masses for the total, blue (κ_B = 0.5 cm²g⁻¹), purple (κ_P = 3 cm²g⁻¹), and red (κ_R = 10 cm²g⁻¹) components. The kilonova of GW170817 is near the sample median in most derived properties. We investigate trends between the ejecta masses and the isotropic-equivalent and beaming-correctedγ-ray energies (E_γ,iso,E_γ), as well as rest-frame durations (T_90,rest). We find long GRB kilonovae have higher median red ejecta masses (M_ej,R ≳ 0.05M_⊙) compared to on-axis short GRB kilonovae (M_ej,R ≲ 0.02M_⊙). We also observe a weak scaling between the total and red ejecta masses withE_γ,isoandE_γ, though a larger sample is needed to establish a significant correlation. These findings imply a connection between merger-driven long GRBs and larger tidal dynamical ejecta masses, which may indicate that their progenitors are asymmetric compact object binaries. We produce representative kilonova light curves, and find that the planned depths and cadences of the Rubin and Roman Observatory surveys will be sufficient for order-of-magnitude constraints onM_ej,B(and, for Roman,M_ej,PandM_ej,R) of future kilonovae atz ≲ 0.1. 

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4. The Long-lived Broadband Afterglow of Short Gamma-Ray Burst 231117A and the Growing Radio-detected Short Gamma-Ray Burst Population

   https://doi.org/10.3847/1538-4357/ada9e5  

   Schroeder, Genevieve; Fong, Wen-fai; Kilpatrick, Charles D; Rouco_Escorial, Alicia; Laskar, Tanmoy; Nugent, Anya E; Rastinejad, Jillian; Alexander, Kate D; Berger, Edo; Brink, Thomas G; et al (March 2025, The Astrophysical Journal)

   Abstract We present multiwavelength observations of the Swift shortγ-ray burst GRB 231117A, localized to an underlying galaxy at redshiftz= 0.257 at a small projected offset (∼2 kpc). We uncover long-lived X-ray Chandra X-ray Observatory and radio/millimeter (VLA, MeerKAT, and ALMA) afterglow emission, detected to ∼37 days and ∼20 days (rest frame), respectively. We measure a wide jet (∼10 $\stackrel{\text{°}}{.}$4) and relatively high circumburst density (∼0.07 cm⁻³) compared to the short GRB population. Our data cannot be easily fit with a standard forward shock model, but they are generally well fit with the incorporation of a refreshed forward shock and a reverse shock at <1 day. We incorporate GRB 231117A into a larger sample of 132 X-ray detected events, 71 of which were radio-observed (17 cm-band detections), for a systematic study of the distributions of redshifts, jet and afterglow properties, galactocentric offsets, and local environments of events with and without detected radio afterglows. Compared to the entire short GRB population, the majority of radio-detected GRBs are at relatively low redshifts (z < 0.6) and have high circumburst densities (>10⁻²cm⁻³), consistent with their smaller (<8 kpc) projected galactocentric offsets. We additionally find that 70% of short GRBs with opening angle measurements were radio-detected, indicating the importance of radio afterglows in jet measurements, especially in the cases of wide (>10°) jets where observational evidence of collimation may only be detectable at radio wavelengths. Owing to improved observing strategies and the emergence of sensitive radio facilities, the number of radio-detected short GRBs has quadrupled in the past decade. 

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5. Photospheric Prompt Emission From Long Gamma Ray Burst Simulations. II. Spectropolarimetry

   https://doi.org/10.3847/1538-4357/ac4093  

   Parsotan, Tyler; Lazzati, Davide (February 2022, The Astrophysical Journal)

   Abstract Although gamma ray bursts (GRBs) have been detected for many decades, the lack of knowledge regarding the radiation mechanism that produces the energetic flash of radiation, or prompt emission, from these events has prevented the full use of GRBs as probes of high-energy astrophysical processes. While there are multiple models that attempt to describe the prompt emission, each model can be tuned to account for observed GRB characteristics in the gamma and X-ray energy bands. One energy range that has not been fully explored for the purpose of prompt emission model comparison is that of the optical band, especially with regard to polarization. Here, we use an improved Monte Carlo radiation transfer code to calculate the expected photospheric optical and gamma-ray polarization signatures (Π_optand Π_γ, respectively) from a set of two relativistic hydrodynamic long GRB simulations, which emulate a constant and variable jet. We find that time-resolved Π_optcan be large (∼75%) while time-integrated Π_optcan be smaller due to integration over the asymmetries in the GRB jet where optical photons originate; Π_γfollows a similar evolution as Π_optwith smaller polarization degrees. We also show that Π_optand Π_γagree well with observations in each energy range. Additionally, we make predictions for the expected polarization of GRBs based on their location within the Yonetoku relationship. While improvements can be made to our analyses and predictions, they exhibit the insight that global radiative transfer simulations of GRB jets can provide with respect to current and future observations. 

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