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1. GRB minimum variability timescale with Insight-HXMT and Swift: Implications for progenitor models, dissipation physics, and GRB classifications

   https://doi.org/10.1051/0004-6361/202245657  

   Camisasca, A. E. ; Guidorzi, C. ; Amati, L. ; Frontera, F. ; Song, X. Y. ; Xiao, S. ; Xiong, S. L. ; Zhang, S. N. ; Margutti, R. ; Kobayashi, S. ; et al ( March 2023 , Astronomy & Astrophysics)

   Context. There has been significant technological and scientific progress in our ability to detect, monitor, and model the physics of γ -ray bursts (GRBs) over the 50 years since their first discovery. However, the dissipation process thought to be responsible for their defining prompt emission is still unknown. Recent efforts have focused on investigating how the ultrarelativistic jet of the GRB propagates through the progenitor’s stellar envelope for different initial composition shapes, jet structures, magnetisation, and, consequently, possible energy dissipation processes. Study of the temporal variability – in particular the shortest duration of an independent emission episode within a GRB – may provide a unique way to distinguish the imprint of the inner engine activity from geometry and propagation related effects. The advent of new high-energy detectors with exquisite time resolution now makes this possible. Aims. We aim to characterise the minimum variability timescale (MVT) defined as the shortest duration of individual pulses that shape a light curve for a sample of GRBs in the keV–MeV energy range and test correlations with other key observables such as the peak luminosity, the Lorentz factor, and the jet opening angle. We compare these correlations with predictions from recent numerical simulations for a relativistic structured – possibly wobbling – jet and assess the value of temporal variability studies as probes of prompt-emission dissipation physics. Methods. We used the peak detection algorithm MEPSA to identify the shortest pulse within a GRB time history and preliminarily calibrated MEPSA to estimate the full width at half maximum duration. We then applied this framework to two sets of GRBs: Swift GRBs (from 2005 to July 2022) and Insight Hard Modulation X-ray Telescope (Insight-HXMT) GRBs (from June 2017 to July 2021, including the exceptional 221009A). We then selected 401 GRBs with measured redshift to test for correlations. Results. We confirm that, on average, short GRBs have significantly shorter MVTs than long GRBs. The MVT distribution of short GRBs with extended emission such as 060614 and 211211A is compatible only with that of short GRBs. This is important because it provides a new clue concerning the progenitor’s nature. The MVT for long GRBs with measured redshift anti-correlates with peak luminosity; our analysis includes careful evaluation of selection effects. We confirm the anti-correlation with the Lorentz factor and find a correlation with the jet opening angle as estimated from the afterglow light curve, along with an inverse correlation with the number of pulses. Conclusions. The MVT can identify the emerging putative new class of long GRBs that are suggested to be produced by compact binary mergers. For otherwise typical long GRBs, the different correlations between MVT and peak luminosity, Lorentz factor, jet opening angle, and number of pulses can be explained within the context of structured, possibly wobbling, weakly magnetised relativistic jets. 

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2. A blast from the infant Universe: The very high- z GRB 210905A

   https://doi.org/10.1051/0004-6361/202243225  

   Rossi, A. ; Frederiks, D. D. ; Kann, D. A. ; De Pasquale, M. ; Pian, E. ; Lamb, G. ; D’Avanzo, P. ; Izzo, L. ; Levan, A. J. ; Malesani, D. B. ; et al ( September 2022 , Astronomy & Astrophysics)

   We present a detailed follow-up of the very energetic GRB 210905A at a high redshift of z  = 6.312 and its luminous X-ray and optical afterglow. Following the detection by Swift and Konus- Wind , we obtained a photometric and spectroscopic follow-up in the optical and near-infrared (NIR), covering both the prompt and afterglow emission from a few minutes up to 20 Ms after burst. With an isotropic gamma-ray energy release of E iso = 1.27 −0.19 +0.20 × 10 54 erg, GRB 210905A lies in the top ∼7% of gamma-ray bursts (GRBs) in the Konus- Wind catalogue in terms of energy released. Its afterglow is among the most luminous ever observed, and, in particular, it is one of the most luminous in the optical at t  ≳ 0.5 d in the rest frame. The afterglow starts with a shallow evolution that can be explained by energy injection, and it is followed by a steeper decay, while the spectral energy distribution is in agreement with slow cooling in a constant-density environment within the standard fireball theory. A jet break at ∼46.2 ± 16.3 d (6.3 ± 2.2 d rest-frame) has been observed in the X-ray light curve; however, it is hidden in the H band due to a constant contribution from the host galaxy and potentially from a foreground intervening galaxy. In particular, the host galaxy is only the fourth GRB host at z  > 6 known to date. By assuming a number density n  = 1 cm −3 and an efficiency η  = 0.2, we derived a half-opening angle of 8.4 ° ±1.0°, which is the highest ever measured for a z  ≳ 6 burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of ≃1 × 10 52 erg is also among the highest ever measured. The moderately large half-opening angle argues against recent claims of an inverse dependence of the half-opening angle on the redshift. The total jet energy is likely too large to be sustained by a standard magnetar, and it suggests that the central engine of this burst was a newly formed black hole. Despite the outstanding energetics and luminosity of both GRB 210905A and its afterglow, we demonstrate that they are consistent within 2 σ with those of less distant bursts, indicating that the powering mechanisms and progenitors do not evolve significantly with redshift. 

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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. The Optical Light Curve of GRB 221009A: The Afterglow and the Emerging Supernova

   https://doi.org/10.3847/2041-8213/acc101  

   Fulton, M. D. ; Smartt, S. J. ; Rhodes, L. ; Huber, M. E. ; Villar, V. A. ; Moore, T. ; Srivastav, S. ; Schultz, A. S. ; Chambers, K. C. ; Izzo, L. ; et al ( March 2023 , The Astrophysical Journal Letters)

   Abstract We present extensive optical photometry of the afterglow of GRB 221009A. Our data cover 0.9–59.9 days from the time of Swift and Fermi gamma-ray burst (GRB) detections. Photometry in rizy -band filters was collected primarily with Pan-STARRS and supplemented by multiple 1–4 m imaging facilities. We analyzed the Swift X-ray data of the afterglow and found a single decline rate power law f ( t ) ∝ t −1.556±0.002 best describes the light curve. In addition to the high foreground Milky Way dust extinction along this line of sight, the data favor additional extinction to consistently model the optical to X-ray flux with optically thin synchrotron emission. We fit the X-ray-derived power law to the optical light curve and find good agreement with the measured data up to 5−6 days. Thereafter we find a flux excess in the riy bands that peaks in the observer frame at ∼20 days. This excess shares similar light-curve profiles to the Type Ic broad-lined supernovae SN 2016jca and SN 2017iuk once corrected for the GRB redshift of z = 0.151 and arbitrarily scaled. This may be representative of an SN emerging from the declining afterglow. We measure rest-frame absolute peak AB magnitudes of M g = −19.8 ± 0.6 and M r = − 19.4 ± 0.3 and M z = −20.1 ± 0.3. If this is an SN component, then Bayesian modeling of the excess flux would imply explosion parameters of M ej = 7.1 − 1.7 + 2.4 M ⊙ , M Ni = 1.0 − 0.4 + 0.6 M ⊙ , and v ej = 33,900 − 5700 + 5900 km s −1 , for the ejecta mass, nickel mass, and ejecta velocity respectively, inferring an explosion energy of E kin ≃ 2.6–9.0 × 10 52 erg. 

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5. The Radio to GeV Afterglow of GRB 221009A

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

   Laskar, Tanmoy ; Alexander, Kate D. ; Margutti, Raffaella ; Eftekhari, Tarraneh ; Chornock, Ryan ; Berger, Edo ; Cendes, Yvette ; Duerr, Anne ; Perley, Daniel A. ; Ravasio, Maria Edvige ; et al ( March 2023 , The Astrophysical Journal Letters)

   Abstract GRB 221009A ( z = 0.151) is one of the closest known long γ -ray bursts (GRBs). Its extreme brightness across all electromagnetic wavelengths provides an unprecedented opportunity to study a member of this still-mysterious class of transients in exquisite detail. We present multiwavelength observations of this extraordinary event, spanning 15 orders of magnitude in photon energy from radio to γ -rays. We find that the data can be partially explained by a forward shock (FS) from a highly collimated relativistic jet interacting with a low-density, wind-like medium. Under this model, the jet’s beaming-corrected kinetic energy ( E K ∼ 4 × 10 50 erg) is typical for the GRB population. The radio and millimeter data provide strong limiting constraints on the FS model, but require the presence of an additional emission component. From equipartition arguments, we find that the radio emission is likely produced by a small amount of mass (≲6 × 10 −7 M ⊙ ) moving relativistically (Γ ≳ 9) with a large kinetic energy (≳10 49 erg). However, the temporal evolution of this component does not follow prescriptions for synchrotron radiation from a single power-law distribution of electrons (e.g., in a reverse shock or two-component jet), or a thermal-electron population, perhaps suggesting that one of the standard assumptions of afterglow theory is violated. GRB 221009A will likely remain detectable with radio telescopes for years to come, providing a valuable opportunity to track the full lifecycle of a powerful relativistic jet. 

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