GW170817 4.5 Yr After Merger: Dynamical Ejecta Afterglow Constraints

Balasubramanian, Arvind; Corsi, A.; Mooley, K. P.; Hotokezaka, Kenta; Kaplan, David L.; Frail, D. A.; Hallinan, Gregg; Lazzati, Davide; Murphy, E. J.

doi:10.3847/1538-4357/ac9133

“…The broadband afterglows of GRB 221009A have been explained in several papers using structured jet models (e.g., Sato et al 2023;O'Connor et al 2023). The structured jet picture has been proposed (e.g., Mészáros et al 1998;Dai & Gou 2001;Rossi et al 2002;Zhang & Mészáros 2002) and supported by GRB 170817A (e.g., Troja et al 2017;Lazzati et al 2018;Ren et al 2020;Makhathini et al 2021;Balasubramanian et al 2022). Complex geometry produces the atypical spectral behaviors, but it also introduces a number of free parameters.…”

Section: Discussionmentioning

confidence: 99%

The Possibility of Modeling the Very High Energy Afterglow of GRB 221009A in a Wind Environment

Ren

¹

,

Wang

²

,

Zhang

³

et al. 2023

ApJ

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In this paper, we model the dynamics and radiation physics of the rarity event GRB 221009A afterglow in detail. By introducing a top-hat jet that propagates in an environment dominated by stellar winds, we explain the publicly available observations of afterglow associated with GRB 221009A over the first week. It is predicted that GRB 221009A emits a luminous very high energy afterglow based on the synchrotron self-Compton (SSC) process in our model. We show the broadband spectral energy distribution (SED) analysis results of GRB 221009A and find that the SSC radiation component of GRB 221009A is very bright in the 0.1–10 TeV band. The integrated SED shows that the SSC emission in the TeV band has detection sensitivity significantly higher than that of LHASSO, MAGIC, and CTA. However, since the release of further observations, deviations from the standard wind environment model have gradually shown up in data. For example, the late-time multiband afterglow cannot be consistently explained under the standard wind environment scenario. It may be necessary to consider modeling with a structured jet with complex geometry or a partial revision of the standard model. Furthermore, we find that the inclusion of GeV observations could break the degeneracy between model parameters, highlighting the significance of high-energy observations in determining accurate parameters for GRB afterglows.

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“…We apply our nonthermal afterglow model described above to the observations of Hajela et al (2022), who presented evidence for a late-time (≈1000 days post-merger) excess of X-ray and radio emission above the GRB broadband afterglow of GW170817. The existence of this late-time rebrightening is still a matter of debate (Balasubramanian et al 2022;) and, if present, could have other possible origins as well (e.g., Metzger & Fernandez 2021).…”

Section: Kilonova Afterglow Light Curves and Gw170817mentioning

confidence: 99%

“…After several years, the broadband synchrotron afterglow originating in the decelerating external shock driven into the ISM has faded to levels at which other possible emission components may be revealed, with first observational indications (Hajela et al 2022;Troja et al 2022). One robust additional emission component expected on timescales of years after the event is the broadband synchrotron emission from a trans-relativistic shock of fast dynamical ejecta expanding into the ISM (e.g., Nakar & Piran 2011;Hotokezaka et al 2018;Nedora et al 2021b;Balasubramanian et al 2022), which we refer to here as the "kilonova afterglow." Late-time fall-back accretion onto a remnant black hole offers a possible alternative explanation to a rebrightening (Metzger & Fernandez 2021).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

GRMHD Simulations of Neutron-star Mergers with Weak Interactions: r-process Nucleosynthesis and Electromagnetic Signatures of Dynamical Ejecta

Combi

¹

,

Siegel

²

2023

ApJ

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Fast neutron-rich material ejected dynamically over ≲10 ms during the merger of a binary neutron star (BNS) can give rise to distinctive electromagnetic counterparts to the system’s gravitational-wave emission that serve as a “smoking gun” to distinguish between a BNS and an NS–black hole merger. We present novel ab initio modeling of the kilonova precursor and kilonova afterglow based on 3D general-relativistic magnetohydrodynamic simulations of BNS mergers with nuclear, tabulated, finite-temperature equations of state (EOSs), weak interactions, and approximate neutrino transport. We analyze dynamical mass ejection from 1.35–1.35 M ⊙ binaries, consistent with properties of the first observed BNS merger GW170817, using three nuclear EOSs that span the range of allowed compactness of 1.35 M ⊙-neutron stars. Nuclear reaction network calculations yield a robust second-to-third-peak r-process. We find few ×10−6 M ⊙ of fast (v > 0.6c) ejecta that give rise to broadband synchrotron emission on ∼years timescales, consistent with tentative evidence for excess X-ray/radio emission following GW170817. We find ≈2 × 10−5 M ⊙ of free neutrons that power a kilonova precursor on ≲ hours timescale. A boost in early UV/optical brightness by a factor of a few due to previously neglected relativistic effects, with enhancements up to ≲10 hr post-merger, is promising for future detection with UV/optical telescopes like Swift or ULTRASAT. We find that a recently predicted opacity boost due to highly ionized lanthanides at ≳70,000 K is unlikely to affect the early kilonova based on the obtained ejecta structures. Azimuthal inhomogeneities in dynamical ejecta composition for soft EOSs found here (“lanthanide/actinide pockets”) may have observable consequences for both early kilonova and late-time nebular emission.

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“…The afterglow light curve detected from GRB170817A has been used to learn about the GRB and its environment. Several different GRB models and parameter estimation techniques have been employed to constrain the radiation, hydrodynamic, and cosmological properties of the burst (Lazzati et al 2018;Wu & MacFadyen 2018;Ghirlanda et al 2019;Hajela et al 2019;Hotokezaka et al 2019;Lamb et al 2019;Troja et al 2019;Ryan et al 2020;Balasubramanian et al 2022). In general, it is agreed that GRB170817A was a relativistic structured jet with a characteristic opening angle, θ j , between 2°a nd 8°viewed off-axis at an angle, θ obs , between 15°and 35° ( Nakar & Piran 2021).…”

Section: Introductionmentioning

confidence: 99%

Revisiting the Parameter Space of Binary Neutron Star Merger Event GW170817

McDowell

¹

,

MacFadyen

²

2023

ApJ

6

0

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Since the gravitational wave event GW170817 and gamma-ray burst GW170817A, there have been numerous studies constraining the burst properties through analysis of the afterglow light curves. Most agree that the burst was viewed off-axis with a ratio of the observer angle to the jet angle (θ obs/θ j ) between 4 and 6. We use a parameterized model and broadband synchrotron data up to ∼800 days post-merger to constrain parameters of the burst. To reproduce the hydrodynamics of a gamma-ray burst outflow, we use a two-parameter “boosted fireball” model. The structure of a boosted fireball is determined by the specific internal energy, η 0, and the bulk Lorentz factor, γ B ( ∼ 1/θ j ), with shapes varying smoothly from a quasi-spherical outflow for low values of γ B to a highly collimated jet for high values. We run simulations with γ B in the range 1–20 and η 0 in the range 2–15. To calculate light curves, we use a synchrotron radiation model characterized by F peak, ν m , and ν c , and calculate millions of spectra at different times and θ obs values using the boxfit radiation code. We can tabulate the spectral parameter values from our spectra and rapidly generate arbitrary light curves for comparison to data in MCMC analysis. We find that our model prefers a gamma-ray burst with jet energy E j ∼ 1050 erg and with an observer angle of θ obs = 0.65 − 0.14 + 0.13 radians and ratio to the jet opening angle of (θ obs/θ j ) = 5.4 − 0.38 + 0.53 .

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GW170817 4.5 Yr After Merger: Dynamical Ejecta Afterglow Constraints

Cited by 22 publications

References 55 publications

The Possibility of Modeling the Very High Energy Afterglow of GRB 221009A in a Wind Environment

The Possibility of Modeling the Very High Energy Afterglow of GRB 221009A in a Wind Environment

GRMHD Simulations of Neutron-star Mergers with Weak Interactions: r-process Nucleosynthesis and Electromagnetic Signatures of Dynamical Ejecta

Revisiting the Parameter Space of Binary Neutron Star Merger Event GW170817

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