Nonthermal afterglow of the binary neutron star merger GW170817: a more natural modeling of electron energy distribution leads to a qualitatively different new solution

Abstract

The observed nonthermal afterglow spectrum of the binary neutron star (BNS) merger GW170817 from radio to X-ray are consistent with synchrotron radiation by shock-accelerated electrons. However, previous afterglow modeling studies were based on a simplified assumption that the acceleration efficiency is extremely high, i.e. all electrons in the shock are accelerated as a nonthermal population. This affects the estimate of the minimum electron energy and hence ${\nu }_m$, the peak frequency of the afterglow spectrum. Here we present Bayesian fitting to the observed data with a more natural electron energy distribution, in which the acceleration efficiency is a free parameter. Interestingly, the maximum likelihood solutions are found with radio flux below ${\nu }_m$ in the early phase, in contrast to previous studies that found the radio frequency always above ${\nu }_m$. Therefore the ${\nu }_m$ passage through the radio band could have been clearly detected for GW170817, if sufficient low-frequency radio data had been taken in early time. In the new solutions, the lowest energy of electrons is found close to equipartition with the post shock protons, but only a small fraction ($<$10\%) of electrons are accelerated as nonthermal particles. The jet energy and interstellar medium density are increased by 1--2 orders of magnitude from the conventional modeling, though these are still consistent with other constraints. We encourage to take densely sampled low-frequency radio data in the early phase for future BNS merger events, which would potentially detect ${\nu }_m$ passage and give a strong constraint on electron energy distribution and particle acceleration efficiency.

arXiv comment15 pages, 9 figures, 2 tables, accepted by MNRAS

References in corpus (61)

• emcee: The MCMC Hammer
• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A
• Spectra and Light Curves of Gamma-Ray Burst Afterglows
• An Ordinary Short Gamma-Ray Burst with Extraordinary Implications: Fermi-GBM Detection of GRB 170817A
• Origin of the heavy elements in binary neutron-star mergers from a gravitational wave event
• Swope Supernova Survey 2017a (SSS17a), the Optical Counterpart to a Gravitational Wave Source
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. II. UV, Optical, and Near-IR Light Curves and Comparison to Kilonova Models
• Spectroscopic identification of r-process nucleosynthesis in a double neutron star merger
• INTEGRAL Detection of the First Prompt Gamma-Ray Signal Coincident with the Gravitational Wave Event GW170817
• The X-ray counterpart to the gravitational wave event GW 170817
• A kilonova as the electromagnetic counterpart to a gravitational-wave source
• Light Curves of the Neutron Star Merger GW170817/SSS17a: Implications for R-Process Nucleosynthesis
• The Emergence of a Lanthanide-Rich Kilonova Following the Merger of Two Neutron Stars
• The Shape of Spectral Breaks in GRB Afterglows
• Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger
• Superluminal motion of a relativistic jet in the neutron star merger GW170817
• The Combined Ultraviolet, Optical, and Near-Infrared Light Curves of the Kilonova Associated with the Binary Neutron Star Merger GW170817: Unified Data Set, Analytic Models, and Physical Implications
• Swift and NuSTAR observations of GW170817: detection of a blue kilonova
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. IV. Detection of Near-infrared Signatures of r-process Nucleosynthesis with Gemini-South
• Systematics of dynamical mass ejection, nucleosynthesis, and radioactively powered electromagnetic signals from neutron-star mergers
• The mass ejection from the merger of binary neutron stars
• A Radio Counterpart to a Neutron Star Merger
• A Decade of Short-duration Gamma-ray Burst Broad-band Afterglows: Energetics, Circumburst Densities, and Jet Opening Angles
• The discovery of the electromagnetic counterpart of GW170817: kilonova AT 2017gfo/DLT17ck
• Particle Acceleration in Relativistic Magnetized Collisionless Electron-Ion Shocks
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. V. Rising X-ray Emission from an Off-Axis Jet
• A mildly relativistic wide-angle outflow in the neutron star merger GW170817
• Late time afterglow observations reveal a collimated relativistic jet in the ejecta of the binary neutron star merger GW170817
• Early Spectra of the Gravitational Wave Source GW170817: Evolution of a Neutron Star Merger
• The Binary Neutron Star event LIGO/VIRGO GW170817 a hundred and sixty days after merger: synchrotron emission across the electromagnetic spectrum
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-Time Emission from the Kilonova Ejecta
• Hot Gas In and Around Elliptical Galaxies
• Fine Structures of Shock of SN 1006 with the Chandra Observation
• The Electromagnetic Signals of Compact Binary Mergers
• Electromagnetic Evidence that SSS17a is the Result of a Binary Neutron Star Merger
• The outflow structure of GW170817 from late time broadband observations
• Images and Spectra From the Interior of a Relativistic Fireball
• Simultaneous Acceleration of Protons and Electrons at Nonrelativistic Quasiparallel Collisionless Shocks
• A Deep Chandra X-ray Study of Neutron Star Coalescence GW170817
• A Decline in the X-ray through Radio Emission from GW170817 Continues to Support an Off-Axis Structured Jet
• A cocoon shock breakout as the origin of the $\gamma$-ray emission in GW170817
• The evolution of the X-ray afterglow emission of GW 170817 / GRB 170817A in XMM-Newton observations
• A turnover in the radio light curve of GW170817
• The efficiency of electron acceleration in collisionless shocks and GRB energetics
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. VII. Properties of the Host Galaxy and Constraints on the Merger Timescale
• Afterglow Imaging and Polarization of Misaligned Structured GRB Jets and Cocoons: Breaking the Degeneracy in GRB 170817A
• Low frequency view of GW 170817/GRB 170817A with the Giant Meterwave Radio Telescope
• Synchrotron radiation from the fast tail of dynamical ejecta of neutron star mergers
• Sub-radian-accuracy gravitational waveforms of coalescing binary neutron stars in numerical relativity
• Ultrarelativistic electromagnetic counterpart to binary neutron star mergers
• From $\gamma$ to Radio - The Electromagnetic Counterpart of GW 170817
• Electrons' energy in GRB afterglows implied by radio peaks
• Implications of the radio and X-ray emission that followed GW170817
• On the electron energy distribution index of Swift Gamma-Ray Burst Afterglows
• Comparison of an approximately isothermal gravitational potentials of elliptical galaxies based on X-ray and optical data
• Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-ray Tail
• A Semi-analytic Formulation for Relativistic Blast Waves with a Long-lived Reverse Shock
• An upper-limit on the linear polarization fraction of the GW170817 radio continuum
• On the Non-existence of a Sharp Cooling Break in GRB Afterglow Spectra
• Optimized Radio Follow Up of Binary Neutron-Star Mergers

Cited by in corpus (4)

• The late afterglow of GW170817/GRB170817A: a large viewing angle and the shift of the Hubble constant to a value more consistent with the local measurements
• Constraining properties of neutron star merger outflows with radio observations
• Detectability of radio afterglows from binary neutron star mergers and implications for fast radio bursts
• On the use of CHIME to Detect Long-Duration Radio Transients from Neutron Star Mergers