Cooling off with a kilonova - Lower Limit on the Expansion Velocity of GW170817

Abstract

GW170817 was the first detection of a binary neutron star merger via gravitational waves. The event was observed over a wide range of the electromagnetic spectrum, revealing a thermal kilonova dominating the optical signal during the first $\sim$15 days, and a non-thermal synchrotron emission that has continued to rise $\sim$200 days post-merger, dominating the radio and x-ray emission. At early times, when the kilonova is still dominant, the synchrotron emitting electrons can efficiently cool by up-scattering the kilonova photos through inverse-Compton. Yet, the cooling frequency is not observed up to the X-ray band. This can only be explained if the source is moving at least at a mildly relativistic velocity. We find a lower limit on the source's bulk Lorentz factor of $\Gamma >2.1$ at $9$ days. This lower limit is model independent, and relies directly on the observed quantities, providing an additional robust evidence to the relativistic motion in this event at early times.

arXiv comment4 pages, accepted for publishing in MNRAS

References in corpus (25)

• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• Spectra and Light Curves of Gamma-Ray Burst Afterglows
• An Ordinary Short Gamma-Ray Burst with Extraordinary Implications: Fermi-GBM Detection of GRB 170817A
• 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
• A kilonova as the electromagnetic counterpart to a gravitational-wave source
• The X-ray counterpart to the gravitational wave event GW 170817
• Light Curves of the Neutron Star Merger GW170817/SSS17a: Implications for R-Process Nucleosynthesis
• Illuminating Gravitational Waves: A Concordant Picture of Photons from a Neutron Star Merger
• The Emergence of a Lanthanide-Rich Kilonova Following the Merger of Two Neutron Stars
• Superluminal motion of a relativistic jet in the neutron star merger GW170817
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. I. Dark Energy Camera Discovery of the Optical Counterpart
• Systematics of dynamical mass ejection, nucleosynthesis, and radioactively powered electromagnetic signals from neutron-star mergers
• A Radio Counterpart to a Neutron Star Merger
• A mildly relativistic wide-angle outflow in the neutron star merger GW170817
• 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
• A Deep Chandra X-ray Study of Neutron Star Coalescence GW170817
• J-GEM observations of an electromagnetic counterpart to the neutron star merger GW170817
• A cocoon shock breakout as the origin of the $\gamma$-ray emission in GW170817
• The cocoon emission - an electromagnetic counterpart to gravitational waves from neutron star mergers
• The spectrum of particles accelerated in relativistic, collisionless shocks
• Synchrotron radiation from the fast tail of dynamical ejecta of neutron star mergers
• Exploring tidal effects of coalescing binary neutron stars in numerical relativity
• Implications of the radio and X-ray emission that followed GW170817
• Constraints on the emitting region of the gamma-rays observed in GW170817

Cited by in corpus (7)

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• MAGIC observations of the nearby short gamma-ray burst GRB 160821B
• GRMHD simulations of neutron-star mergers with weak interactions: r-process nucleosynthesis and electromagnetic signatures of dynamical ejecta
• Continued radio observations of GW170817 3.5 years post-merger
• Constraining properties of neutron star merger outflows with radio observations
• Synchrotron break frequencies of mildly-to-highly relativistic outflows observed off-axis
• The impact of kilonova seed photons on GRB VHE emission